method for a literature review

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Writing a Literature Review

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A literature review is a document or section of a document that collects key sources on a topic and discusses those sources in conversation with each other (also called synthesis ). The lit review is an important genre in many disciplines, not just literature (i.e., the study of works of literature such as novels and plays). When we say “literature review” or refer to “the literature,” we are talking about the research ( scholarship ) in a given field. You will often see the terms “the research,” “the scholarship,” and “the literature” used mostly interchangeably.

Where, when, and why would I write a lit review?

There are a number of different situations where you might write a literature review, each with slightly different expectations; different disciplines, too, have field-specific expectations for what a literature review is and does. For instance, in the humanities, authors might include more overt argumentation and interpretation of source material in their literature reviews, whereas in the sciences, authors are more likely to report study designs and results in their literature reviews; these differences reflect these disciplines’ purposes and conventions in scholarship. You should always look at examples from your own discipline and talk to professors or mentors in your field to be sure you understand your discipline’s conventions, for literature reviews as well as for any other genre.

A literature review can be a part of a research paper or scholarly article, usually falling after the introduction and before the research methods sections. In these cases, the lit review just needs to cover scholarship that is important to the issue you are writing about; sometimes it will also cover key sources that informed your research methodology.

Lit reviews can also be standalone pieces, either as assignments in a class or as publications. In a class, a lit review may be assigned to help students familiarize themselves with a topic and with scholarship in their field, get an idea of the other researchers working on the topic they’re interested in, find gaps in existing research in order to propose new projects, and/or develop a theoretical framework and methodology for later research. As a publication, a lit review usually is meant to help make other scholars’ lives easier by collecting and summarizing, synthesizing, and analyzing existing research on a topic. This can be especially helpful for students or scholars getting into a new research area, or for directing an entire community of scholars toward questions that have not yet been answered.

What are the parts of a lit review?

Most lit reviews use a basic introduction-body-conclusion structure; if your lit review is part of a larger paper, the introduction and conclusion pieces may be just a few sentences while you focus most of your attention on the body. If your lit review is a standalone piece, the introduction and conclusion take up more space and give you a place to discuss your goals, research methods, and conclusions separately from where you discuss the literature itself.

Introduction:

• An introductory paragraph that explains what your working topic and thesis is
• A forecast of key topics or texts that will appear in the review
• Potentially, a description of how you found sources and how you analyzed them for inclusion and discussion in the review (more often found in published, standalone literature reviews than in lit review sections in an article or research paper)
• Summarize and synthesize: Give an overview of the main points of each source and combine them into a coherent whole
• Analyze and interpret: Don’t just paraphrase other researchers – add your own interpretations where possible, discussing the significance of findings in relation to the literature as a whole
• Critically Evaluate: Mention the strengths and weaknesses of your sources
• Write in well-structured paragraphs: Use transition words and topic sentence to draw connections, comparisons, and contrasts.

Conclusion:

• Summarize the key findings you have taken from the literature and emphasize their significance
• Connect it back to your primary research question

How should I organize my lit review?

Lit reviews can take many different organizational patterns depending on what you are trying to accomplish with the review. Here are some examples:

• Chronological : The simplest approach is to trace the development of the topic over time, which helps familiarize the audience with the topic (for instance if you are introducing something that is not commonly known in your field). If you choose this strategy, be careful to avoid simply listing and summarizing sources in order. Try to analyze the patterns, turning points, and key debates that have shaped the direction of the field. Give your interpretation of how and why certain developments occurred (as mentioned previously, this may not be appropriate in your discipline — check with a teacher or mentor if you’re unsure).
• Thematic : If you have found some recurring central themes that you will continue working with throughout your piece, you can organize your literature review into subsections that address different aspects of the topic. For example, if you are reviewing literature about women and religion, key themes can include the role of women in churches and the religious attitude towards women.
• Qualitative versus quantitative research
• Empirical versus theoretical scholarship
• Divide the research by sociological, historical, or cultural sources
• Theoretical : In many humanities articles, the literature review is the foundation for the theoretical framework. You can use it to discuss various theories, models, and definitions of key concepts. You can argue for the relevance of a specific theoretical approach or combine various theorical concepts to create a framework for your research.

What are some strategies or tips I can use while writing my lit review?

Any lit review is only as good as the research it discusses; make sure your sources are well-chosen and your research is thorough. Don’t be afraid to do more research if you discover a new thread as you’re writing. More info on the research process is available in our "Conducting Research" resources .

As you’re doing your research, create an annotated bibliography ( see our page on the this type of document ). Much of the information used in an annotated bibliography can be used also in a literature review, so you’ll be not only partially drafting your lit review as you research, but also developing your sense of the larger conversation going on among scholars, professionals, and any other stakeholders in your topic.

Usually you will need to synthesize research rather than just summarizing it. This means drawing connections between sources to create a picture of the scholarly conversation on a topic over time. Many student writers struggle to synthesize because they feel they don’t have anything to add to the scholars they are citing; here are some strategies to help you:

• It often helps to remember that the point of these kinds of syntheses is to show your readers how you understand your research, to help them read the rest of your paper.
• Writing teachers often say synthesis is like hosting a dinner party: imagine all your sources are together in a room, discussing your topic. What are they saying to each other?
• Look at the in-text citations in each paragraph. Are you citing just one source for each paragraph? This usually indicates summary only. When you have multiple sources cited in a paragraph, you are more likely to be synthesizing them (not always, but often
• Read more about synthesis here.

The most interesting literature reviews are often written as arguments (again, as mentioned at the beginning of the page, this is discipline-specific and doesn’t work for all situations). Often, the literature review is where you can establish your research as filling a particular gap or as relevant in a particular way. You have some chance to do this in your introduction in an article, but the literature review section gives a more extended opportunity to establish the conversation in the way you would like your readers to see it. You can choose the intellectual lineage you would like to be part of and whose definitions matter most to your thinking (mostly humanities-specific, but this goes for sciences as well). In addressing these points, you argue for your place in the conversation, which tends to make the lit review more compelling than a simple reporting of other sources.

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• What is a Literature Review? | Guide, Template, & Examples

What is a Literature Review? | Guide, Template, & Examples

Published on 22 February 2022 by Shona McCombes . Revised on 7 June 2022.

What is a literature review? A literature review is a survey of scholarly sources on a specific topic. It provides an overview of current knowledge, allowing you to identify relevant theories, methods, and gaps in the existing research.

There are five key steps to writing a literature review:

• Search for relevant literature
• Evaluate sources
• Identify themes, debates and gaps
• Outline the structure
• Write your literature review

A good literature review doesn’t just summarise sources – it analyses, synthesises, and critically evaluates to give a clear picture of the state of knowledge on the subject.

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Table of contents

Why write a literature review, examples of literature reviews, step 1: search for relevant literature, step 2: evaluate and select sources, step 3: identify themes, debates and gaps, step 4: outline your literature review’s structure, step 5: write your literature review, frequently asked questions about literature reviews, introduction.

• Quick Run-through
• Step 1 & 2

When you write a dissertation or thesis, you will have to conduct a literature review to situate your research within existing knowledge. The literature review gives you a chance to:

• Demonstrate your familiarity with the topic and scholarly context
• Develop a theoretical framework and methodology for your research
• Position yourself in relation to other researchers and theorists
• Show how your dissertation addresses a gap or contributes to a debate

You might also have to write a literature review as a stand-alone assignment. In this case, the purpose is to evaluate the current state of research and demonstrate your knowledge of scholarly debates around a topic.

The content will look slightly different in each case, but the process of conducting a literature review follows the same steps. We’ve written a step-by-step guide that you can follow below.

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Writing literature reviews can be quite challenging! A good starting point could be to look at some examples, depending on what kind of literature review you’d like to write.

• Example literature review #1: “Why Do People Migrate? A Review of the Theoretical Literature” ( Theoretical literature review about the development of economic migration theory from the 1950s to today.)
• Example literature review #2: “Literature review as a research methodology: An overview and guidelines” ( Methodological literature review about interdisciplinary knowledge acquisition and production.)
• Example literature review #3: “The Use of Technology in English Language Learning: A Literature Review” ( Thematic literature review about the effects of technology on language acquisition.)
• Example literature review #4: “Learners’ Listening Comprehension Difficulties in English Language Learning: A Literature Review” ( Chronological literature review about how the concept of listening skills has changed over time.)

You can also check out our templates with literature review examples and sample outlines at the links below.

Download Word doc Download Google doc

Before you begin searching for literature, you need a clearly defined topic .

If you are writing the literature review section of a dissertation or research paper, you will search for literature related to your research objectives and questions .

If you are writing a literature review as a stand-alone assignment, you will have to choose a focus and develop a central question to direct your search. Unlike a dissertation research question, this question has to be answerable without collecting original data. You should be able to answer it based only on a review of existing publications.

Make a list of keywords

Start by creating a list of keywords related to your research topic. Include each of the key concepts or variables you’re interested in, and list any synonyms and related terms. You can add to this list if you discover new keywords in the process of your literature search.

• Social media, Facebook, Instagram, Twitter, Snapchat, TikTok
• Body image, self-perception, self-esteem, mental health
• Generation Z, teenagers, adolescents, youth

Search for relevant sources

Use your keywords to begin searching for sources. Some databases to search for journals and articles include:

• Your university’s library catalogue
• Google Scholar
• Project Muse (humanities and social sciences)
• Medline (life sciences and biomedicine)
• EconLit (economics)
• Inspec (physics, engineering and computer science)

You can use boolean operators to help narrow down your search:

Read the abstract to find out whether an article is relevant to your question. When you find a useful book or article, you can check the bibliography to find other relevant sources.

To identify the most important publications on your topic, take note of recurring citations. If the same authors, books or articles keep appearing in your reading, make sure to seek them out.

You probably won’t be able to read absolutely everything that has been written on the topic – you’ll have to evaluate which sources are most relevant to your questions.

For each publication, ask yourself:

• What question or problem is the author addressing?
• What are the key concepts and how are they defined?
• What are the key theories, models and methods? Does the research use established frameworks or take an innovative approach?
• What are the results and conclusions of the study?
• How does the publication relate to other literature in the field? Does it confirm, add to, or challenge established knowledge?
• How does the publication contribute to your understanding of the topic? What are its key insights and arguments?
• What are the strengths and weaknesses of the research?

Make sure the sources you use are credible, and make sure you read any landmark studies and major theories in your field of research.

You can find out how many times an article has been cited on Google Scholar – a high citation count means the article has been influential in the field, and should certainly be included in your literature review.

The scope of your review will depend on your topic and discipline: in the sciences you usually only review recent literature, but in the humanities you might take a long historical perspective (for example, to trace how a concept has changed in meaning over time).

Remember that you can use our template to summarise and evaluate sources you’re thinking about using!

Take notes and cite your sources

As you read, you should also begin the writing process. Take notes that you can later incorporate into the text of your literature review.

It’s important to keep track of your sources with references to avoid plagiarism . It can be helpful to make an annotated bibliography, where you compile full reference information and write a paragraph of summary and analysis for each source. This helps you remember what you read and saves time later in the process.

You can use our free APA Reference Generator for quick, correct, consistent citations.

To begin organising your literature review’s argument and structure, you need to understand the connections and relationships between the sources you’ve read. Based on your reading and notes, you can look for:

• Trends and patterns (in theory, method or results): do certain approaches become more or less popular over time?
• Themes: what questions or concepts recur across the literature?
• Debates, conflicts and contradictions: where do sources disagree?
• Pivotal publications: are there any influential theories or studies that changed the direction of the field?
• Gaps: what is missing from the literature? Are there weaknesses that need to be addressed?

This step will help you work out the structure of your literature review and (if applicable) show how your own research will contribute to existing knowledge.

• Most research has focused on young women.
• There is an increasing interest in the visual aspects of social media.
• But there is still a lack of robust research on highly-visual platforms like Instagram and Snapchat – this is a gap that you could address in your own research.

There are various approaches to organising the body of a literature review. You should have a rough idea of your strategy before you start writing.

Depending on the length of your literature review, you can combine several of these strategies (for example, your overall structure might be thematic, but each theme is discussed chronologically).

Chronological

The simplest approach is to trace the development of the topic over time. However, if you choose this strategy, be careful to avoid simply listing and summarising sources in order.

Try to analyse patterns, turning points and key debates that have shaped the direction of the field. Give your interpretation of how and why certain developments occurred.

If you have found some recurring central themes, you can organise your literature review into subsections that address different aspects of the topic.

For example, if you are reviewing literature about inequalities in migrant health outcomes, key themes might include healthcare policy, language barriers, cultural attitudes, legal status, and economic access.

Methodological

If you draw your sources from different disciplines or fields that use a variety of research methods , you might want to compare the results and conclusions that emerge from different approaches. For example:

• Look at what results have emerged in qualitative versus quantitative research
• Discuss how the topic has been approached by empirical versus theoretical scholarship
• Divide the literature into sociological, historical, and cultural sources

Theoretical

A literature review is often the foundation for a theoretical framework . You can use it to discuss various theories, models, and definitions of key concepts.

You might argue for the relevance of a specific theoretical approach, or combine various theoretical concepts to create a framework for your research.

Like any other academic text, your literature review should have an introduction , a main body, and a conclusion . What you include in each depends on the objective of your literature review.

The introduction should clearly establish the focus and purpose of the literature review.

If you are writing the literature review as part of your dissertation or thesis, reiterate your central problem or research question and give a brief summary of the scholarly context. You can emphasise the timeliness of the topic (“many recent studies have focused on the problem of x”) or highlight a gap in the literature (“while there has been much research on x, few researchers have taken y into consideration”).

Depending on the length of your literature review, you might want to divide the body into subsections. You can use a subheading for each theme, time period, or methodological approach.

As you write, make sure to follow these tips:

• Summarise and synthesise: give an overview of the main points of each source and combine them into a coherent whole.
• Analyse and interpret: don’t just paraphrase other researchers – add your own interpretations, discussing the significance of findings in relation to the literature as a whole.
• Critically evaluate: mention the strengths and weaknesses of your sources.
• Write in well-structured paragraphs: use transitions and topic sentences to draw connections, comparisons and contrasts.

In the conclusion, you should summarise the key findings you have taken from the literature and emphasise their significance.

If the literature review is part of your dissertation or thesis, reiterate how your research addresses gaps and contributes new knowledge, or discuss how you have drawn on existing theories and methods to build a framework for your research. This can lead directly into your methodology section.

A literature review is a survey of scholarly sources (such as books, journal articles, and theses) related to a specific topic or research question .

It is often written as part of a dissertation , thesis, research paper , or proposal .

There are several reasons to conduct a literature review at the beginning of a research project:

• To familiarise yourself with the current state of knowledge on your topic
• To ensure that you’re not just repeating what others have already done
• To identify gaps in knowledge and unresolved problems that your research can address
• To develop your theoretical framework and methodology
• To provide an overview of the key findings and debates on the topic

Writing the literature review shows your reader how your work relates to existing research and what new insights it will contribute.

The literature review usually comes near the beginning of your  dissertation . After the introduction , it grounds your research in a scholarly field and leads directly to your theoretical framework or methodology .

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How To Write An A-Grade Literature Review

3 straightforward steps (with examples) + free template.

By: Derek Jansen (MBA) | Expert Reviewed By: Dr. Eunice Rautenbach | October 2019

Quality research is about building onto the existing work of others , “standing on the shoulders of giants”, as Newton put it. The literature review chapter of your dissertation, thesis or research project is where you synthesise this prior work and lay the theoretical foundation for your own research.

Long story short, this chapter is a pretty big deal, which is why you want to make sure you get it right . In this post, I’ll show you exactly how to write a literature review in three straightforward steps, so you can conquer this vital chapter (the smart way).

Overview: The Literature Review Process

• Understanding the “ why “
• Finding the relevant literature
• Cataloguing and synthesising the information
• Outlining & writing up your literature review
• Example of a literature review

But first, the “why”…

Before we unpack how to write the literature review chapter, we’ve got to look at the why . To put it bluntly, if you don’t understand the function and purpose of the literature review process, there’s no way you can pull it off well. So, what exactly is the purpose of the literature review?

Well, there are (at least) four core functions:

• For you to gain an understanding (and demonstrate this understanding) of where the research is at currently, what the key arguments and disagreements are.
• For you to identify the gap(s) in the literature and then use this as justification for your own research topic.
• To help you build a conceptual framework for empirical testing (if applicable to your research topic).
• To inform your methodological choices and help you source tried and tested questionnaires (for interviews ) and measurement instruments (for surveys ).

Most students understand the first point but don’t give any thought to the rest. To get the most from the literature review process, you must keep all four points front of mind as you review the literature (more on this shortly), or you’ll land up with a wonky foundation.

Okay – with the why out the way, let’s move on to the how . As mentioned above, writing your literature review is a process, which I’ll break down into three steps:

• Finding the most suitable literature
• Understanding , distilling and organising the literature
• Planning and writing up your literature review chapter

Importantly, you must complete steps one and two before you start writing up your chapter. I know it’s very tempting, but don’t try to kill two birds with one stone and write as you read. You’ll invariably end up wasting huge amounts of time re-writing and re-shaping, or you’ll just land up with a disjointed, hard-to-digest mess . Instead, you need to read first and distil the information, then plan and execute the writing.

Step 1: Find the relevant literature

Naturally, the first step in the literature review journey is to hunt down the existing research that’s relevant to your topic. While you probably already have a decent base of this from your research proposal , you need to expand on this substantially in the dissertation or thesis itself.

Essentially, you need to be looking for any existing literature that potentially helps you answer your research question (or develop it, if that’s not yet pinned down). There are numerous ways to find relevant literature, but I’ll cover my top four tactics here. I’d suggest combining all four methods to ensure that nothing slips past you:

Method 1 – Google Scholar Scrubbing

Google’s academic search engine, Google Scholar , is a great starting point as it provides a good high-level view of the relevant journal articles for whatever keyword you throw at it. Most valuably, it tells you how many times each article has been cited, which gives you an idea of how credible (or at least, popular) it is. Some articles will be free to access, while others will require an account, which brings us to the next method.

Method 2 – University Database Scrounging

Generally, universities provide students with access to an online library, which provides access to many (but not all) of the major journals.

So, if you find an article using Google Scholar that requires paid access (which is quite likely), search for that article in your university’s database – if it’s listed there, you’ll have access. Note that, generally, the search engine capabilities of these databases are poor, so make sure you search for the exact article name, or you might not find it.

Method 3 – Journal Article Snowballing

At the end of every academic journal article, you’ll find a list of references. As with any academic writing, these references are the building blocks of the article, so if the article is relevant to your topic, there’s a good chance a portion of the referenced works will be too. Do a quick scan of the titles and see what seems relevant, then search for the relevant ones in your university’s database.

Method 4 – Dissertation Scavenging

Similar to Method 3 above, you can leverage other students’ dissertations. All you have to do is skim through literature review chapters of existing dissertations related to your topic and you’ll find a gold mine of potential literature. Usually, your university will provide you with access to previous students’ dissertations, but you can also find a much larger selection in the following databases:

• Open Access Theses & Dissertations
• Stanford SearchWorks

Keep in mind that dissertations and theses are not as academically sound as published, peer-reviewed journal articles (because they’re written by students, not professionals), so be sure to check the credibility of any sources you find using this method. You can do this by assessing the citation count of any given article in Google Scholar. If you need help with assessing the credibility of any article, or with finding relevant research in general, you can chat with one of our Research Specialists .

Alright – with a good base of literature firmly under your belt, it’s time to move onto the next step.

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Step 2: Log, catalogue and synthesise

Once you’ve built a little treasure trove of articles, it’s time to get reading and start digesting the information – what does it all mean?

While I present steps one and two (hunting and digesting) as sequential, in reality, it’s more of a back-and-forth tango – you’ll read a little , then have an idea, spot a new citation, or a new potential variable, and then go back to searching for articles. This is perfectly natural – through the reading process, your thoughts will develop , new avenues might crop up, and directional adjustments might arise. This is, after all, one of the main purposes of the literature review process (i.e. to familiarise yourself with the current state of research in your field).

As you’re working through your treasure chest, it’s essential that you simultaneously start organising the information. There are three aspects to this:

• Logging reference information
• Building an organised catalogue
• Distilling and synthesising the information

I’ll discuss each of these below:

2.1 – Log the reference information

As you read each article, you should add it to your reference management software. I usually recommend Mendeley for this purpose (see the Mendeley 101 video below), but you can use whichever software you’re comfortable with. Most importantly, make sure you load EVERY article you read into your reference manager, even if it doesn’t seem very relevant at the time.

2.2 – Build an organised catalogue

In the beginning, you might feel confident that you can remember who said what, where, and what their main arguments were. Trust me, you won’t. If you do a thorough review of the relevant literature (as you must!), you’re going to read many, many articles, and it’s simply impossible to remember who said what, when, and in what context . Also, without the bird’s eye view that a catalogue provides, you’ll miss connections between various articles, and have no view of how the research developed over time. Simply put, it’s essential to build your own catalogue of the literature.

I would suggest using Excel to build your catalogue, as it allows you to run filters, colour code and sort – all very useful when your list grows large (which it will). How you lay your spreadsheet out is up to you, but I’d suggest you have the following columns (at minimum):

• Author, date, title – Start with three columns containing this core information. This will make it easy for you to search for titles with certain words, order research by date, or group by author.
• Categories or keywords – You can either create multiple columns, one for each category/theme and then tick the relevant categories, or you can have one column with keywords.
• Key arguments/points – Use this column to succinctly convey the essence of the article, the key arguments and implications thereof for your research.
• Context – Note the socioeconomic context in which the research was undertaken. For example, US-based, respondents aged 25-35, lower- income, etc. This will be useful for making an argument about gaps in the research.
• Methodology – Note which methodology was used and why. Also, note any issues you feel arise due to the methodology. Again, you can use this to make an argument about gaps in the research.
• Quotations – Note down any quoteworthy lines you feel might be useful later.
• Notes – Make notes about anything not already covered. For example, linkages to or disagreements with other theories, questions raised but unanswered, shortcomings or limitations, and so forth.

If you’d like, you can try out our free catalog template here (see screenshot below).

2.3 – Digest and synthesise

Most importantly, as you work through the literature and build your catalogue, you need to synthesise all the information in your own mind – how does it all fit together? Look for links between the various articles and try to develop a bigger picture view of the state of the research. Some important questions to ask yourself are:

• What answers does the existing research provide to my own research questions ?
• Which points do the researchers agree (and disagree) on?
• How has the research developed over time?
• Where do the gaps in the current research lie?

To help you develop a big-picture view and synthesise all the information, you might find mind mapping software such as Freemind useful. Alternatively, if you’re a fan of physical note-taking, investing in a large whiteboard might work for you.

Step 3: Outline and write it up!

Once you’re satisfied that you have digested and distilled all the relevant literature in your mind, it’s time to put pen to paper (or rather, fingers to keyboard). There are two steps here – outlining and writing:

3.1 – Draw up your outline

Having spent so much time reading, it might be tempting to just start writing up without a clear structure in mind. However, it’s critically important to decide on your structure and develop a detailed outline before you write anything. Your literature review chapter needs to present a clear, logical and an easy to follow narrative – and that requires some planning. Don’t try to wing it!

Naturally, you won’t always follow the plan to the letter, but without a detailed outline, you’re more than likely going to end up with a disjointed pile of waffle , and then you’re going to spend a far greater amount of time re-writing, hacking and patching. The adage, “measure twice, cut once” is very suitable here.

In terms of structure, the first decision you’ll have to make is whether you’ll lay out your review thematically (into themes) or chronologically (by date/period). The right choice depends on your topic, research objectives and research questions, which we discuss in this article .

Once that’s decided, you need to draw up an outline of your entire chapter in bullet point format. Try to get as detailed as possible, so that you know exactly what you’ll cover where, how each section will connect to the next, and how your entire argument will develop throughout the chapter. Also, at this stage, it’s a good idea to allocate rough word count limits for each section, so that you can identify word count problems before you’ve spent weeks or months writing!

PS – check out our free literature review chapter template…

3.2 – Get writing

With a detailed outline at your side, it’s time to start writing up (finally!). At this stage, it’s common to feel a bit of writer’s block and find yourself procrastinating under the pressure of finally having to put something on paper. To help with this, remember that the objective of the first draft is not perfection – it’s simply to get your thoughts out of your head and onto paper, after which you can refine them. The structure might change a little, the word count allocations might shift and shuffle, and you might add or remove a section – that’s all okay. Don’t worry about all this on your first draft – just get your thoughts down on paper.

Once you’ve got a full first draft (however rough it may be), step away from it for a day or two (longer if you can) and then come back at it with fresh eyes. Pay particular attention to the flow and narrative – does it fall fit together and flow from one section to another smoothly? Now’s the time to try to improve the linkage from each section to the next, tighten up the writing to be more concise, trim down word count and sand it down into a more digestible read.

Once you’ve done that, give your writing to a friend or colleague who is not a subject matter expert and ask them if they understand the overall discussion. The best way to assess this is to ask them to explain the chapter back to you. This technique will give you a strong indication of which points were clearly communicated and which weren’t. If you’re working with Grad Coach, this is a good time to have your Research Specialist review your chapter.

Finally, tighten it up and send it off to your supervisor for comment. Some might argue that you should be sending your work to your supervisor sooner than this (indeed your university might formally require this), but in my experience, supervisors are extremely short on time (and often patience), so, the more refined your chapter is, the less time they’ll waste on addressing basic issues (which you know about already) and the more time they’ll spend on valuable feedback that will increase your mark-earning potential.

Literature Review Example

In the video below, we unpack an actual literature review so that you can see how all the core components come together in reality.

Let’s Recap

In this post, we’ve covered how to research and write up a high-quality literature review chapter. Let’s do a quick recap of the key takeaways:

• It is essential to understand the WHY of the literature review before you read or write anything. Make sure you understand the 4 core functions of the process.
• The first step is to hunt down the relevant literature . You can do this using Google Scholar, your university database, the snowballing technique and by reviewing other dissertations and theses.
• Next, you need to log all the articles in your reference manager , build your own catalogue of literature and synthesise all the research.
• Following that, you need to develop a detailed outline of your entire chapter – the more detail the better. Don’t start writing without a clear outline (on paper, not in your head!)
• Write up your first draft in rough form – don’t aim for perfection. Remember, done beats perfect.
• Refine your second draft and get a layman’s perspective on it . Then tighten it up and submit it to your supervisor.

Psst… there’s more!

This post is an extract from our bestselling short course, Literature Review Bootcamp . If you want to work smart, you don't want to miss this .

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38 Comments

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• UConn Library
• Literature Review: The What, Why and How-to Guide
• Introduction

Literature Review: The What, Why and How-to Guide — Introduction

• Getting Started
• How to Pick a Topic
• Strategies to Find Sources
• Evaluating Sources & Lit. Reviews
• Tips for Writing Literature Reviews
• Writing Literature Review: Useful Sites
• Citation Resources
• Other Academic Writings

What are Literature Reviews?

So, what is a literature review? "A literature review is an account of what has been published on a topic by accredited scholars and researchers. In writing the literature review, your purpose is to convey to your reader what knowledge and ideas have been established on a topic, and what their strengths and weaknesses are. As a piece of writing, the literature review must be defined by a guiding concept (e.g., your research objective, the problem or issue you are discussing, or your argumentative thesis). It is not just a descriptive list of the material available, or a set of summaries." Taylor, D.  The literature review: A few tips on conducting it . University of Toronto Health Sciences Writing Centre.

Goals of Literature Reviews

What are the goals of creating a Literature Review?  A literature could be written to accomplish different aims:

• To develop a theory or evaluate an existing theory
• To summarize the historical or existing state of a research topic
• Identify a problem in a field of research 

Baumeister, R. F., & Leary, M. R. (1997). Writing narrative literature reviews .  Review of General Psychology , 1 (3), 311-320.

What kinds of sources require a Literature Review?

• A research paper assigned in a course
• A thesis or dissertation
• A grant proposal
• An article intended for publication in a journal

All these instances require you to collect what has been written about your research topic so that you can demonstrate how your own research sheds new light on the topic.

Types of Literature Reviews

What kinds of literature reviews are written?

Narrative review: The purpose of this type of review is to describe the current state of the research on a specific topic/research and to offer a critical analysis of the literature reviewed. Studies are grouped by research/theoretical categories, and themes and trends, strengths and weakness, and gaps are identified. The review ends with a conclusion section which summarizes the findings regarding the state of the research of the specific study, the gaps identify and if applicable, explains how the author's research will address gaps identify in the review and expand the knowledge on the topic reviewed.

• Example : Predictors and Outcomes of U.S. Quality Maternity Leave: A Review and Conceptual Framework:  10.1177/08948453211037398  

Systematic review : "The authors of a systematic review use a specific procedure to search the research literature, select the studies to include in their review, and critically evaluate the studies they find." (p. 139). Nelson, L. K. (2013). Research in Communication Sciences and Disorders . Plural Publishing.

• Example : The effect of leave policies on increasing fertility: a systematic review:  10.1057/s41599-022-01270-w

Meta-analysis : "Meta-analysis is a method of reviewing research findings in a quantitative fashion by transforming the data from individual studies into what is called an effect size and then pooling and analyzing this information. The basic goal in meta-analysis is to explain why different outcomes have occurred in different studies." (p. 197). Roberts, M. C., & Ilardi, S. S. (2003). Handbook of Research Methods in Clinical Psychology . Blackwell Publishing.

• Example : Employment Instability and Fertility in Europe: A Meta-Analysis:  10.1215/00703370-9164737

Meta-synthesis : "Qualitative meta-synthesis is a type of qualitative study that uses as data the findings from other qualitative studies linked by the same or related topic." (p.312). Zimmer, L. (2006). Qualitative meta-synthesis: A question of dialoguing with texts .  Journal of Advanced Nursing , 53 (3), 311-318.

• Example : Women’s perspectives on career successes and barriers: A qualitative meta-synthesis:  10.1177/05390184221113735

Literature Reviews in the Health Sciences

• UConn Health subject guide on systematic reviews Explanation of the different review types used in health sciences literature as well as tools to help you find the right review type
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• Last Updated: Sep 21, 2022 2:16 PM
• URL: https://guides.lib.uconn.edu/literaturereview

Research Methods

• Getting Started
• Literature Review Research
• Research Design
• Research Design By Discipline
• SAGE Research Methods
• Teaching with SAGE Research Methods

Literature Review

• What is a Literature Review?
• What is NOT a Literature Review?
• Purposes of a Literature Review
• Types of Literature Reviews
• Literature Reviews vs. Systematic Reviews
• Systematic vs. Meta-Analysis

Literature Review  is a comprehensive survey of the works published in a particular field of study or line of research, usually over a specific period of time, in the form of an in-depth, critical bibliographic essay or annotated list in which attention is drawn to the most significant works.

Also, we can define a literature review as the collected body of scholarly works related to a topic:

• Summarizes and analyzes previous research relevant to a topic
• Includes scholarly books and articles published in academic journals
• Can be an specific scholarly paper or a section in a research paper

The objective of a Literature Review is to find previous published scholarly works relevant to an specific topic

• Help gather ideas or information
• Keep up to date in current trends and findings
• Help develop new questions

A literature review is important because it:

• Explains the background of research on a topic.
• Demonstrates why a topic is significant to a subject area.
• Helps focus your own research questions or problems
• Discovers relationships between research studies/ideas.
• Suggests unexplored ideas or populations
• Identifies major themes, concepts, and researchers on a topic.
• Tests assumptions; may help counter preconceived ideas and remove unconscious bias.
• Identifies critical gaps, points of disagreement, or potentially flawed methodology or theoretical approaches.
• Indicates potential directions for future research.

All content in this section is from Literature Review Research from Old Dominion University 

Keep in mind the following, a literature review is NOT:

Not an essay 

Not an annotated bibliography  in which you summarize each article that you have reviewed.  A literature review goes beyond basic summarizing to focus on the critical analysis of the reviewed works and their relationship to your research question.

Not a research paper   where you select resources to support one side of an issue versus another.  A lit review should explain and consider all sides of an argument in order to avoid bias, and areas of agreement and disagreement should be highlighted.

A literature review serves several purposes. For example, it

• provides thorough knowledge of previous studies; introduces seminal works.
• helps focus one’s own research topic.
• identifies a conceptual framework for one’s own research questions or problems; indicates potential directions for future research.
• suggests previously unused or underused methodologies, designs, quantitative and qualitative strategies.
• identifies gaps in previous studies; identifies flawed methodologies and/or theoretical approaches; avoids replication of mistakes.
• helps the researcher avoid repetition of earlier research.
• suggests unexplored populations.
• determines whether past studies agree or disagree; identifies controversy in the literature.
• tests assumptions; may help counter preconceived ideas and remove unconscious bias.

As Kennedy (2007) notes*, it is important to think of knowledge in a given field as consisting of three layers. First, there are the primary studies that researchers conduct and publish. Second are the reviews of those studies that summarize and offer new interpretations built from and often extending beyond the original studies. Third, there are the perceptions, conclusions, opinion, and interpretations that are shared informally that become part of the lore of field. In composing a literature review, it is important to note that it is often this third layer of knowledge that is cited as "true" even though it often has only a loose relationship to the primary studies and secondary literature reviews.

Given this, while literature reviews are designed to provide an overview and synthesis of pertinent sources you have explored, there are several approaches to how they can be done, depending upon the type of analysis underpinning your study. Listed below are definitions of types of literature reviews:

Argumentative Review      This form examines literature selectively in order to support or refute an argument, deeply imbedded assumption, or philosophical problem already established in the literature. The purpose is to develop a body of literature that establishes a contrarian viewpoint. Given the value-laden nature of some social science research [e.g., educational reform; immigration control], argumentative approaches to analyzing the literature can be a legitimate and important form of discourse. However, note that they can also introduce problems of bias when they are used to to make summary claims of the sort found in systematic reviews.

Integrative Review      Considered a form of research that reviews, critiques, and synthesizes representative literature on a topic in an integrated way such that new frameworks and perspectives on the topic are generated. The body of literature includes all studies that address related or identical hypotheses. A well-done integrative review meets the same standards as primary research in regard to clarity, rigor, and replication.

Historical Review      Few things rest in isolation from historical precedent. Historical reviews are focused on examining research throughout a period of time, often starting with the first time an issue, concept, theory, phenomena emerged in the literature, then tracing its evolution within the scholarship of a discipline. The purpose is to place research in a historical context to show familiarity with state-of-the-art developments and to identify the likely directions for future research.

Methodological Review      A review does not always focus on what someone said [content], but how they said it [method of analysis]. This approach provides a framework of understanding at different levels (i.e. those of theory, substantive fields, research approaches and data collection and analysis techniques), enables researchers to draw on a wide variety of knowledge ranging from the conceptual level to practical documents for use in fieldwork in the areas of ontological and epistemological consideration, quantitative and qualitative integration, sampling, interviewing, data collection and data analysis, and helps highlight many ethical issues which we should be aware of and consider as we go through our study.

Systematic Review      This form consists of an overview of existing evidence pertinent to a clearly formulated research question, which uses pre-specified and standardized methods to identify and critically appraise relevant research, and to collect, report, and analyse data from the studies that are included in the review. Typically it focuses on a very specific empirical question, often posed in a cause-and-effect form, such as "To what extent does A contribute to B?"

Theoretical Review      The purpose of this form is to concretely examine the corpus of theory that has accumulated in regard to an issue, concept, theory, phenomena. The theoretical literature review help establish what theories already exist, the relationships between them, to what degree the existing theories have been investigated, and to develop new hypotheses to be tested. Often this form is used to help establish a lack of appropriate theories or reveal that current theories are inadequate for explaining new or emerging research problems. The unit of analysis can focus on a theoretical concept or a whole theory or framework.

* Kennedy, Mary M. "Defining a Literature."  Educational Researcher  36 (April 2007): 139-147.

All content in this section is from The Literature Review created by Dr. Robert Larabee USC

Robinson, P. and Lowe, J. (2015),  Literature reviews vs systematic reviews.  Australian and New Zealand Journal of Public Health, 39: 103-103. doi: 10.1111/1753-6405.12393

What's in the name? The difference between a Systematic Review and a Literature Review, and why it matters . By Lynn Kysh from University of Southern California

Systematic review or meta-analysis?

A  systematic review  answers a defined research question by collecting and summarizing all empirical evidence that fits pre-specified eligibility criteria.

A  meta-analysis  is the use of statistical methods to summarize the results of these studies.

Systematic reviews, just like other research articles, can be of varying quality. They are a significant piece of work (the Centre for Reviews and Dissemination at York estimates that a team will take 9-24 months), and to be useful to other researchers and practitioners they should have:

• clearly stated objectives with pre-defined eligibility criteria for studies
• explicit, reproducible methodology
• a systematic search that attempts to identify all studies
• assessment of the validity of the findings of the included studies (e.g. risk of bias)
• systematic presentation, and synthesis, of the characteristics and findings of the included studies

Not all systematic reviews contain meta-analysis. 

Meta-analysis is the use of statistical methods to summarize the results of independent studies. By combining information from all relevant studies, meta-analysis can provide more precise estimates of the effects of health care than those derived from the individual studies included within a review.  More information on meta-analyses can be found in  Cochrane Handbook, Chapter 9 .

A meta-analysis goes beyond critique and integration and conducts secondary statistical analysis on the outcomes of similar studies.  It is a systematic review that uses quantitative methods to synthesize and summarize the results.

An advantage of a meta-analysis is the ability to be completely objective in evaluating research findings.  Not all topics, however, have sufficient research evidence to allow a meta-analysis to be conducted.  In that case, an integrative review is an appropriate strategy. 

Some of the content in this section is from Systematic reviews and meta-analyses: step by step guide created by Kate McAllister.

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• Next: Research Design >>
• Last Updated: Aug 21, 2023 4:07 PM
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Research Methods: Literature Reviews

• Annotated Bibliographies
• Literature Reviews
• Scoping Reviews
• Systematic Reviews
• Scholarship of Teaching and Learning
• Persuasive Arguments
• Subject Specific Methodology

A literature review involves researching, reading, analyzing, evaluating, and summarizing scholarly literature (typically journals and articles) about a specific topic. The results of a literature review may be an entire report or article OR may be part of a article, thesis, dissertation, or grant proposal. A literature review helps the author learn about the history and nature of their topic, and identify research gaps and problems.

Steps & Elements

Problem formulation

• Determine your topic and its components by asking a question
• Research: locate literature related to your topic to identify the gap(s) that can be addressed
• Read: read the articles or other sources of information
• Analyze: assess the findings for relevancy
• Evaluating: determine how the article are relevant to your research and what are the key findings
• Synthesis: write about the key findings and how it is relevant to your research

Elements of a Literature Review

• Summarize subject, issue or theory under consideration, along with objectives of the review
• Divide works under review into categories (e.g. those in support of a particular position, those against, those offering alternative theories entirely)
• Explain how each work is similar to and how it varies from the others
• Conclude which pieces are best considered in their argument, are most convincing of their opinions, and make the greatest contribution to the understanding and development of an area of research

Writing a Literature Review Resources

• How to Write a Literature Review From the Wesleyan University Library
• Write a Literature Review From the University of California Santa Cruz Library. A Brief overview of a literature review, includes a list of stages for writing a lit review.
• Literature Reviews From the University of North Carolina Writing Center. Detailed information about writing a literature review.
• Undertaking a literature review: a step-by-step approach Cronin, P., Ryan, F., & Coughan, M. (2008). Undertaking a literature review: A step-by-step approach. British Journal of Nursing, 17(1), p.38-43

Literature Review Tutorial

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Research Methods and Design

• Action Research
• Case Study Design

Literature Review

• Quantitative Research Methods
• Qualitative Research Methods
• Mixed Methods Study
• Indigenous Research and Ethics This link opens in a new window
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• Research Ethics and Quality
• Data Literacy
• Get Help with Writing Assignments

A literature review is a discussion of the literature (aka. the "research" or "scholarship") surrounding a certain topic. A good literature review doesn't simply summarize the existing material, but provides thoughtful synthesis and analysis. The purpose of a literature review is to orient your own work within an existing body of knowledge. A literature review may be written as a standalone piece or be included in a larger body of work.

You can read more about literature reviews, what they entail, and how to write one, using the resources below. 

Am I the only one struggling to write a literature review?

Dr. Zina O'Leary explains the misconceptions and struggles students often have with writing a literature review. She also provides step-by-step guidance on writing a persuasive literature review.

An Introduction to Literature Reviews

Dr. Eric Jensen, Professor of Sociology at the University of Warwick, and Dr. Charles Laurie, Director of Research at Verisk Maplecroft, explain how to write a literature review, and why researchers need to do so. Literature reviews can be stand-alone research or part of a larger project. They communicate the state of academic knowledge on a given topic, specifically detailing what is still unknown.

This is the first video in a whole series about literature reviews. You can find the rest of the series in our SAGE database, Research Methods:

Videos covering research methods and statistics

Identify Themes and Gaps in Literature (with real examples) | Scribbr

Finding connections between sources is key to organizing the arguments and structure of a good literature review. In this video, you'll learn how to identify themes, debates, and gaps between sources, using examples from real papers.

4 Tips for Writing a Literature Review's Intro, Body, and Conclusion | Scribbr

While each review will be unique in its structure--based on both the existing body of both literature and the overall goals of your own paper, dissertation, or research--this video from Scribbr does a good job simplifying the goals of writing a literature review for those who are new to the process. In this video, you’ll learn what to include in each section, as well as 4 tips for the main body illustrated with an example.

• Literature Review This chapter in SAGE's Encyclopedia of Research Design describes the types of literature reviews and scientific standards for conducting literature reviews.
• UNC Writing Center: Literature Reviews This handout from the Writing Center at UNC will explain what literature reviews are and offer insights into the form and construction of literature reviews in the humanities, social sciences, and sciences.
• Purdue OWL: Writing a Literature Review The overview of literature reviews comes from Purdue's Online Writing Lab. It explains the basic why, what, and how of writing a literature review.

Organizational Tools for Literature Reviews

One of the most daunting aspects of writing a literature review is organizing your research. There are a variety of strategies that you can use to help you in this task. We've highlighted just a few ways writers keep track of all that information! You can use a combination of these tools or come up with your own organizational process. The key is choosing something that works with your own learning style.

Citation Managers

Citation managers are great tools, in general, for organizing research, but can be especially helpful when writing a literature review. You can keep all of your research in one place, take notes, and organize your materials into different folders or categories. Read more about citations managers here:

• Manage Citations & Sources

Concept Mapping

Some writers use concept mapping (sometimes called flow or bubble charts or "mind maps") to help them visualize the ways in which the research they found connects.

There is no right or wrong way to make a concept map. There are a variety of online tools that can help you create a concept map or you can simply put pen to paper. To read more about concept mapping, take a look at the following help guides:

• Using Concept Maps From Williams College's guide, Literature Review: A Self-guided Tutorial

Synthesis Matrix

A synthesis matrix is is a chart you can use to help you organize your research into thematic categories. By organizing your research into a matrix, like the examples below, can help you visualize the ways in which your sources connect. 

• Walden University Writing Center: Literature Review Matrix Find a variety of literature review matrix examples and templates from Walden University.
• Writing A Literature Review and Using a Synthesis Matrix An example synthesis matrix created by NC State University Writing and Speaking Tutorial Service Tutors. If you would like a copy of this synthesis matrix in a different format, like a Word document, please ask a librarian. CC-BY-SA 3.0
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• Last Updated: May 7, 2024 9:51 AM

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What is a Literature Review? How to Write It (with Examples)

A literature review is a critical analysis and synthesis of existing research on a particular topic. It provides an overview of the current state of knowledge, identifies gaps, and highlights key findings in the literature. 1 The purpose of a literature review is to situate your own research within the context of existing scholarship, demonstrating your understanding of the topic and showing how your work contributes to the ongoing conversation in the field. Learning how to write a literature review is a critical tool for successful research. Your ability to summarize and synthesize prior research pertaining to a certain topic demonstrates your grasp on the topic of study, and assists in the learning process. 

Table of Contents

• What is the purpose of literature review? 
• a. Habitat Loss and Species Extinction: 
• b. Range Shifts and Phenological Changes: 
• c. Ocean Acidification and Coral Reefs: 
• d. Adaptive Strategies and Conservation Efforts: 
• How to write a good literature review 
• Choose a Topic and Define the Research Question: 
• Decide on the Scope of Your Review: 
• Select Databases for Searches: 
• Conduct Searches and Keep Track: 
• Review the Literature: 
• Organize and Write Your Literature Review: 
• Frequently asked questions 

What is a literature review?

A well-conducted literature review demonstrates the researcher’s familiarity with the existing literature, establishes the context for their own research, and contributes to scholarly conversations on the topic. One of the purposes of a literature review is also to help researchers avoid duplicating previous work and ensure that their research is informed by and builds upon the existing body of knowledge.

What is the purpose of literature review?

A literature review serves several important purposes within academic and research contexts. Here are some key objectives and functions of a literature review: 2  

• Contextualizing the Research Problem: The literature review provides a background and context for the research problem under investigation. It helps to situate the study within the existing body of knowledge. 
• Identifying Gaps in Knowledge: By identifying gaps, contradictions, or areas requiring further research, the researcher can shape the research question and justify the significance of the study. This is crucial for ensuring that the new research contributes something novel to the field. 
• Understanding Theoretical and Conceptual Frameworks: Literature reviews help researchers gain an understanding of the theoretical and conceptual frameworks used in previous studies. This aids in the development of a theoretical framework for the current research. 
• Providing Methodological Insights: Another purpose of literature reviews is that it allows researchers to learn about the methodologies employed in previous studies. This can help in choosing appropriate research methods for the current study and avoiding pitfalls that others may have encountered. 
• Establishing Credibility: A well-conducted literature review demonstrates the researcher’s familiarity with existing scholarship, establishing their credibility and expertise in the field. It also helps in building a solid foundation for the new research. 
• Informing Hypotheses or Research Questions: The literature review guides the formulation of hypotheses or research questions by highlighting relevant findings and areas of uncertainty in existing literature. 

Literature review example

Let’s delve deeper with a literature review example: Let’s say your literature review is about the impact of climate change on biodiversity. You might format your literature review into sections such as the effects of climate change on habitat loss and species extinction, phenological changes, and marine biodiversity. Each section would then summarize and analyze relevant studies in those areas, highlighting key findings and identifying gaps in the research. The review would conclude by emphasizing the need for further research on specific aspects of the relationship between climate change and biodiversity. The following literature review template provides a glimpse into the recommended literature review structure and content, demonstrating how research findings are organized around specific themes within a broader topic. 

Literature Review on Climate Change Impacts on Biodiversity:

Climate change is a global phenomenon with far-reaching consequences, including significant impacts on biodiversity. This literature review synthesizes key findings from various studies: 

a. Habitat Loss and Species Extinction:

Climate change-induced alterations in temperature and precipitation patterns contribute to habitat loss, affecting numerous species (Thomas et al., 2004). The review discusses how these changes increase the risk of extinction, particularly for species with specific habitat requirements. 

b. Range Shifts and Phenological Changes:

Observations of range shifts and changes in the timing of biological events (phenology) are documented in response to changing climatic conditions (Parmesan & Yohe, 2003). These shifts affect ecosystems and may lead to mismatches between species and their resources. 

c. Ocean Acidification and Coral Reefs:

The review explores the impact of climate change on marine biodiversity, emphasizing ocean acidification’s threat to coral reefs (Hoegh-Guldberg et al., 2007). Changes in pH levels negatively affect coral calcification, disrupting the delicate balance of marine ecosystems. 

d. Adaptive Strategies and Conservation Efforts:

Recognizing the urgency of the situation, the literature review discusses various adaptive strategies adopted by species and conservation efforts aimed at mitigating the impacts of climate change on biodiversity (Hannah et al., 2007). It emphasizes the importance of interdisciplinary approaches for effective conservation planning. 

How to write a good literature review

Writing a literature review involves summarizing and synthesizing existing research on a particular topic. A good literature review format should include the following elements. 

Introduction: The introduction sets the stage for your literature review, providing context and introducing the main focus of your review. 

• Opening Statement: Begin with a general statement about the broader topic and its significance in the field. 
• Scope and Purpose: Clearly define the scope of your literature review. Explain the specific research question or objective you aim to address. 
• Organizational Framework: Briefly outline the structure of your literature review, indicating how you will categorize and discuss the existing research. 
• Significance of the Study: Highlight why your literature review is important and how it contributes to the understanding of the chosen topic. 
• Thesis Statement: Conclude the introduction with a concise thesis statement that outlines the main argument or perspective you will develop in the body of the literature review. 

Body: The body of the literature review is where you provide a comprehensive analysis of existing literature, grouping studies based on themes, methodologies, or other relevant criteria. 

• Organize by Theme or Concept: Group studies that share common themes, concepts, or methodologies. Discuss each theme or concept in detail, summarizing key findings and identifying gaps or areas of disagreement. 
• Critical Analysis: Evaluate the strengths and weaknesses of each study. Discuss the methodologies used, the quality of evidence, and the overall contribution of each work to the understanding of the topic. 
• Synthesis of Findings: Synthesize the information from different studies to highlight trends, patterns, or areas of consensus in the literature. 
• Identification of Gaps: Discuss any gaps or limitations in the existing research and explain how your review contributes to filling these gaps. 
• Transition between Sections: Provide smooth transitions between different themes or concepts to maintain the flow of your literature review. 

Conclusion: The conclusion of your literature review should summarize the main findings, highlight the contributions of the review, and suggest avenues for future research. 

• Summary of Key Findings: Recap the main findings from the literature and restate how they contribute to your research question or objective. 
• Contributions to the Field: Discuss the overall contribution of your literature review to the existing knowledge in the field. 
• Implications and Applications: Explore the practical implications of the findings and suggest how they might impact future research or practice. 
• Recommendations for Future Research: Identify areas that require further investigation and propose potential directions for future research in the field. 
• Final Thoughts: Conclude with a final reflection on the importance of your literature review and its relevance to the broader academic community. 

Conducting a literature review

Conducting a literature review is an essential step in research that involves reviewing and analyzing existing literature on a specific topic. It’s important to know how to do a literature review effectively, so here are the steps to follow: 1  

Choose a Topic and Define the Research Question:

• Select a topic that is relevant to your field of study. 
• Clearly define your research question or objective. Determine what specific aspect of the topic do you want to explore? 

Decide on the Scope of Your Review:

• Determine the timeframe for your literature review. Are you focusing on recent developments, or do you want a historical overview? 
• Consider the geographical scope. Is your review global, or are you focusing on a specific region? 
• Define the inclusion and exclusion criteria. What types of sources will you include? Are there specific types of studies or publications you will exclude? 

Select Databases for Searches:

• Identify relevant databases for your field. Examples include PubMed, IEEE Xplore, Scopus, Web of Science, and Google Scholar. 
• Consider searching in library catalogs, institutional repositories, and specialized databases related to your topic. 

Conduct Searches and Keep Track:

• Develop a systematic search strategy using keywords, Boolean operators (AND, OR, NOT), and other search techniques. 
• Record and document your search strategy for transparency and replicability. 
• Keep track of the articles, including publication details, abstracts, and links. Use citation management tools like EndNote, Zotero, or Mendeley to organize your references. 

Review the Literature:

• Evaluate the relevance and quality of each source. Consider the methodology, sample size, and results of studies. 
• Organize the literature by themes or key concepts. Identify patterns, trends, and gaps in the existing research. 
• Summarize key findings and arguments from each source. Compare and contrast different perspectives. 
• Identify areas where there is a consensus in the literature and where there are conflicting opinions. 
• Provide critical analysis and synthesis of the literature. What are the strengths and weaknesses of existing research? 

Organize and Write Your Literature Review:

• Literature review outline should be based on themes, chronological order, or methodological approaches. 
• Write a clear and coherent narrative that synthesizes the information gathered. 
• Use proper citations for each source and ensure consistency in your citation style (APA, MLA, Chicago, etc.). 
• Conclude your literature review by summarizing key findings, identifying gaps, and suggesting areas for future research. 

The literature review sample and detailed advice on writing and conducting a review will help you produce a well-structured report. But remember that a literature review is an ongoing process, and it may be necessary to revisit and update it as your research progresses. 

Frequently asked questions

A literature review is a critical and comprehensive analysis of existing literature (published and unpublished works) on a specific topic or research question and provides a synthesis of the current state of knowledge in a particular field. A well-conducted literature review is crucial for researchers to build upon existing knowledge, avoid duplication of efforts, and contribute to the advancement of their field. It also helps researchers situate their work within a broader context and facilitates the development of a sound theoretical and conceptual framework for their studies.

Literature review is a crucial component of research writing, providing a solid background for a research paper’s investigation. The aim is to keep professionals up to date by providing an understanding of ongoing developments within a specific field, including research methods, and experimental techniques used in that field, and present that knowledge in the form of a written report. Also, the depth and breadth of the literature review emphasizes the credibility of the scholar in his or her field.  

Before writing a literature review, it’s essential to undertake several preparatory steps to ensure that your review is well-researched, organized, and focused. This includes choosing a topic of general interest to you and doing exploratory research on that topic, writing an annotated bibliography, and noting major points, especially those that relate to the position you have taken on the topic. 

Literature reviews and academic research papers are essential components of scholarly work but serve different purposes within the academic realm. 3 A literature review aims to provide a foundation for understanding the current state of research on a particular topic, identify gaps or controversies, and lay the groundwork for future research. Therefore, it draws heavily from existing academic sources, including books, journal articles, and other scholarly publications. In contrast, an academic research paper aims to present new knowledge, contribute to the academic discourse, and advance the understanding of a specific research question. Therefore, it involves a mix of existing literature (in the introduction and literature review sections) and original data or findings obtained through research methods. 

Literature reviews are essential components of academic and research papers, and various strategies can be employed to conduct them effectively. If you want to know how to write a literature review for a research paper, here are four common approaches that are often used by researchers.  Chronological Review: This strategy involves organizing the literature based on the chronological order of publication. It helps to trace the development of a topic over time, showing how ideas, theories, and research have evolved.  Thematic Review: Thematic reviews focus on identifying and analyzing themes or topics that cut across different studies. Instead of organizing the literature chronologically, it is grouped by key themes or concepts, allowing for a comprehensive exploration of various aspects of the topic.  Methodological Review: This strategy involves organizing the literature based on the research methods employed in different studies. It helps to highlight the strengths and weaknesses of various methodologies and allows the reader to evaluate the reliability and validity of the research findings.  Theoretical Review: A theoretical review examines the literature based on the theoretical frameworks used in different studies. This approach helps to identify the key theories that have been applied to the topic and assess their contributions to the understanding of the subject.  It’s important to note that these strategies are not mutually exclusive, and a literature review may combine elements of more than one approach. The choice of strategy depends on the research question, the nature of the literature available, and the goals of the review. Additionally, other strategies, such as integrative reviews or systematic reviews, may be employed depending on the specific requirements of the research.

The literature review format can vary depending on the specific publication guidelines. However, there are some common elements and structures that are often followed. Here is a general guideline for the format of a literature review:  Introduction:   Provide an overview of the topic.  Define the scope and purpose of the literature review.  State the research question or objective.  Body:   Organize the literature by themes, concepts, or chronology.  Critically analyze and evaluate each source.  Discuss the strengths and weaknesses of the studies.  Highlight any methodological limitations or biases.  Identify patterns, connections, or contradictions in the existing research.  Conclusion:   Summarize the key points discussed in the literature review.  Highlight the research gap.  Address the research question or objective stated in the introduction.  Highlight the contributions of the review and suggest directions for future research.

Both annotated bibliographies and literature reviews involve the examination of scholarly sources. While annotated bibliographies focus on individual sources with brief annotations, literature reviews provide a more in-depth, integrated, and comprehensive analysis of existing literature on a specific topic. The key differences are as follows: 

References 

• Denney, A. S., & Tewksbury, R. (2013). How to write a literature review.  Journal of criminal justice education ,  24 (2), 218-234. 
• Pan, M. L. (2016).  Preparing literature reviews: Qualitative and quantitative approaches . Taylor & Francis. 
• Cantero, C. (2019). How to write a literature review.  San José State University Writing Center . 

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• Steps in Conducting a Literature Review

What is a literature review?

A literature review is an integrated analysis -- not just a summary-- of scholarly writings and other relevant evidence related directly to your research question.  That is, it represents a synthesis of the evidence that provides background information on your topic and shows a association between the evidence and your research question.

A literature review may be a stand alone work or the introduction to a larger research paper, depending on the assignment.  Rely heavily on the guidelines your instructor has given you.

Why is it important?

A literature review is important because it:

• Explains the background of research on a topic.
• Demonstrates why a topic is significant to a subject area.
• Discovers relationships between research studies/ideas.
• Identifies major themes, concepts, and researchers on a topic.
• Identifies critical gaps and points of disagreement.
• Discusses further research questions that logically come out of the previous studies.

APA7 Style resources

APA Style Blog - for those harder to find answers

1. Choose a topic. Define your research question.

Your literature review should be guided by your central research question.  The literature represents background and research developments related to a specific research question, interpreted and analyzed by you in a synthesized way.

• Make sure your research question is not too broad or too narrow.  Is it manageable?
• Begin writing down terms that are related to your question. These will be useful for searches later.
• If you have the opportunity, discuss your topic with your professor and your class mates.

2. Decide on the scope of your review

How many studies do you need to look at? How comprehensive should it be? How many years should it cover? 

• This may depend on your assignment.  How many sources does the assignment require?

3. Select the databases you will use to conduct your searches.

Make a list of the databases you will search. 

Where to find databases:

• use the tabs on this guide
• Find other databases in the Nursing Information Resources web page
• More on the Medical Library web page
• ... and more on the Yale University Library web page

4. Conduct your searches to find the evidence. Keep track of your searches.

• Use the key words in your question, as well as synonyms for those words, as terms in your search. Use the database tutorials for help.
• Save the searches in the databases. This saves time when you want to redo, or modify, the searches. It is also helpful to use as a guide is the searches are not finding any useful results.
• Review the abstracts of research studies carefully. This will save you time.
• Use the bibliographies and references of research studies you find to locate others.
• Check with your professor, or a subject expert in the field, if you are missing any key works in the field.
• Ask your librarian for help at any time.
• Use a citation manager, such as EndNote as the repository for your citations. See the EndNote tutorials for help.

Review the literature

Some questions to help you analyze the research:

• What was the research question of the study you are reviewing? What were the authors trying to discover?
• Was the research funded by a source that could influence the findings?
• What were the research methodologies? Analyze its literature review, the samples and variables used, the results, and the conclusions.
• Does the research seem to be complete? Could it have been conducted more soundly? What further questions does it raise?
• If there are conflicting studies, why do you think that is?
• How are the authors viewed in the field? Has this study been cited? If so, how has it been analyzed?

Tips: 

• Review the abstracts carefully.  
• Keep careful notes so that you may track your thought processes during the research process.
• Create a matrix of the studies for easy analysis, and synthesis, across all of the studies.
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How to write the methods section of a systematic review

Home | Blog | How To | How to write the methods section of a systematic review

Covidence breaks down how to write a methods section

The methods section of your systematic review describes what you did, how you did it, and why. Readers need this information to interpret the results and conclusions of the review. Often, a lot of information needs to be distilled into just a few paragraphs. This can be a challenging task, but good preparation and the right tools will help you to set off in the right direction 🗺️🧭.

Systematic reviews are so-called because they are conducted in a way that is rigorous and replicable. So it’s important that these methods are reported in a way that is thorough, clear, and easy to navigate for the reader – whether that’s a patient, a healthcare worker, or a researcher. 

Like most things in a systematic review, the methods should be planned upfront and ideally described in detail in a project plan or protocol. Reviews of healthcare interventions follow the PRISMA guidelines for the minimum set of items to report in the methods section. But what else should be included? It’s a good idea to consider what readers will want to know about the review methods and whether the journal you’re planning to submit the work to has expectations on the reporting of methods. Finding out in advance will help you to plan what to include.

Describe what happened

While the research plan sets out what you intend to do, the methods section is a write-up of what actually happened. It’s not a simple case of rewriting the plan in the past tense – you will also need to discuss and justify deviations from the plan and describe the handling of issues that were unforeseen at the time the plan was written. For this reason, it is useful to make detailed notes before, during, and after the review is completed. Relying on memory alone risks losing valuable information and trawling through emails when the deadline is looming can be frustrating and time consuming! 

Keep it brief

The methods section should be succinct but include all the noteworthy information. This can be a difficult balance to achieve. A useful strategy is to aim for a brief description that signposts the reader to a separate section or sections of supporting information. This could include datasets, a flowchart to show what happened to the excluded studies, a collection of search strategies, and tables containing detailed information about the studies.This separation keeps the review short and simple while enabling the reader to drill down to the detail as needed. And if the methods follow a well-known or standard process, it might suffice to say so and give a reference, rather than describe the process at length. 

Follow a structure

A clear structure provides focus. Use of descriptive headings keeps the writing on track and helps the reader get to key information quickly. What should the structure of the methods section look like? As always, a lot depends on the type of review but it will certainly contain information relating to the following areas:

• Selection criteria ⭕
• Data collection and analysis 👩‍💻
• Study quality and risk of bias ⚖️

Let’s look at each of these in turn.

1. Selection criteria ⭕

The criteria for including and excluding studies are listed here. This includes detail about the types of studies, the types of participants, the types of interventions and the types of outcomes and how they were measured. 

2. Search 🕵🏾‍♀️

Comprehensive reporting of the search is important because this means it can be evaluated and replicated. The search strategies are included in the review, along with details of the databases searched. It’s also important to list any restrictions on the search (for example, language), describe how resources other than electronic databases were searched (for example,  non-indexed journals), and give the date that the searches were run. The PRISMA-S extension provides guidance on reporting literature searches. 

Systematic reviewer pro-tip:

 Copy and paste the search strategy to avoid introducing typos

3. Data collection and analysis 👩‍💻

This section describes:

• how studies were selected for inclusion in the review
• how study data were extracted from the study reports
• how study data were combined for analysis and synthesis

To describe how studies were selected for inclusion , review teams outline the screening process. Covidence uses reviewers’ decision data to automatically populate a PRISMA flow diagram for this purpose. Covidence can also calculate Cohen’s kappa to enable review teams to report the level of agreement among individual reviewers during screening.

To describe how study data were extracted from the study reports , reviewers outline the form that was used, any pilot-testing that was done, and the items that were extracted from the included studies. An important piece of information to include here is the process used to resolve conflict among the reviewers. Covidence’s data extraction tool saves reviewers’ comments and notes in the system as they work. This keeps the information in one place for easy retrieval ⚡.

To describe how study data were combined for analysis and synthesis, reviewers outline the type of synthesis (narrative or quantitative, for example), the methods for grouping data, the challenges that came up, and how these were dealt with. If the review includes a meta-analysis, it will detail how this was performed and how the treatment effects were measured.

4. Study quality and risk of bias ⚖️

Because the results of systematic reviews can be affected by many types of bias, reviewers make every effort to minimise it and to show the reader that the methods they used were appropriate. This section describes the methods used to assess study quality and an assessment of the risk of bias across a range of domains. 

Steps to assess the risk of bias in studies include looking at how study participants were assigned to treatment groups and whether patients and/or study assessors were blinded to the treatment given. Reviewers also report their assessment of the risk of bias due to missing outcome data, whether that is due to participant drop-out or non-reporting of the outcomes by the study authors.

Covidence’s default template for assessing study quality is Cochrane’s risk of bias tool but it is also possible to start from scratch and build a tool with a set of custom domains if you prefer.

Careful planning, clear writing, and a structured approach are key to a good methods section. A methodologist will be able to refer review teams to examples of good methods reporting in the literature. Covidence helps reviewers to screen references, extract data and complete risk of bias tables quickly and efficiently. Sign up for a free trial today!

Laura Mellor. Portsmouth, UK

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A Comprehensive Review of Bias in Deep Learning Models: Methods, Impacts, and Future Directions

• Review article
• Published: 08 May 2024

Cite this article

• Milind Shah   ORCID: orcid.org/0009-0001-6077-3924 1 &
• Nitesh Sureja   ORCID: orcid.org/0000-0002-4429-1597 1  

This comprehensive review and analysis delve into the intricate facets of bias within the realm of deep learning. As artificial intelligence and machine learning technologies become increasingly integrated into our lives, understanding and mitigating bias in these systems is of paramount importance. This paper scrutinizes the multifaceted nature of bias, encompassing data bias, algorithmic bias, and societal bias, and explores the interconnectedness among these dimensions. Through an exploration of existing literature and recent advancements in the field, this paper offers a critical assessment of various bias mitigation techniques. It examines the challenges faced in addressing bias and emphasizes the need for an intersectional and inclusive approach to effectively rectify disparities. Furthermore, this review underscores the importance of ethical considerations in the development and deployment of deep learning models. It highlights the necessity of diverse representation in data, fairness-aware algorithms, and interpretability as key elements in creating bias-free AI systems. By synthesizing existing research and providing a holistic overview of bias in deep learning, this paper aims to contribute to the ongoing discourse on mitigating bias and fostering equity in artificial intelligence systems. The insights presented herein can serve as a foundation for future research and as a guide for practitioners, policymakers, and stakeholders to navigate the complex landscape of bias in deep learning.

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Shah, M., Sureja, N. A Comprehensive Review of Bias in Deep Learning Models: Methods, Impacts, and Future Directions. Arch Computat Methods Eng (2024). https://doi.org/10.1007/s11831-024-10134-2

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Research Article

Cover crops support the climate change mitigation potential of agroecosystems

Roles Conceptualization, Data curation, Formal analysis, Methodology, Writing – original draft, Writing – review & editing

Affiliation Weihenstephan-Triesdorf University of Applied Sciences, Triesdorf, Germany

Roles Funding acquisition, Methodology, Writing – original draft, Writing – review & editing

Affiliation Institute of Earth System Science, Section Soil Science, Leibniz Universität Hannover, Hannover, Germany

Roles Conceptualization, Funding acquisition, Methodology, Supervision, Writing – original draft, Writing – review & editing

* E-mail: [email protected]

• Jonas Schön, 
• Norman Gentsch, 
• Peter Breunig

• Published: May 8, 2024
• https://doi.org/10.1371/journal.pone.0302139
• Peer Review
• Reader Comments

Cover crops have the potential to mitigate climate change by reducing negative impacts of agriculture on ecosystems. This study is first to quantify the net climate change mitigation impact of cover crops including land-use effects. A systematic literature and data review was conducted to identify major drivers for climate benefits and costs of cover crops in maize ( Zea maize L .) production systems. The results indicate that cover crops lead to a net climate change mitigation impact (NCCMI) of 3.30 Mg CO 2 e ha -1 a -1 . We created four scenarios with different impact weights of the drivers and all of them showing a positive NCCMI. Carbon land benefit, the carbon opportunity costs based on maize yield gains following cover crops, is the major contributor to the NCCMI (34.5% of all benefits). Carbon sequestration is the second largest contributor (33.8%). The climate costs of cover crops are mainly dominated by emissions from their seed production and foregone benefits due to land use for cover crops seeds. However, these two costs account for only 15.8% of the benefits. Extrapolating these results, planting cover crops before all maize acreage in the EU results in a climate change mitigation of 49.80 million Mg CO 2 e a -1 , which is equivalent to 13.0% of the EU’s agricultural emissions. This study highlights the importance of incorporating cover crops into sustainable cropping systems to minimize the agricultural impact to climate change.

Citation: Schön J, Gentsch N, Breunig P (2024) Cover crops support the climate change mitigation potential of agroecosystems. PLoS ONE 19(5): e0302139. https://doi.org/10.1371/journal.pone.0302139

Editor: Abhay Omprakash Shirale, IISS: Indian Institute of Soil Science, INDIA

Received: November 29, 2023; Accepted: March 28, 2024; Published: May 8, 2024

Copyright: © 2024 Schön et al. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: All relevant data are within the manuscript and its Supporting Information files.

Funding: The research was funded by the German Federal Ministry of Education and Research within the Project "CATCHY", project number: 031B1060C. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: The authors have declared that no competing interests exist.

1. Introduction

Mitigating climate change through measures in agriculture and food systems is essential for achieving the 1.50° and 2.00°C climate change targets of the Paris Agreement [ 1 ]. Worldwide, agriculture and food production systems are responsible for about one-third of global anthropogenic greenhouse gas (GHG) emissions, of which 40.0% are caused by agricultural production and 32.0% by land use and land use change [ 2 ]. However, agricultural systems also offer opportunities for climate change mitigation. Agroecosystems have significant potential to reduce global net emissions [ 3 ] and could even act as a net carbon sink [ 4 ]. To achieve these emission reduction and carbon sequestration opportunities, a transformation of agricultural systems is required. An essential component of climate-smart agricultural systems is planting cover crops. Cover crops are plants or plant mixtures that are not harvested for revenue, but are grown for the purpose of soil nutrient management, organic matter input, soil protection, and soil health improvement [ 5 ]. Cover crops are associated with multiple ecosystem services, such as closer nutrient cycling [ 5 , 6 ], activation of soil biology, biodiversity improvement [ 7 ], soil water management, and restoration of soil structures [ 8 ]. In addition, the mentioned effects of cover crops could mitigate climate change by soil carbon sequestration [ 9 ], biomass carbon storage, or reduced fertilizer losses to aquatic ecosystems. Recent studies show that these effects can lead to agronomic benefits that translate into higher yields and reduced agricultural input [ 10 ].

Despite these benefits, there exists a knowledge gap. The research problem addressed in this article is that there are no complete climate impact assessments of the cultivation of cover crops that also fully include land use effects. Cover crops reduces land use requirements by increasing yields of the subsequent crop. As land use change is still the largest carbon emissions source in the agri-food-system, yield increasing measures can be considered as a climate change mitigation option. According to Searchinger et al. [ 11 ], these carbon land benefits can be calculated as a “carbon opportunity cost,” which is the foregone carbon storage potential from natural vegetation to produce a certain agricultural product in kg CO 2 e kg -1 . When yields rise on one piece of land, this carbon storage potential can be maintained or restored on other locations since land use change is prevented or reverted. Kovak et al. [ 12 ] used this approach to quantify the climate change mitigation potential of a yield increase based on genetic modified crops. This is the first study to include carbon opportunity cost in the evaluation of the climate impact of cover cropping. As an additional climate benefit, cover crops remove carbon from the atmosphere during their growth while their residues and rhizodeposits are transformed to stable soil organic matter by retaining soil moisture. Further climate benefits of cover crops can be attributed to the supply of nutrients to the subsequent crop, which allows for fertilizer savings [ 13 ], reduced N 2 O emissions due to less nitrogen leaching, reduced solar radiation due to plant growth [ 14 ], and reduced soil erosion [ 15 ]. We define all factors above as “climate benefits”.

While cultivating cover crops can accrue these climate benefits, it can also generate GHG emissions that we refer as “climate costs”. The cultivation of cover crops can increase N 2 O emissions in periods of anaerobic microbial conversion of cover crop decomposition products [ 16 ]. Emissions are also generated in the production of cover crop seeds through a number of avenues, such as the land required for seed production and the production process of the seeds. In addition, produced seeds need to be packed and delivered, which also generates emissions[ 17 ]. Lastly, tilling, seeding, and termination of cover crops requires the use of GHG emitting machinery and resources [ 18 ]. Both climate benefits and climate costs of cover crops need to be carefully considered to calculate the NCCMI.

This article addresses the research problem by developing a comprehensive framework to quantify the NCCMI of cover crops including land use effects based on a systematic literature review. Using selected keywords, a systematic search was conducted in “Web of Science” and “Google Scholar” and resulted in 1269 publications. We define a list of relevant climate benefits and climate costs from which the NCCMI of cover crops is derived. We apply the framework to cover crops for maize in the European Union (EU-27, which is all EU Member States excluding the United Kingdom) to quantify its total NCCMI.

The following analyses are based on a typical Central European crop rotation, namely winter cover crops that are incorporated into the soil and maize as a cash crop following the cover crops in the spring. To quantify the impact of cover crop cultivation on the climate mitigation potential, a systematic literature review was conducted. The research platforms "Web of Science" and "Google Scholar" were used to explore the literature. Based on the initial research, we defined five factors that support climate change mitigation (“climate benefits”) and five factors leading to additional GHG emissions (“climate costs”) due to cover cropping ( Table 1 ). The individual research terms of the individual impact factors are described below in detail.

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https://doi.org/10.1371/journal.pone.0302139.t001

To calculate the NCCMI of cover crops, the values of all impact factors were converted to the same unit, namely megagram of CO 2 equivalents per hectare and year (Mg CO 2 e ha -1 a -1 ). From the values in the literature, a weighted average value that takes into account the number of measurements in the specific study was calculated. Only data from the literature that indicated the number of measurement points were considered in the calculation of the NCCMI. Thus, the climate benefits and climate costs could be added together in each case before subtracting the costs from the benefits. This resulted in a value for the NCCMI per hectare of cover crops given in Mg CO 2 e ha -1 a -1 . The 95% confidence interval was calculated from all study means and presents a range of estimates. To extrapolate the results to the EU-27 maize area the respective impact factors and the climate balances were subsequently multiplied with the maize acreage in the EU-27. The result was then divided by the total emissions from agriculture in the EU-27. This calculation resulted in the share of the NCCMI of cover crops before maize relative to all EU-27 agricultural emissions. The average maize acreage in 22 of the EU-27 countries was calculated as an average based on the years 2011 to 2022 (five of the EU-27 countries do not grow relevant quantities of maize and were thus not considered.)

The following search terms were used in Web of Science:

Cover crop & albedo, Cover crop & C sequestration, Cover crop & Yield corn, Cover crop & Yield maize, Cover crop & life cycle assessment, Cover crop & N-fixation, Cover crop & N-leaching, Fertilizer & Greenhouse gas emissions, Greenhouse gas emissions & fertilizer production, Cover crop & N 2 O, Catch crop & N 2 O. To enable a phrase search, all search terms were used with quotation marks to obtain search results exactly in this word order.

A total of 883 publications were extracted and checked for their suitability. Criteria for the required data were an absolute value from the respective study and the number of measurements. Only 44 articles met these criteria and were finally included in the review. In addition, we used 31 publications from Google Scholar and data from 26 other Internet sources that were selected based on the above criteria. In selecting sources, cover crops were considered both consisting of individual components and mixtures, overwintering as well as freezing, and legumes as well as non-legumes. All data sources are presented in the supplemental material ( S1 File ) and sorted based on the subsections below.

2.1. Carbon land benefit based on yield gain

2.2. Carbon sequestration

Carbon sequestration through cover crops is usually provided in units of soil organic carbon. This value was multiplied by a factor of 3.667 (i.e., the ratio of molar masses of CO 2 and C) to obtain the value of CO 2 stored per ha land.

2.3. Nitrogen fertilizer savings

For nutrient savings, we considered only the main nutrient, namely nitrogen since its production is highly energy intensive. To quantify fertilizer savings, we only considered emissions of mineral fertilizers and not that of organic fertilizers. The overall nitrogen fertilizer savings were quantified as the sum of the mean literature values on nitrogen scavenging and the mean literature values of nitrogen fixation from cover crops.

2.4. Reduced indirect N 2 O emissions due to less leaching

Reducing nitrogen leaching due to cover crops leads to lower indirect N 2 O emissions since less nitrogen is deposited to rivers and other water bodies. To quantify this effect, we use the approach suggested in the 2006 IPCC Guidelines for National Greenhouse Gas Inventories: The amount of nitrogen leaching reduction is multiplied by the emission factor EF 5 of 0.0075 [ 20 ] to obtain the reduction in indirect N 2 O emissions per hectare. This value is then converted from kg N 2 O-N ha -1 to kg CO 2 -eq ha -1 using the conversion factor 273 and divided by 1000 to obtain the unit Mg CO 2 -eq ha -1 [ 21 ].

2.5 Albedo change

The reduced adsorption and thus increased reflection of solar radiation by cover crops’ plant cover compared to fallow land leads to a climate change mitigation effect. We found, however, only two studies evaluating the impact of albedo change from cover crops (Kaye and Quemada [ 22 ] and another by Carrer et al. [ 14 ]).

2.6. Nitrous oxide emissions

In the case of nitrous oxide emissions, the cumulative emissions that occur during the period of fallow or the period from sowing to incorporation of cover crops are considered. The values with the unit kg N 2 O-N ha -1 were multiplied by a factor of 273 and 1000 to obtain the unit in Mg CO 2 e ha -1 .

2.7. Foregone benefits due to cover crop seed land use

Table 2 shows the mean input values derived from the literature for the land use requirements for cover crop seeds.

https://doi.org/10.1371/journal.pone.0302139.t002

2.8. Production emissions for cover crop seed production, dispatch and additional machinery costs

Product emissions for cover crop seed production are not available in literature and therefore derived from maize, rapeseed, and spring wheat seed production. This is a conservative assumption since the nitrogen input and machinery operations as key drivers of emissions are usually higher in these crops than in cover crops. Emissions data is collected from the literature in kg CO 2 e kg -1 of seed and is multiplied with the seeding rate of cover crops. This results in an emissions value per hectare.

Emissions for processing, packaging, and transporting cover crop seeds are also not available and derived from maize seed production emissions and are expressed in kg CO 2 e kg -1 of seed. This is again a conservative assumption since maize seed has to fulfill much higher quality standards than cover crop seed.

The emissions of additional machinery operations for tillage, seeding, and mechanical termination of cover crops were included only in terms of additional diesel fuel usage. Average diesel fuel consumption of all additional operations was calculated and multiplied by the average GHG emissions of diesel fuel.

The impact of all analyzed factors on the NCCMI of cover crops is summarized in Table 3 . The results refer to a typical European crop rotation using a winter cover crop and maize as the following crop in spring. Individual impact factors are described in the following sections.

https://doi.org/10.1371/journal.pone.0302139.t003

3.1. Carbon land benefit based on yield gain

The literature review on the impact of winter cover crop on maize yields revealed that yield effects are highly dependent on the cover crop species or the composition of the cover crop mixture. The yield effect of cover crops on the following crop is also highly dependent on the type of land management. The lower the tillage intensity, the higher the yield increase [ 18 ]. The yield increase effect is also much higher in organic farming than in conventional systems. Marcillo and Miguez [ 24 ] find a yield increase of 8.0% for a conventional intensive tillage system, while for an organic reduced tillage system, yields increased by 61.0% when cover cropping was implemented. In summary, the considered studies showed a weighted mean maize yield gain due to cover cropping of 8.8% (95% CI, 2.9, 14.8).

Fig 1 shows the included yield effect results, confidence intervals and the pooled effect. If more than one result is shown from one study in Fig 1 this literature showed results from multiple experiments.

https://doi.org/10.1371/journal.pone.0302139.g001

Multiplying the weighted mean yield gain with the average EU-27 maize yield and the carbon opportunity cost factor for maize (2.10 kg CO 2 e kg -1 maize [ 11 ]) results in a mean carbon land benefit increase of 1.46 Mg CO 2 e ha -1 .

3.2. Carbon sequestration

Cover crops have a positive impact on soil organic carbon sequestration in arable soils. The literature research revealed two meta-analyses with 139 and 1195 studies included. In addition, 26 studies that were not included in the above-mentioned meta-analyses were also included in our calculations. Up to 12.0% of carbon from cover crop biomass can be sequestered as soil organic carbon [ 13 ]. Similar to the crop yield effects shown above, the potential of soil organic carbon stock increases depending on the type of cover crop. For example, a soil organic carbon increase of 0.26 Mg CO 2 e ha -1 a -1 has been found for a winter vetch cover crop [ 13 ]. A much larger increase in soil carbon of 5.12 Mg CO 2 e ha -1 a -1 was found by Abdalla et al. [ 25 ] for non-legume cover crops. The weighted mean of all studies investigated shows an average sequestration of 1.43 Mg CO 2 e ha -1 a -1 (95% CI, 0.86, 2.01). Based on the included studies, grass cover crops lead to a 2.3-times higher soil organic carbon sequestration than legume cover crops. Cover crop mixtures of grasses, legumes, and various other species are in the middle of the field in sequestration performance [ 26 ].

Fig 2 shows the included carbon sequestration results, confidence intervals and the pooled effect. If more than one result is shown from one study in Fig 2 this literature showed results from multiple experiments.

https://doi.org/10.1371/journal.pone.0302139.g002

3.3. Nitrogen fixation

Legume cover crops are able to fix atmospheric nitrogen and transfer it to the rhizosphere or utilize it for their own biomass. The decay of nitrogen-rich legume litter or rhizosphere products contribute to soil nitrogen fertilization and plant nutrition of the crop following the cover crop [ 13 ]. The amount of nitrogen fixed depends on the legume species and/or the companion crop species in the mixture. For example, red clover fixes 29.00 kg N ha -1 as a single species cover crop, but 52.00 kg N ha -1 in a mixture with ryegrass [ 13 ]. The same was observed with winter vetch, which fixes 85.00 kg N ha -1 as single species and 113.00 kg N ha -1 in a mixture with winter rye [ 27 ]. A much wider range from 50.00 kg N ha -1 to 300.00 kg N ha -1 was found by Kaye and Quemada [ 22 ]. Wittwer et al. [ 18 ] showed that legumes as a single species cover crop can increase nitrogen uptake of maize by 32.00 kg N ha -1 and as part of cover crop mixtures by 28.00 kg N ha -1 . The summarized literature resulted in a weighted mean of 52.49 kg N ha -1 (95% CI, 32.06, 72.92) that can be contributed by legumes from nitrogen fixation.

3.4. Reduced nitrogen leaching

Cover crops prevent nutrient leaching over winter, especially of mineral nitrogen (N min ) that otherwise would be leached from the soil profile. However, this leaching reduction strongly depends on the timing of when cover crop are planted and type of cover crop. Early sown oil radish reduces leaching in the fall and winter months [ 28 ]. Rye as an over-wintering cover crop can better protect nitrogen from leaching in the spring [ 28 ]. Non-legume cover crops and mixtures can protect up to 100.00 kg N ha -1 from leaching, while pure legumes can protect between 60.00 to 70.00 kg N ha -1 [ 6 , 22 ]. Abdalla et al. [ 25 ] show that legume cover crops prevent 18.00 kg N ha -1 from leaching, non-legume cover crops 38.00 kg N ha -1 , and cover crop mixtures 19.00 kg N ha -1 . A study by McCracken et al. [ 29 ] resulted in an average leaching reduction of 35.80 kg N ha -1 . The amount of scavenged nitrogen depends heavily on water availability and soil type. Early sowing and sufficient precipitation enable cover crops to develop faster, establish more biomass, and absorb more nitrogen in the fall. The tillage system also affects the amount of scavenged nitorgen. The faster mineralization in conventional tillage systems in autumn resulted in higher N min stocks and higher nitrogen uptake from cover crops than in no-till systems [ 30 ]. Literature research resulted in a weighted mean value of 38.82 kg N ha -1 (95% CI, 19.13, 58.51) of scavenged nitrogen available to the crop following the cover crop.

3.5. Fertilizer emissions

Nitrogen fertilizer emissions are caused by different factors and vary between fertilizer types. About half of the CO 2 e emissions of nitrogen fertilizer is N 2 O emissions from soil after application and about one-third of all emissions occur during the production process of fertilizer itself. The remaining emissions are divided into N 2 O emissions from NH 4 + and NO 3 - losses and CO 2 from lime [ 31 ]. Total emissions per kg of nitrogen also vary between fertilizer types and the literature. For example, Hasler et al. [ 32 ] reported 8.69 kg CO 2 e kg -1 N for urea fertilizer to 11.64 kg CO 2 e kg -1 N for CaH 4 N 4 O 9 fertilizer. A much higher emission factor of 15.00 to 31.00 kg CO 2 e kg -1 N is used by Kahrl et al. [ 33 ]. The mean value of all studies considered was 11.23 kg CO 2 e kg -1 N (95% CI, 9.02, 13.43). Emissions from organic fertilizers were not considered.

3.6. Reduced indirect N 2 O emissions due to less leaching

Leached nitrogen is deposited into aquatic ecosystems and represents a globally significant source of N 2 O through indirect emissions. These emissions can be calculated using the IPCC emission factor EF 5 , which incorporates three components: EF 5g (groundwater and surface drainage), EF 5r (rivers) and EF 5e (estuaries) [ 34 ].

The IPCC emission factor EF 5 amounts to 0.0075 and is the sum of EF 5g , EF 5r and EF 5e with a value of 0.0025 each. Tian et al. [ 34 ] suggest a higher emission factor for EF 5g of 0.0060, the other two factors are almost the same as those suggested by IPCC in 2006. Here, we use the mean of two results: One using the IPPC factor which results in 0.08 Mg CO 2 -eq ha -1 and the other one using the factor of Tian et al. [ 34 ] which results in 0.12 Mg CO 2 -eq ha -1 . The mean of both values leads to an average climate benefit caused by reduced indirect N 2 O emissions due to less leaching of 0.10 Mg CO 2 -eq ha -1 (95% CI, -0.15, 0.35).

3.7. Albedo change

When soil is covered with cover crops, less solar radiation reaches the soil surface compared to fallow land. Thus, the albedo value of plants is higher than that of fallow soil. Due to this reduced net radiation, there is a cooling effect that impacts the global climate. However, the albedo value of fallow soil strongly depends on soil type and condition. In particular, the color and moisture content of the soil play important roles. Dark and moist soils have a lower albedo than light dry soils, so there is a wide variation in the albedo value of fallow land. Since there is often a high soil moisture level during the growth period of cover crops (in fall and winter) the albedo effect of covering the soil with cover crops is stronger than for the yearly average [ 14 ]. Unfortunately, research on albedo effects of cover crops is in its infancy and only two adequate publications were found. The albedo effect of cover crops ranges from -4.00 to 10.00 W m -2 was calculated by Kaye and Quemada [ 22 ]. Converted to CO 2 e, this corresponds to a climate benefit of 0.25 Mg CO 2 e ha -1 a -1 . Carrer et al. [ 14 ] show that three months of cover cropping in Europe results in a climate benefit of 0.16 Mg CO 2 e ha -1 a -1 . Similarly, this study found that longer time periods with cover cropping would increase the mitigation potential by 27.0%. Overall, a mean climate benefit of 0.20 Mg CO 2 e ha -1 a -1 (95% CI, -0.37, 0.78) was identified in the considered literature for the albedo change of cover crops.

3.8. Nitrous oxide emissions

Cover cropping can increase N 2 O emissions under certain weather conditions compared to fallow land depending on the fertilizer N rate, soil incorporation, and the period of measurement and rainfall [ 35 ].There are many factors that influence the level of N 2 O emissions which leads to high variability from this GHG emission source. A prerequisite for cover crops is high soil moisture and water saturation, or at least water saturation of the upper soil layers. Another factor is the C/N ratio of the cover crop, which influences how fast cover crop residues decompose. Cover crops with low C/N ratios emit more N 2 O than cover crops with high C/N ratios [ 36 , 37 ]. Another factor influencing N 2 O emissions is the treatment of cover crop residues. Incorporating residues into the soil leads to higher N 2 O emissions as compared to leaving cover crop residues decaying on the soil surface [ 36 ]. Air temperature also effects N 2 O emissions. As temperatures increase, N 2 O emissions from cover crops also increase. Overall, air temperature accounts for 22.0% of the variability in N 2 O emissions [ 38 ]. In a meta-analysis, Basche et al. [ 35 ] found that N 2 O emissions increased in 60.0% and decreased in 40.0% of the publications when cover cropping was practiced. The overall conclusion of the meta-analysis is that cover cropping increases N 2 O emissions only to a very small extent and that the variability is very large. For example, compared to a chemical fallow system, Olofsson and Ernfors [ 39 ] identified the highest value with 1.80 kg N 2 O-N ha -1 for oilseed radish, whereas Preza-Fontes et al. [ 40 ] identified the lowest emissions with -0.54 kg N 2 O-N ha -1 for a sorghum-sudan-grass-mixture. The result of our calculation based on a systematic literature search also shows only a very small increase in N 2 O emissions from cover cropping. The determined weighted mean value of emissions, as displayed in Table 2 , is 0.13 kg N 2 O-N ha -1 , which is equivalent to 0.04 Mg CO 2 e ha -1 (95% CI, -0.02, 0.09).

3.9. Foregone benefits due to cover crop seed land use

Land use for producing seeds for cover crops was calculated for the species phacelia, mustard, clover, and oat as representatives of the most important plant families used as cover crops. Their average was used for further calculations. The average land use requirement to produce one ha of cover crop seeds was calculated as 0.02 ha (95% CI, 0.00, 0.04).

We assume that the land used for the production of cover crop seeds would otherwise be used for wheat production. Wheat production on land in the EU offers a carbon benefit that consists of: (1) the opportunity that the wheat output enables storing carbon elsewhere (yield × carbon opportunity cost); and (2) savings in global production emissions due to lower production emissions in the EU compared to the global average (here, the difference between Swedish and global wheat production emissions is based on Searchinger et al. [ 11 ]). Other factors, like changes in soil organic carbon or bioenergy benefits are not considered here. The climate benefit of wheat consisting of factors (1) and (2) is forgone due to land requirements for cover crops’ seed requirements and can be quantified accordingly to a mean value of 0.28 Mg CO 2 e ha -1 (95% CI, 0.27, 0.29).

3.10. Production emissions for cover crop seed production

Since no specific literature sources for seed production emissions could be found for typical cover crop species, emissions from corn, spring wheat, and canola seed production are used in this study. Hybrid corn seed production in China leads to emissions of 1,459.00 kg CO 2 e Mg -1 , whereas in the U.S. it causes 2,250.50 kg CO 2 e Mg -1 [ 17 , 41 ] of emissions. Seed production of rapeseed in Poland causes slightly lower emissions of 1,014.00 kg CO 2 e Mg -1 [ 42 ]. In contrast, for spring wheat, it is only 580.00 kg CO 2 e Mg -1 in Southwest Finland and 680.00 kg CO 2 e Mg -1 in Northern Savonia (Finland) [ 43 ]. These emissions mostly depend on the amount of fertilizer used since fertilization is responsible for the majority of production emissions, especially the production and application of nitrogen fertilizers [ 17 ]. The weighted average value used for our calculation is 1,066.26 kg CO 2 e Mg -1 (95% CI, 219.56, 1912.95). For the emissions of processing, packaging, and transporting cover crop seeds, only one literature source for corn seed could be found. The emissions here are 213.00 kg CO 2 e Mg -1 [ 44 ].

3.11. Additional machinery operations for cover crop establishment

We assume that cover crop establishment requires either tillage with a plow or cultivator followed by seeding. In addition, it is assumed that the only relevant emission from these machinery operations is from burning fossil diesel fuel. Based on data of an internet database from KTBL [ 45 ], plowing plus seeding is estimated at 55.00 liter diesel fuel ha -1 , while cultivator tillage and seeding is estimated at 45.00 liter diesel fuel ha -1 . The variability of the machinery operation intensity and corresponding fuel consumption was analyzed by Wittwer et al. [ 18 ]. In this study, diesel fuel consumption ranges from 22.00 l ha -1 for no-till systems to 61.00 l ha -1 for conventional intensive tillage systems. In organic farming, diesel fuel consumption ranges from 32.00 l ha -1 for reduced tillage to 55.00 l ha -1 for intensive tillage. The average diesel fuel consumption used in this study was calculated as 45.29 l ha -1 (95% CI, 32.43, 58.13).

The emission factor per liter of diesel fuel ranges from 3.00 kg CO 2 e l -1 [ 44 ] to 3.31 kg CO 2 e l -1 [ 46 ]. The average emissions were calculated as 3.13 kg CO 2 e l -1 (95% CI, 2.92, 3.34) diesel fuel. The additional machinery operations thus cause emissions of 0.14 Mg CO 2 e ha -1 (95% CI, 0.10, 0.18).

3.12. A factor not considered: Erosion reduction

According to Lugato et al. [ 47 ], soil erosion can lead to a loss of soil carbon, lowering the carbon sink capacity of soils. Additionally, soil erosion can disturb the soil structure and reduce soil fertility, which disconnects soil element cycles and further contributes to GHG emissions. However, the extent to which soil erosion leads to higher carbon fluxes out of the system depends on the specific context and interplay of many factors [ 48 ].

Cover crops protect soil erosion during the fall and winter and also after the establishment of the following crop, such as maize, due to remaining cover crop residues [ 49 ]. According to Laloy and Bielders [ 49 ] and Gentsch et al. [ 6 ], Cover crops increase the infiltration rate and improve soil structure, allowing the soil to absorb more water without eroding. According to Panagos et al. [ 15 ], cover crops reduce soil erosion by at least 20.0% in Europe and the United Kingdom. Machiwal et al. [ 50 ] found a reduction in soil loss of 33.0% to 77.0%, depending on the cover crop type for India. The reduction in soil erosion is highly dependent on the slope and management of the land. In addition, the soil type also plays a major role.

Given the uncertainty between soil erosion and fluxes from the soil system as well as the varying factors leading to soil erosion, we have not included climate benefits of soil erosion prevention from cover crops in our calculations.

3.13. Net climate change mitigation impact (NCCMI) of cover crops

We calculate the summary of all climate benefits from cover crops as 4.23 Mg CO 2 e ha -1 a -1 (95%CI, 1.25, 7.49) and all climate costs from cover crops as 0.93 Mg CO 2 -eq ha -1 a -1 (95% CI 0.51, 1.34). The NCCMI of cover crops was calculated as 3.30 Mg CO 2 e ha -1 a -1 ( Table 3 ). The NCCMI was then extrapolated to the maize harvest area for all EU-27 countries. Here, we assume a scenario in which cover crops are grown before maize on all of the EU-27’s harvested maize areas, which is 15,092 x 10 6 ha, on average [ 23 ]. Based on this calculation, planting cover crops before maize in the EU-27 results in a climate change mitigation potential of 49.80 million Mg CO 2 e a -1 . This is equivalent to 13.0% of the EU-27’s agricultural GHG emissions [ 51 ].

3.14 Sensitivity analysis based NCCMI scenarios

The impact factors “carbon land benefit based on yield gain” and “carbon sequestration” have a high impact on NCCMI but also show a high uncertainty. To address this uncertainty, we use four scenarios for NCCMI to analyze the sensitivity of the results:

A) Base scenario as shown in section 3.13

B) Base scenario less carbon land benefit based on yield gain

C) Base scenario less carbon sequestration

D) Base scenario less carbon land benefit based on yield gain and less carbon sequestration

4. Discussion

4.1. review of results.

The results of this study indicate that the climate benefits of cover crops significantly exceed their climate costs. This resulted in a positive NCCMI of 3.30 Mg CO 2 e ha -1 a -1 that can be achieved if cover crops were incorporated in crop rotations of maize production. The major contributor to climate benefits is through carbon land benefits based on yield gain for maize following a cover crop, which alone contributed to 33.8% of all climate benefits. Several meta studies [ 9 , 10 ] showed that cover crops increase soil organic carbon stocks, with an average organic carbon sequestration rate of 1.43 Mg ha -1 a -1 . The most important mechanism behind this is the stimulation of microbial activity from cover crops’ organic matter input. Litter from shoots and roots, rhizodeposits, and the transport of photoassimilate to the rhizosphere during the growth of cover crops results in a higher and more active microbial biomass [ 52 ]. These processes resulted in microbial derived organic substances that are key to build up mineral associated organic matter fractions with prolonged turnover times [ 53 ]. The activation of microbial cycling that derived from higher organic matter input rates is therefore a strong indirect factor of the climate benefits from cover crops. Carbon sequestration was the second largest contributor (34.5%) to climate benefits. The results indicate a maize yield increase of 8.8% by following a cover crop. Since maize yield gains are higher following legume cover crops and in organic systems, it can be assumed that increased nutrient availability is one of the key factors driving crop yield gains from cover crops. Cover crops might help in closing the yield gap between organic and conventional systems, as well as in improving the resilience of arable production under climate change. However, yield benefits depend on the quality of cover crop residues and reach its maximum at litter C/N ratios <25 [ 54 ]. Closer nutrient cycling and reduction of leaching losses resulted in another 24.3% of cover crops’ climate benefits, including nitrogen fixation of legume cover crops. The highest maize yield gains were found for legume-based cover crops while graminoid cover crops showed only minor effects. For example, Marcillo and Miguez [ 24 ] found that a grass cover crop has no influence on maize yield. In the same study, a legume-only mixture shows up to a 21.0% yield gain in the maize crop following the cover crop. Wittwer et al. [ 18 ] confirmed that a legume-free cover crop mixture leads to only a 3.0% increase in maize yield. Mixtures of legumes and grasses as cover crops enable yield gains from 1.3% [ 55 ] to 21.0% [ 56 ], respectively. However, cover crops can also have negative effects on the subsequent maize yield. Hunter et al. [ 57 ] showed that a high carbon to nitrogen (C/N) ratio in spring cover crops resulted in a lower yield of silage maize. Nutrient use efficiency has been demonstrated to increase through cover crops that are in crop rotations for several crops, including maize [ 58 – 60 ]. The inclusion of legumes increases cover crops’ litter quality and crop yield benefits; however, pure legume stands are less effective to prevent nitrogen leaching over the winter [ 6 ]. The inclusion of cover crops in crop rotations, therefore, requires a high degree of management by selection of suitable cover crop species or mixtures to maximize their green manure and environmental benefits.

Cover crops are able to change the amount of incoming shortwave radiation that is reflected back to the atmosphere and thereby mitigate warming [ 22 ]. Albedo change through cover crops’ land cover has the smallest contribution (4.5%) to the climate benefits from cover crops. The systematic literature search resulted in only two publications that analyze albedo change through cover crops. Both found a slight climate change mitigating effect through radiative forcing changes induced by an increase in surface albedo.

Climate costs of cover crops were nearly six times lower than climate benefits and are mainly dominated by cover crops’ seed production emissions and foregone benefits due to cover crops’ seed land use (accounting for 41.9% and 30.1%, respectively, of the total climate costs of cover crops). Interestingly, additional machinery operations and N 2 O emissions from cover crops’ decomposition contributed minorly to climate costs (15.0% and 4.3%, respectively). Emissions of N 2 O have a 280 times higher global warming potential compared to CO 2 . Therefore, the latest studies were alarmed about the emission potential that can appear from decomposition of cover crop residues [ 35 ]. However, N 2 O emissions are of episodic nature and depend on extended precipitation events. Despite this, N 2 O emissions from cover crops’ decomposition can be further reduced if cover crop residues were not incorporated into the soil [ 35 ]. With this respect, no-till operations would further reduce the N 2 O emissions factor.

We created four scenarios where we assumed that not all positive impacts of cover cropping might be present at the same time. We removed the major contributors: C sequestration (B) and yield gain (C) or both together (D). All scenarios kept a positive NCCMI ( Fig 3 ). Therefore, we conclude that positive aspects of cover cropping always superimpose the negative impacts.

Numbers are given as Mg CO 2 e ha -1 .

https://doi.org/10.1371/journal.pone.0302139.g003

All scenarios show a positive NCCMI.

4.2. Comparing overall results to existing literature

In contrast to Kaye and Quemada [ 22 ], this study is first to include land use effects based on the yield gain caused by cover crops. Furthermore, our study uses a systematic literature review and quantifies the climate change mitigation impact for all of EU-27, which demonstrates the magnitude of climate change mitigation opportunities of cover crops. So far no other literature has extrapolated the climate change mitigation impact of cover crops to a country or region. Overall, Kaye and Quemada [ 22 ] found a climate change mitigation effect of 1.16–1.35 Mg CO 2 e ha -1 a -1 from cover crops, which is lower than that calculated in this study. This discrepancy is a result of different analyzed literature and the inclusion of land use effects in our research. In addition, Abdalla et al. [ 25 ] showed a slightly lower climate change mitigation impact from cover crops compared to our results of 2.06 Mg CO 2 e ha −1 a −1 (95% CI -0.04, 4.16). The study, however, does not consider land use change effects (1.51 Mg CO 2 e ha -1 a -1 in our study), albedo change, emissions for cover crop seed production, and additional machinery operations. If the study would have considered these factors, their results would be more comparable to those in our study.

4.3. Limitations

A limitation of this study is that we did not include erosion reduction of cover crops as a climate benefit due to the high data variability and uncertainty of its climate impact. Furthermore, the use of carbon opportunity cost as suggested by Searchinger at al. [ 11 ] is criticized by some authors for separating supply and demand side efficiencies and including supply and demand side interactions [ 61 , 62 ]. Despite this criticism, carbon opportunity cost is a widely used approach to quantify changes in efficiency of land use for mitigating climate change. Given that land use change is still the highest source of emissions in the global agri-food sector [ 2 ], carbon opportunity cost based on Searchinger at al. [ 11 ] is an adequate land use change quantification approach for changes in maize yield.

For the climate impact of land use for cover crop seed production, we use the forgone climate benefit of wheat based on Searchinger et al. [ 11 ]. Here, the same arguments discussed above for using carbon opportunity costs are relevant. We assume wheat as the displaced crop when cover crop seeds are produced since wheat is the crop with the largest acreage in the European Union [ 23 ]. For additional machinery operations to establish cover crops, we only consider diesel fuel emissions. There are also emissions for producing, maintaining, and repairing agricultural machinery, but in the majority of cases, existing machinery is used for cover crop establishment, i.e., no additional machinery is produced to establish cover crops. In this study, we only evaluated maize as the crop to follow cover crops. Maize is the largest spring crop by acreage in the European Union [ 23 ] and the most investigated crop in the scientific literature with a good data base. Despite this, other spring crops like oats, spring barley, sunflower, or sugar beet are investigated for the benefits of winter cover crops in the European Union. More data is needed to complete the view on climate benefits of cover crops based on different main crops.

4.4. Conclusion and policy recommendations

This is the first study to include all relevant climate change impacting effects of cover crops in a comprehensive calculation and to extrapolate the NCCMI of cover crops to the whole maize cropping area of the EU-27. We show that cover crops are powerful tools to mitigate climate change impacts from European agriculture, which make our findings very relevant for political decisions in the EU. If all maize cropping acreage in the EU-27 were to include cover crops, GHG emissions from EU-27 agriculture could be reduced by 13.0%. Based on this result, we recommend that the Common Agricultural Policy (CAP) of the European Union should continue to accelerate the integration of cover crops into farming. The current CAP (2023–2027) does not include regulations that make cover crops before spring crops mandatory or provide incentives so the area devoted to cover crops in the EU is maximized. There are conditionality requirements, such as obligatory soil cover in the winter for 80% of a farm’s arable land. Therefore, if a farm has 80% winter crops and 20% spring crops in its rotation, there is no requirement to grow cover crops under the current CAP for this farm [ 63 ].

Our results recommend a stronger commitment from the EU’s agricultural policy to make cover crops before spring crops a common practice on all farms. This will not only help to achieve the EU’s climate protection goals, but also global targets in the Paris Agreement.

5. Statistics

The weighted mean was calculated from study values using the number of observations as the weighting factor. Mean values are followed by the 95% confidence intervals (CI) and are shown in brackets (lower CI limit, upper CI limit). All data collected from the literature as well as the calculations shown in Table 3 are provided as supplementary material ( S1 File ).

Forest plots were produced with the R package [ 64 ] with R version 4.3.2 [ 65 ]. Detailed description of the statistic parameters are outlined in [ 66 ].

Supporting information

S1 file. calculations..

https://doi.org/10.1371/journal.pone.0302139.s001

S2 File. Included literature.

https://doi.org/10.1371/journal.pone.0302139.s002

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• http://orcid.org/0000-0001-8708-9324 Krystel Aouad 1 ,
• http://orcid.org/0000-0002-8428-6354 Maarten de Wit 2 ,
• Muriel Elhai 3 ,
• http://orcid.org/0000-0001-9119-5330 Diego Benavent 4 ,
• Heidi Bertheussen 5 ,
• Condruta Zabalan 6 ,
• http://orcid.org/0000-0002-1049-4150 Jette Primdahl 7 ,
• http://orcid.org/0000-0002-8895-6941 Paul Studenic 8 , 9 ,
• http://orcid.org/0000-0002-4528-310X Laure Gossec 10 , 11
• 1 Rheumatology Division, Saint George University of Beirut , Saint George Hospital University Medical Center , Beirut , Lebanon
• 2 EULAR Study Group for collaborative research , Patient Research Partner , Amsterdam , The Netherlands
• 3 Rheumatology Department, University of Zurich , University Hospital Zurich , Zurich , Switzerland
• 4 Rheumatology Department , Hospital Universitari de Bellvitge , Barcelona , Spain
• 5 Patient Research Partner , Oslo , Norway
• 6 Patient Research Partner , Bucharest , Romania
• 7 Danish Hospital for Rheumatic Diseases , University Hospital of Southern Denmark , Sønderborg , Denmark
• 8 Rheumatology Division, Department of Medicine(Solna) , Karolinska Institutet , Stockholm , Sweden
• 9 Rheumatology Division, Internal Medicine Department , Medical University of Vienna , Vienna , Austria
• 10 Rheumatology Department , University Hospital Pitié Salpêtrière , Paris , France
• 11 INSERM, Institut Pierre Louis d'Epidémiologie et de Santé Publique , Sorbonne Université , Paris , France
• Correspondence to Dr Krystel Aouad, Rheumatology Department, Saint George University of Beirut, Saint George Hospital University Medical Center, PO box 166378, Beirut, Lebanon; krystel.aouad{at}hotmail.com

Background Patient research partners (PRPs) are people with a disease who collaborate in a research team as partners. The aim of this systematic literature review (SLR) was to assess barriers and facilitators to PRP involvement in rheumatology research.

Methods The SLR was conducted in PubMed/Medline for articles on PRP involvement in rheumatology research, published between 2017 and 2023; websites were also searched in rheumatology and other specialties. Data were extracted regarding the definition of PRPs, their role and added value, as well as barriers and facilitators to PRP involvement. The quality of the articles was assessed. Quantitative data were analysed descriptively, and principles of thematic content analysis was applied to qualitative data.

Results Of 1016 publications, 53 articles were included; the majority of these studies were qualitative studies (26%), opinion articles (21%), meeting reports (17%) and mixed-methods studies (11%). Roles of PRPs ranged from research partners to patient advocates, advisors and patient reviewers. PRPs were reported/advised to be involved early in the project (32% of articles) and in all research phases (30%), from the conception stage to the implementation of research findings. The main barriers were challenges in communication and support for both PRPs and researchers. Facilitators of PRP involvement included more than one PRP per project, training of PRPs and researchers, a supportive environment for PRPs (including adequate communication, acknowledgement and compensation of PRPs) and the presence of a PRP coordinator.

Conclusion This SLR identified barriers and facilitators to PRP involvement, and was key to updating the European Alliance of Associations for Rheumatology recommendations for PRP–researcher collaboration based on scientific evidence.

• Health services research
• Health-Related Quality Of Life
• Outcome and Process Assessment, Health Care
• Quality Indicators, Health Care

Data availability statement

Data are available upon reasonable request. All data relevant to the study are included in the article or uploaded as supplemental information. Additional data are available on reasonable request.

This is an open access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0).

https://doi.org/10.1136/ard-2024-225567

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WHAT IS ALREADY KNOWN ON THIS TOPIC

Patient research partners (PRPs) are increasingly integrated into medical research, particularly in rheumatology.

Major global health organisations recognise the central role of PRPs’ involvement in research.

Previous recommendations have guided researchers and PRPs to build collaborative relations but lack a strong evidence base.

WHAT THIS STUDY ADDS

This systematic literature review provides for the first time a comprehensive overview of the emerging role of PRPs in rheumatology research, emphasising their expanding roles, contributions and the value they bring to the research process.

The review identified key barriers to PRP involvement, ranging from personal factors to challenges in training, communication and collaboration, and also identified strategies to enhance PRP involvement.

Early and sustained involvement of PRPs, as well as a supportive environment and effective communication, were found to be essential to enhance the relevance and impact of PRP contribution to research.

HOW THIS STUDY MIGHT AFFECT RESEARCH, PRACTICE OR POLICY

Recognising and addressing the barriers to PRP involvement can lead to better support for PRPs, enhancing their involvement in research.

Some facilitators identified include involvement of PRPs since the early stages of research, a supportive environment for PRPs and encouraging researchers to adopt more flexible strategies and behaviours to maximise the benefit of PRP involvement.

This literature review informed European Alliance of Associations for Rheumatology recommendations, highlighting the importance of active collaboration, training, mutual respect, and transparent communication between PRPs and researchers.

Introduction

Patient research partners (PRPs) are described as individuals living with a health condition who ‘provide input to research, through active collaboration as equal partners with researchers’. 1 Their involvement is essential to make research more patient centred, for instance, by capturing outcomes that matter to patients. Over the past two decades, the magnitude of PRP involvement and their roles in research has grown substantially. 2–8 Patients have transitioned from passive subjects to active collaborators and equal partners, bringing their unique perspectives and valuable insights to the forefront of medical research. 5 This change has not only profoundly modified research practices but has also underscored the integral role PRPs play in shaping the future of medical practice. 9 The importance of PRP involvement in research has become widely recognised as an essential component of high-quality patient care, highlighted by organisations such as the WHO 4 and European Medicine Agency (EMA), 10 and is acknowledged across various medical specialties. 11–13

In rheumatology, this paradigm shift has been significant. In 2011, the European Alliance of Associations for Rheumatology (EULAR) developed recommendations for the involvement of patient representatives in scientific projects based on expert opinion. 14 These recommendations marked a pivotal step, setting the stage for the involvement of PRPs in research projects. Since then, these EULAR recommendations have guided other organisations such as Outcome Measures in Rheumatology (OMERACT), Group for Research and Assessment of Psoriasis and Psoriatic Arthritis (GRAPPA) and the Foundation for Research in Rheumatology (FOREUM), to recognise the important role of PRPs or to develop their own guidelines for collaborative research. 15–18

As the landscape of patient involvement in research evolves, the literature has witnessed a great surge in data and studies concerning PRP involvement. 2 4 5 15 19–25 These studies not only shed light on the benefits of PRP participation but also highlighted the challenges encountered in this collaborative effort and solutions proposed to overcome barriers. 21 22 25–29

In 2022, EULAR decided to update the 2011 recommendations for PRP involvement in research, focusing specifically on PRPs in the context of chronic conditions. 14 In accordance with the EULAR standardised operational procedures (SOPs) supporting this update, we conducted a systematic literature review (SLR) to inform the EULAR taskforce.

To support the update of the EULAR recommendations, we conducted in 2023 an SLR that encompassed both qualitative and semiquantitative analyses of recent publications in rheumatology, with the goal of identifying factors that affect PRP involvement, including barriers and facilitators.

Literature search

The SLR aimed to identify publications reporting PRP involvement in rheumatology research published between 1 January 2017 and 1 January 2023. We searched the electronic database PubMed MEDLINE using the terms “patient research partner”, “patient expert”, “patient and public involvement (PPI)”, their synonyms and related concepts. Details of the search terms and search strategy can be found in online supplemental table 1 . Two authors (KA, LG) independently assessed the title, abstract and keywords of every publication identified. In the event of disagreement between the reviewers, disparities were discussed and resolved. Additionally, we performed a scoping review of databases to assess PRP involvement and explored six websites from rheumatology: OMERACT, GRAPPA, American College of Rheumatology, EULAR, FOREUM and Osteoarthritis Research Society International. We also searched 2 regulator websites: Food and Drug Administration and EMA, and 10 websites of three selected specialties recognised for significant PRP involvement: cardiology, oncology, endocrinology (diabetes) ( online supplemental table 2 ). A specific search was done in two websites focusing on patient and public involvement: INVOLVE UK by the National Institute for Health Research and Education that empowers (European Patients’ Academy on Therapeutic Innovation), and in orphan diseases to answer specific research questions about training, involvement in grant applications and remuneration of PRPs ( online supplemental table 3 ).

Supplemental material

The scope of the literature search was defined by the EULAR taskforce steering group, 1 and addressed 11 specific research questions ( Box 1 ).

Research topics included in the systematic literature review

1. Definition of patient research partners (PRPs)

How to define a PRP? Is the current definition of PRPs still adequate?

2. Roles and activities undertaken by PRPs

What are the roles and activities of PRPs in rheumatic musculoskeletal disease research?

3. Benefits and added value of PRP involvement for PRPs themselves, researchers, the research itself

What is the added value of PRPs in different types of research and groups?

4. Types of scientific projects that involved PRPs and the stages of the projects in which they participated

What types of projects are (or should) PRPs (be) involved in?

What phases of a project are (or should) PRPs (be) involved in?

5. Selection and recruitment processes for PRPs

How are (or should) PRPs (be) recruited and selected?

How many PRPs are (or should be) involved in the research?

6. Insights into the experiences and feedback provided by PRPs

What are the PRP feedback and experiences, in terms of facilitators and barriers to PRP involvement?

How can we improve the PRP experience and involvement overall?

7. Roles of a coordinator for PRPs in research

Are PRP facilitators involved, if so how, and is it useful?

Is a facilitator/PRP coordinator recommended?

What is the reported usefulness of a facilitator ?

8. Training provided to PRPs or researchers

Do the PRPs involved have a specific training (previously/during the study)?

How should researchers be educated, trained, supported to enhance PRP involvement?

9. Evaluation and monitoring related to PRP involvement

How should PRP involvement be monitored or evaluated? At which time points and by whom?

How should PRP involvement evaluation/monitoring be reported?

10. Recognition, compensation and acknowledgement of PRPs during their involvement in a scientific project

How should PRP involvement be recognised and acknowledged?

Is (should) compensation (be) proposed?

11. Barriers encountered and proposed solutions to enhance PRP involvement

What are the barriers encountered during PRP involvement?

Which strategies and contextual factors enable optimal engagement of PRPs?

Inclusion and exclusion criteria

We included all types of articles reporting PRP involvement in all types of research, including trials and observational studies, qualitative studies, mixed-methods studies and reports of meetings, opinion papers and reviews. We did not exclude published articles from any country, aiming to enhance the generalisability of our findings. Recommendations and guidelines on PRPs were also analysed and were used as supportive information. Articles not focused on rheumatology research or not bringing any information on PRPs (ie, not answering one or more research questions), as well as not in English, were excluded. Articles only mentioning PRPs or their involvement, without providing any details (eg, on their roles, contributions or barriers/facilitators), were excluded as well. Articles with duplicate information (ie, multiple publications reporting on a single study) were excluded if they did not provide additional information relevant to our research questions.

We also identified relevant articles by hand search of the references cited in the included studies, extending the inclusion period to the date of publication of the previous recommendations (2011–2023).

Data extraction

Data collection encompassed both quantitative and qualitative data, addressing various aspects of PRP involvement and providing answers to our research questions ( Box 1 ). Data were extracted and checked independently by two authors (KA and MdW). Discrepancies were resolved by discussion among the core team (KA, MdW, PS, LG).

Quality assessment

Papers were assessed for quality only if they reported original data. Review papers, recommendation papers, opinion papers, case studies, study protocols, report papers and qualitative studies not primarily focused on PRPs were excluded from quality assessment. Given the diversity of study types, we used the Critical Appraisal Skills Programme (CASP) checklist for qualitative studies, literature reviews and cross-sectional studies as described in the EULAR SOP. 30 31 This tool, originally developed for qualitative studies, assesses elements such as the clarity of research aims, appropriateness of methodology, suitability of the research design, adequacy of data collection and clarity in reporting outcomes. For mixed-methods studies, we used the Mixed Methods Appraisal Tool (MMAT, a critical appraisal tool that is designed for the appraisal stage of systematic mixed-studies reviews, that is, reviews that include qualitative, quantitative and mixed-methods studies 32 (see online supplemental tables 4 and 5 for quality assessment). To facilitate interpretation, an overall quality assessment for the level of evidence (LoE) was conducted by evaluating the number of items on the score checklist and on the key items. Subsequently, the authors reached a consensus on classification of the articles’ quality as high, medium or low quality.

This SLR was not considered appropriate by PROSPERO for registration due to the mixed-methods study analyses involved.

Patient and public involvement

This SLR study is the result of a co-production of three PRPs (MdW, CZ, HB) and five researchers, all being members of the EULAR steering committee responsible for updating the EULAR recommendations on PRP involvement. 1 The three PRPs actively contributed to all meetings and discussions within the steering committee. They were involved at the early stage of formulating the research questions until reviewing and agreeing on the final manuscript. They were also actively engaged in planning dissemination and implementation of the study findings within the wider community and patient associations. The recruitment of the PRPs was coordinated by one of the PRPs (MdW), the convenor of the project.

For quantitative data, a descriptive analysis of findings is reported, including characteristics of studies (study design, population, country, study objectives), characteristics of PRPs, selection process of PRPs, type of involvement, phases of the research where their involvement occurred, with numbers and percentages using frequency tables and charts.

The number of PRPs involved in the studies was quantified using two distinct methods: first, coauthorship count: direct examination of the research articles’ authorship lists. PRPs were identified based on explicit mentioning of their role as ‘PRP’ or other specific identification. Second, participation count: this approach assessed the number of broader involvements of PRPs in activities of the research project. For instance, in a GRAPPA meeting report, the number of PRPs who actively participated was counted. 8

Qualitative data were analysed according to the principles of thematic content analysis (more details in online supplemental table 6 ). 33 The results were discussed within the EULAR taskforce, 1 and any disagreements on the interpretation of the findings were resolved by a consensus of the core group (MdW, LG, PS, KA).

Search strategy

The SLR yielded a total of 1016 records of which 941 (92.6%) were excluded based on titles and abstracts. We conducted a full-text screening of 75 papers and 46 (61.3%) were included. The main reasons for exclusion were papers not related to rheumatology, lacking reports of PRP involvement in research, being irrelevant to our research questions, or being duplicates or conference abstracts ( figure 1 ). Additionally, 7 papers were identified by hand search, resulting in a total number of 53 included articles.

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Flow chart of selected article search on PRP involvement in rheumatology research. PRP, patient research partner.

Quality assessment (LoE) of the papers

Nineteen articles were assessed for quality using the predefined scores according to the study type. Overall, 79% (15 of 19) were classified as high quality, 11% (2 of 19) as medium quality and 11% as low quality ( online supplemental table 4 ).

Study characteristics

The included studies were qualitative studies (14 of 53, 26%), opinion articles (11 of 53, 21%), meeting reports (9 of 53, 17%), mixed-methods studies (6 of 53, 11%), recommendation articles (4 of 53, 8%), reviews (SLR or scoping review; 3 of 53, 6%), cross-sectional (2 of 53, 4%), case studies (2 of 53, 4%), observational (1 of 53, 2%) and study protocol (1 of 53, 2%) ( online supplemental tables 4 and 5 ).

Overall, 62% were published in rheumatology journals. Geographically, most of the studies were from Europe (50%), followed by North America (31%).

Identification of barriers encountered and proposed solutions to enhance PRP involvement

Barriers to PRP involvement ( table 1 and online supplemental table 7 ) included emotional and personal factors, communication and relationship challenges, inadequate training and support, difficulties in the research process and pace, as well as collaboration and engagement issues. 2–4 21 22 24–27 34–42 Effective strategies to enhance PRP involvement ( table 1 ) included early involvement, a supportive environment, effective communication and trust, and providing support and training for PRPs and researchers. 7 21 22 26 29 38 40 43 44

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Barriers and strategies to enhance PRP involvement in rheumatology research

Definitions of PRP

Among the 53 included papers, 62% provided a definition of PRP. Importantly, a significant portion (30%) of these papers 4 6 15 26 27 34–36 45 46 adopted the 2011 EULAR definition of PRP as ‘persons with a relevant disease who operate as active research team members on an equal basis with professional researchers, adding the benefit of their experiential knowledge to any phase of the project’. 14 These papers consistently emphasised the importance of active involvement and fostering equal partnerships between PRP and researchers.

Additionally, seven papers (13%) expanded upon this definition by incorporating informal caregivers into the PRP definition, 20 28 37 38 47–49 known as persons, usually family members, who provide unpaid care to someone with whom they share a personal relationship.

The roles and activities of PRPs

The roles and activities of PRPs covered a wide spectrum, extending from research partners to patient advocates, advisory roles and participation as patient reviewers (as detailed in table 2 and online supplemental table 8 ). Their contributions encompassed a diverse range of activities, including providing input in guideline development, shaping research agendas, and actively advocating in scientific and clinical committees.

Activities and roles of PRPs

The added value of PRP involvement

The literature reported that PRPs added significant value across various aspects of research ( table 3 ). Specifically, 53% of the articles indicated that PRP involvement brought benefits for the PRP themselves, that is, better understanding of their medical condition, acquisition of practical skills, improved comprehension of the research process and increased self-confidence. 2 21 25 36 39 Furthermore, 26% of the articles highlighted advantages for the research process, that is, heightened relevance of the research, enhancement of its overall impact and enrichment of the results by adding experiential knowledge. 2 7 21 25 29 36 38 39 45 The positive impact on researchers, reported in 15% of the articles, encompassed deeper insights into research priorities, increased motivation, innovative ideas, awareness of the impact of their work, a comprehensive approach to addressing patients’ needs and improved communication in lay language ( table 3 ). 2 21 25 34 36 38 40 The added value of PRP involvement was also reported as advantageous for the wider community by enhancing the acceptance of research that prioritises community benefits. 2 21 25 36

Articles reporting on added value of PRP involvement in research for PRPs, for researchers and for the research

Types of research that involved PRPs

PRPs were actively involved in a wide range of scientific projects, including basic, translational and clinical research. 50 Although the benefits of PRP involvement were less apparent in basic and translational research, some researchers and PRPs recognised the substantial advantages of collaborative partnerships in this area. 3 25 34 A scoping review highlighted the benefits of PRP engagement in preclinical research, including enhanced understanding of basic science research for PRPs, broadened perspectives for researchers, and positive influence on study questions and methods, along with fostering mutual learning, new collaborations, and improved research quality and efficiency. 40 One study reported that researchers were committed to finding more meaningful ways to integrate PRPs into basic scientific research and dissemination of the project results. 3 Strategies to enhance PRP involvement (ie, training, support, PRP-focused tasks) were also reported. 3

Research phases in which PRP participated

Early involvement of PRPs in the research was reported or recommended in 32% of the included articles, emphasising engaging PRPs from the inception of a research project. 2 19–22 27–29 34 36–38 43 45 47 51 This early engagement was reported to enable PRPs to actively shape research questions and methodologies in line with their priorities. Additionally, 30% of the articles stressed the importance of PRPs’ continuous participation throughout all research stages ( table 4 ). 4 15 21 22 26 35 43 52–54

Articles reporting or recommending PRP involvement in different phases of the research project

Number of PRPs

The number of PRPs involved in research is shown in online supplemental figure 1 . When considering the coauthorship lists, the majority of articles clearly specified the name and identity of PRPs; subsequently, the number of PRPs involved in the writing and reviewing of the article could be easily deducted. Yet, in 19% of cases, the identification of a coauthor as a PRP was unclear. In cases where PRP involvement was explicitly highlighted by coauthorship, 34% of the articles included one or two PRPs per project, 17% of articles included three or four PRPs, and 25% of articles involved more than five PRPs. Notably, single-centre studies commonly involved one or two PRPs as coauthors. One study, which engaged four PRPs, found this number to be beneficial due to the diverse perspectives they brought. 45 Larger-scale international consortia projects recruited a higher number of PRPs, with around six PRPs being identified as an effective group size for facilitating participation and decision-making. 2

On the other hand, when reporting all PRP involvement and activities in a research project, 36% of the articles reported a number of PRP higher than nine ( online supplemental figure 1 ). Therefore, the number of PRPs involved in research can be higher than the number of PRPs mentioned as coauthors.

Selection and recruitment processes for PRPs

The selection process of PRPs was reported in 34% of articles ( figure 2 ). PRP selection criteria were mainly language proficiency (11%), research knowledge (6%), disease diagnosis (9%), communication skills and constructive assertiveness (9%), motivation (8%), educational background (6%), experiential knowledge and expertise (6%) as well as travel capability (4%). 2 3 15 19 21 23 24 27 34 35 55–58 Recruitment methods for PRPs were diverse, relying on patient organisations, marketing companies, rheumatology associations, social media, community outreach, clinic visits, personal connections with patients or researchers, word-of-mouth referrals and volunteering. 2 21 34 38 41 44 53 59 Furthermore, 28% of studies emphasised the importance of clarifying patient roles through clear goal-setting and exchanging mutual expectations early in the project initiation phase. 15 19–21 27 29 36 42 45 47 Additionally, 28% of studies highlighted the need for inclusivity and diverse representation in PRP recruitment. 2 4 15 35 41 42 52

The selection criteria of PRPs reported in the studies. PRP, patient research partner.

Creating a supportive environment for PRPs

A supportive environment for PRPs was reported to depend on several key principles ( table 1 ) 4 19 20 25 36 42 52 53 60 : ensuring a balanced and manageable workload that respects PRP abilities, providing adequate resources and time for PRP involvement, offering support to overcome language barriers, promoting flexibility and offering accessible accommodation to participate in meetings and scientific conferences. 7 21 38 40 43 44 Equal relationships and co-leadership between PRPs and researchers were cited in several papers as crucial, emphasising mutual respect, trust, and open, transparent communication. 7 15 19 Building strong team communication, and establishing informal personal relationships between PRPs and researchers were also found to be important factors to enhance collaboration. 20 38 47 Regular feedback and discussions about the quality of collaboration, combined with ongoing adjustments to meet the needs and preferences of PRPs, were proposed in two papers. 34 45

Roles of a PRP coordinator

A PRP ‘coordinator’ was defined in some papers, as an individual or a role within a research team responsible for facilitating and supporting the collaboration between researchers and PRPs. 2 20 25 47 61 The presence of a PRP coordinator was reported or advised in 29% of the included articles. 2 3 19 28 34 35 40 42 44 48 61 PRP coordinators were reported to be helpful in facilitating effective communication among PRPs, researchers and stakeholders, aligning expectations, organising logistics, moderating group discussions, providing ongoing education and support, and assisting in the recruitment and selection of PRPs in projects ( table 5 ). 2 20 25 35 36 42 47 This role was reported to be taken by a member of the research team, a PRP or a designated person within a patient organisation or academic institution. 2 38

Potential roles of a PRP coordinator

Training of researchers

We found that 34% of the included articles included in the SLR reported or advised training or education of researchers. 4 7 19 21 25 28 29 38–40 44 Researchers could receive training concerning various aspects of working with PRPs ( table 6 ).

Reported training content for researchers and PRPs

Training of PRPs

Educating and training PRPs was proposed in many papers to enhance the quality of their collaboration with researchers. Notably, nearly half of the publications emphasised the importance of training, with 21% recommending it and 25% providing it. 25 28 29 35 37 45 51 62–64 PRP training and support included various aspects ( table 6 ). Training of PRPs was reported to foster well-prepared and empowered PRPs ready to engage effectively in research collaborations. 22 26 29

Evaluation and monitoring related to PRP involvement

Around 21% of the included publications recommended or reported some form of evaluation, 3 4 15 19–21 25 34 35 with 28% collecting feedback from PRPs on their involvement. Regular discussions and evaluations of the quality and impact of PRPs’ collaboration and contributions were reported to enhance understanding, satisfaction and impact, allowing for adjustments and improvements as needed. 4 5 37 60 Some tools were reported for monitoring such as the Patient-Centered Outcomes Research Institute conceptual framework, an evaluative framework for research engagement, 19 surveys to evaluate the impact of PRPs in the project, 3 26 the Public Involvement Impact Assessment Framework Guidance, 53 and the Guidance for Reporting Involvement of Patients and the Public. 25 34

Recognition, compensation and acknowledgement of PRPs

Recognising, compensating and acknowledging the contributions of PRPs during their involvement in a scientific project were reported to be essential components of equal and meaningful partnerships. 27

In the context of recognition, coauthorship was cited as proof of PRP involvement and equality in research collaborations. 5 39 The SLR revealed a growing trend in recognising PRPs through coauthorship in 68% of articles, 2–6 8 15 19–21 23–26 28 29 34 36–40 42 43 45 47–50 52 54 56 58 60 63 65 and acknowledgement in 45% of articles. 3 6 7 25 27 28 34 37 43–45 48 51 53 56–61 63 65–67

Compensation refers to the payment of salary, wages, honorarium, fees or allowances for the time commitment and expertise of PRPs; this is different from reimbursing PRPs for expenses (eg, travel expenses and accommodation). 49 Non-compensation for PRPs was reported as a limitation and challenge for their effective involvement. 4 While PRPs can opt out of payment, several papers reported that researchers should consider compensation in their budget planning. 2 39 49 Some articles advised that institutions should simplify processes for fair PRP payment, and funders should enable researchers to allocate resources for PRP involvement. 5

The role of PRPs in rheumatology research has significantly expanded over recent years. The findings of this SLR underscore the important roles and contributions of PRPs in research projects, and the added value of PRP involvement, not only in clinical research, but also in basic, translational, registry and longitudinal observational studies. This review also highlighted current challenges and barriers, and pulled together proposals of strategies to overcome them.

The exact definition and roles of PRPs remain unclear for some researchers. A wide proportion of the reviewed studies had adopted the 2011 EULAR definition of PRP which reflects the global acknowledgement of the importance of PRP involvement in rheumatology research and the need for specific recommendations. 14 PRPs hold a crucial position in recognising and actively integrating the patient perspective, their voice and needs into research decision-making processes. Diverse roles and activities were undertaken by PRPs in this SLR, from research partners to patient advocates, reflecting the many ways PRPs can contribute. Their involvement, as evident in recent papers shaping research priorities, guideline development, and scientific and clinical committees, suggests a trend towards more inclusive and patient-centred research practices.

Our review revealed specific barriers and challenges in communication, training, research processes and collaboration. These challenges highlight difficulties in communication and relationship dynamics during research, the necessity for training and support for both PRPs and researchers, concerns about the research process and its pace, and obstacles in PRP collaboration, including issues of recognition and diversity. Inclusivity and diversity are important topics for future research. To address these challenges effectively, targeted strategies such as fostering open communication, creating a supportive environment, ensuring early and sustained involvement, using a PRP coordinator and providing appropriate training and support for PRPs and researchers are crucial. These findings underscore the ongoing need for refining and implementing these strategies to enhance PRP involvement more efficiently. 26

A key observation from the SLR is the importance of early and sustained PRP involvement in research projects. Engaging PRPs from the research project’s inception ensures that research questions and methodologies are aligned with patients’ priorities and perspectives right from the start. Sustained involvement further reinforces the trust and collaboration between PRPs and researchers, leading to research outcomes that are more relevant and impactful. The OMERACT recommendations proposed that the level and timing of PRP involvement should vary based on the scope and type of project, emphasising adaptability as a key factor for successful involvement. 15

Evaluation and monitoring are also integral aspects of PRP involvement. This ongoing reflection and feedback process is vital for fostering effective and meaningful PRP involvement in research. Recognition, compensation and acknowledgement of PRPs stand as key elements for fostering a meaningful partnership. Coauthorship serves not only to document the PRP’s contribution but also reinforces the idea of collaborative research. Of note, we observed disparity between the involvement of PRPs in research activities versus their acknowledgement as coauthors. This disparity may arise from some PRPs not prioritising or desiring coauthorship, or being unable to participate in producing and writing a research paper due to health-related challenges such as disease flare-ups or fatigue. In ensuring equitable recognition, a collective effort is essential to guarantee that PRPs receive due acknowledgement and compensation for their valuable contributions to scientific research.

Our study has strengths and weaknesses. One important strength of this SLR is that the findings will equip researchers, healthcare professionals and other stakeholders with evidence-based solutions to improve PRP involvement in medical research. To this end, the findings have supported the process of updating the EULAR recommendations for PRP involvement and made them more evidence based. 1 Another strength is the obtention of a more nuanced understanding of the challenges and complexities surrounding PRP involvement in rheumatology research. Furthermore, our study stands out for its comprehensive approach, analysing a broad spectrum of study types, including quantitative and qualitative studies, reviews, opinion pieces and information from websites. The inclusion of various rheumatic musculoskeletal disease conditions, encompassing both paediatric and adult populations, enhances the robustness of our findings. Another notable strength lies in the co-production of this work by three PRPs. The project was initiated and led by a PRP (MdW) who gave the work direction, participated in article screening, article analysis, overall interpretation and manuscript writing. The two other PRPs brought important insights into PRP roles, facilitators and barriers.

A limitation of the study might be the heterogeneity of the included papers. Because of the expected limited reporting of PRP involvement in rheumatology research, we decided to include a diversity of papers in the SLR, varying from qualitative studies, case studies and original research papers to conference reports and opinion articles. This heterogeneity did not allow for any form of meta-analysis, nor for identifying themes that would benefit individual groups of PRPs such as people with rare diseases, children or young adults, or people with different cultural or ethnic backgrounds. Furthermore, quality assessments could not be uniformly applied across all study types. It is important to note that the traditional evidence hierarchy may not be applicable to this SLR, given the expected absence of randomised controlled studies. Despite this, certain papers were assessed to be of high quality of evidence within their respective study types. While the systematic approach ensured a comprehensive gathering of data, there might be relevant grey literature or non-English-language publications that were not included. Another limitation might be the time period of the last 6 years, including data from articles published between January 2017 and January 2023. This time frame was chosen to reflect studies performed after the 2011 EULAR recommendations were published, taking into account the implementation time gap. 14 Furthermore, the chosen time span resulted in 53 articles which was deemed sufficient for gathering relevant data related to our research questions.

In conclusion, this SLR identified numerous publications reporting on PRP involvement in rheumatology research. Most authors reported that PRP involvement not only enriches the research process but also ensures that research outcomes are more relevant, meaningful and patient centred. However, for this involvement to be genuinely effective, it is essential to address the barriers and challenges that PRPs and researchers are facing. By updating the EULAR 2011 recommendations, based on the findings of this SLR, we can look forward to a future where research is more inclusive, collaborative, and aligned with patient needs and perspectives.

Ethics statements

Patient consent for publication.

Not applicable.

Ethics approval

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Supplementary materials

Supplementary data.

This web only file has been produced by the BMJ Publishing Group from an electronic file supplied by the author(s) and has not been edited for content.

• Data supplement 1
• Data supplement 2

Handling editor Kimme L Hyrich

X @krystelaouad, @Stiddyo, @LGossec

Contributors All authors have contributed to this work and approved the final version. KA, MDW and LG accept full responsibility for the work and/or the conduct of thestudy, had access to the data and controlled the decision to publish.

Funding Funded by EULAR grant RES005.

Competing interests KA—research grant (EULAR grant RES005); over the last 3 years, research grants from UCB; consulting fees from Novartis. MdW—over the last 3 years, Stichting Tools has received fees for lectures or consultancy provided by MdW from UCB. ME—congress travel support from Janssen and AstraZeneca outside of the submitted work. DB—research grants from Novartis; speakers bureau from AbbVie, BMS, Galapagos, Janssen and Lilly; consulting fees from Pfizer, Sandoz and UCB. PS—speakers bureau from AstraZeneca; consulting fees from AbbVie; travel support from Janssen and Galapagos. LG—research grants from AbbVie, Biogen, Lilly, Novartis and UCB; consulting fees from AbbVie, Amgen, BMS, Celltrion, Galapagos, Janssen, Lilly, MSD, Novartis, Pfizer, Sandoz and UCB.

Patient and public involvement Patients and/or the public were involved in the design, or conduct, or reporting, or dissemination plans of this research. Refer to the Methods section for further details.

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Supplemental material This content has been supplied by the author(s). It has not been vetted by BMJ Publishing Group Limited (BMJ) and may not have been peer-reviewed. Any opinions or recommendations discussed are solely those of the author(s) and are not endorsed by BMJ. BMJ disclaims all liability and responsibility arising from any reliance placed on the content. Where the content includes any translated material, BMJ does not warrant the accuracy and reliability of the translations (including but not limited to local regulations, clinical guidelines, terminology, drug names and drug dosages), and is not responsible for any error and/or omissions arising from translation and adaptation or otherwise.

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• Published: 30 April 2024

Application of exercise therapy in patients with chronic kidney disease-induced muscle atrophy: a scoping review

• Jiawei Yin 1 ,
• Xiaotu Zhang 1 ,
• Zilin Wang 1 ,
• Zihan Qu 1 ,
• Xuefeng Sun 1 ,
• Yuqing Song 1 &
• Hongshi Zhang 1  

BMC Sports Science, Medicine and Rehabilitation volume  16 , Article number:  100 ( 2024 ) Cite this article

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The prevalence of muscle atrophy in patients suffering from chronic kidney disease (CKD) presents a significant challenge to healthcare providers, necessitating innovative approaches to management and care. Against this backdrop, this study embarks on a comprehensive review of literature concerning the application of exercise interventions in the nursing care of these patients. Such interventions are critical in addressing the debilitating effects of the condition, which include progressive loss of muscle mass and strength, adversely affecting patient mobility, quality of life, and overall survival. This review aims to identify the specific exercise modalities, contents, outcome indicators, and application effects associated with this intervention, in the context of the complex interplay of metabolic, inflammatory, and hormonal factors contributing to muscle wasting in CKD patients. By examining the efficacy of various exercise interventions, this study seeks to elucidate optimal strategies for mitigating the impact of CKD-induced muscle atrophy, thereby informing clinical practices and improving patient outcomes.

According to the method of a scoping review, nine databases (Cochrane, PubMed, EMBASE, Web of Science, ProQuest, Ovid, CNKI, Wanfang Data, and VIP) were searched until September 28, 2023. The included literature was screened, summarized, and analyzed.

A total of 20 pieces of literature were included. Some types include aerobic exercise, resistance exercise, and aerobic combined resistance exercise. The exercise intensity primarily falls within the mild to moderate range, with a recommended frequency of 2 − 3 times a week, lasting 30 − 60 min each time. The types of outcomes encompassed in this study include body composition, functional testing, strength measurements, laboratory examinations, cardiopulmonary function assessments, and patient-reported outcomes. To varying degrees, exercise intervention positively impacts the subjects' physical activity ability, body composition, and skeletal muscle status. Currently, resistance training is the primary type of intervention used for preventing and treating CKD patients induced by muscle atrophy.

Exercise intervention can improve muscle strength, physical function, and quality of life in patients with CKD muscle atrophy. Therefore, patients should be fully informed of the effect of exercise intervention in the treatment of chronic kidney disease-induced muscle atrophy in future, so as to promote the standardized implementation of exercise intervention.

Peer Review reports

Introduction

Chronic kidney disease (CKD) significantly impacts global health, with its management complicated by the high prevalence of sarcopenia, identified as a loss of skeletal muscle mass and strength. Duarte et al. [ 1 ] report a notable prevalence of sarcopenia in CKD patients at 24.5%, with a higher incidence observed in those undergoing dialysis. This condition, especially severe in 26.2% of dialysis patients compared to 3.0% in non-dialysis patients, underscores the critical need for early identification and intervention.Chronic Kidney Disease (CKD) is associated with various pathophysiological processes, including mitochondrial dysfunction. Studies reported that muscle atrophy in CKD patients is related to mitochondrial dysfunction [ 2 ]. Exercise intervention can reduce oxidative stress, increase mitochondrial capacity, and enhance mitochondrial function [ 3 ]. Furthermore, it is particularly noteworthy that research has identified the combination of moderate-intensity continuous exercise (MICE) with blood flow restriction (BFR) as effective in suppressing the appetite of obese adults through the promotion of Lac-Phe and ghrelin secretion [ 4 ]. This finding underscores the significant implications of exercise not only for CKD but also for diabetes management, highlighting its potential as a pivotal intervention in the treatment and prevention strategies for these conditions. Mitochondrial DNA copy number (mtDNA-CN) is considered a novel biomarker for CKD risk, as higher levels of mtDNA-CN are associated with a lower risk of CKD. This relationship is independent of traditional CKD risk factors, suggesting that mtDNA-CN could serve as an important indicator for identifying CKD progression and severity [ 5 ]. Furthermore, the onset of diabetes in patients with CKD is significant, as mitochondrial dysfunction can exacerbate metabolic dysregulation. The relationship between mitochondrial function and CKD highlights the potential for targeted interventions to improve mitochondrial health and manage CKD, particularly in the context of comorbid conditions like diabetes [ 6 ].

Recent meta-analyses showed that exercise training can increase cardiorespiratory endurance, improve muscle strength and muscle volume, reduce the risk of cardiovascular disease, delay the progression of CKD, and improve the quality of life of CKD patients [ 7 ]. Therefore, we should identify muscle atrophy in CKD patients as early as possible and implement the intervention. While patients diagnosed with CKD may participate in resistance training, flexibility exercises, and aerobic exercise [ 8 ], it is crucial to consider the diverse physical capacities and tolerances of subjects at various phases of CKD. Furthermore, the lack of clear evaluation criteria for the type, specific content, and outcomes of exercise interventions for CKD patients leads to significant heterogeneity in the type, frequency, and intensity of exercise interventions [ 7 ].

A scoping review assists researchers in elucidating their research inquiries, presenting the scope and extent of research involved, summarizing research results, identifying the limitations of existing research, and finding research progress in a particular knowledge field [ 9 ]. This study used a scoping review methodology to analyze and synthesize the application research on exercise intervention in chronic nephritis muscle atrophy. Our objective is to furnish medical and nursing professionals with a comprehensive summary of exercise intervention types, content elements, evaluation indices, and efficacy, serving as a valuable resource.

Research question

① What are the methods, intensity, frequency, and time of exercise intervention for chronic kidney disease-related muscle atrophy? ② How does the application affect the exercise intervention program?

Identifying relevant studies

Search strategy.

Using a computer, a search was conducted in the Cochrane Library, PubMed, EMBASE, Ovid, ProQuest, Web of Science, CNKI, Wanfang, and VIP. The search time was covered from the establishment of the database until September 2023. The search words were combined with the MeSH words and entry terms in the PubMed database, including the search terms "Sarcopenias," "Muscle wasting," "Muscle atrophy," "Diabetes Mellitus," "Diabetes Insipidus" "Diet, Diabetic," "Prediabetic State" "Exercise Therapy" "Remedial Exercise," "Rehabilitation Exercises," "Physical Activities," "Aerobic Exercise," "Acute Exercise," "Isometric Exercise," "Exercise Training." The relevant references were tracked and noted.

Inclusion criteria and Exclusion criteria

Inclusion criteria: (1) Population: The study subjects included CKD patients with muscle atrophy or sarcopenia. (2) Intervention:Articles that employ exercise therapy as an intervention strategy are included, covering a range of modalities such as aerobic training, resistance training, and stretching exercises. Studies focusing on traditional physical therapy interventions, such as electrostimulation, and pharmacological treatments are excluded from this review. (3) Comparison: The control arm was subjected to conventional care, incorporating stretching routines or engaging in physical activities quantitatively inferior to the normative daily exertion levels. (4) Outcomes: Muscle Mass, Muscle Strength, Physical Performance, Biochemical Markers, Functional Status were measured at baseline and post-exercise. (5) Study: The literature categories comprised a variety of original research encompassing randomized controlled trials, quantitative studies, qualitative studies, and mixed studies.

Exclusion criteria: Reviews, conference abstracts, thematic summaries, protocols, duplicate publications, and full-text articles that were not accessible were excluded from the analysis. Literature that missed any discussion of the specific details regarding the execution, substance, or impact of exercise intervention in individuals afflicted with muscular atrophy due to chronic renal illness was likewise disregarded.

Study selection

Two proficient researchers conducted an exhaustive review of titles and abstracts based on the inclusion and exclusion criteria, completing preliminary literature screening. Subsequently, they reviewed the complete text to make a final selection. The screening was conducted independently, and then the results were compared. The issue was referred to the research team for discussion in a dispute.Through open dialogue and a rigorous examination of the inclusion and exclusion criteria, we reconciled differing viewpoints and reached a mutually agreed-upon selection of literature. This process highlights the critical role of collaborative discussion and meticulous scrutiny in achieving a consensus on literature screening outcomes, ensuring the integrity and quality of our research methodology.

Data extraction and synthesis

Two researchers independently extracted all the information from the included literature as follows: (1) authors, research type, country and year of publication, and sample size; (2) intervention method, intervention content and type, intervention duration of the experimental group and the control group; (3) outcome indicators, and measurement content of outcome indicators.

Study characteristics

Following the initial retrieval of 875 articles, a final selection of 20 articles was made [ 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 ]. Figure 1 depicts the specific screening process. These included studies were conducted in China ( n  = 9), United Kingdom( n  = 3), Brazil ( n  = 2), Australia ( n  = 1), Germany ( n  = 1), Italy ( n  = 1), South Korea (= 1), Sweden ( n  = 1), and the USA ( n  = 1). Regarding the publication years of the studies included in this article, the distribution is as follows: 2023 ( n  = 3), 2022 ( n  = 4), 2021 ( n  = 4), 2019 ( n  = 3), 2018 ( n  = 1), 2016 ( n  = 1), 2015 ( n  = 1), 2014 ( n  = 2), and 2007 ( n  = 1). The distribution of the studies by year and geographic location is presented in the supplementary figure. Among the 20 pieces of literature included in this study, 14 literatures described RCTs [ 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 ] and six literatures are quantitative non-random studies [ 10 , 11 , 13 , 19 , 22 , 29 ]. Table 1 lists the characteristics of the included literature and is discussed below.

Types of Exercise Intervention Programs

The exercise intervention scheme mainly includes resistance exercise [ 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 ], aerobic exercise [ 23 , 29 ], and aerobic exercise combined with resistance exercise [ 13 , 15 , 22 , 24 ]. (1) Currently, resistance training is the most prevalent form of exercise intervention. The common resistance exercise intervention scheme involves strength training of the upper and lower limbs. Upper limb strength training includes biceps curl with dumbbells, elastic ball movement on the non-internal fistula side, pull-ups, and chest and shoulder compression. However, lower limb strength training focuses on the muscle groups around the knee joint, including lunge, squat, sitting leg raising, knee flexion and extension, and quadriceps strength training with elastic belts. Furthermore, there are resistance exercises for the hip joint, such as leg compression and hip flexion. (2) The aerobic exercise intervention scheme primarily consists of several modalities, such as bicycle modalities [ 11 , 16 ], treadmill exercise [ 13 ], Baduanjin [ 29 ], and steady-state walking [ 23 ]. In this context, Luca et al. [ 23 ] used steady-state walking as an intervention plan for aerobic exercise and examined the impact of this simple aerobic exercise on the subjects. (3) The intervention program mostly incorporates resistance exercise with low-to-moderate intensity aerobic exercise. The three most prevalent forms of aerobic exercise are cycling, treadmill, and rowing. Liming et al. [ 19 ] used psychological theory to guide patients in resistance exercise, helping patients develop healthy behaviors through the healthy action process orientation model.

Intensity, frequency, and duration of exercise intervention

The intervention intensity and frequency varied among the studies. (1) Exercise intensity: Eight studies [ 10 , 11 , 12 , 18 , 19 , 22 , 24 , 26 ] set the exercise intensity according to the scores of the subjects' perceived exertion (RPE). The intensity target of exercise training was established in two studies [ 14 , 26 ] based on the patient's capacity to tolerate the quantity of exercise. The training intensity progressively escalated until the patients reached the optimal tolerance level. Two studies [ 21 , 27 ] assessed the exercise intensity of the subjects based on maximum repetitions (RM) and the percentage of maximum loading (1RM) of the exercise load. Four studies [ 13 , 15 , 16 , 19 ] set the exercise intensity based on the subjects' cardiopulmonary function indexes. This was achieved by measuring the heart rate of the patients during exercise as a reference, setting the appropriate exercise intensity after evaluating the heart rate at the maximum exercise intensity they could tolerate. Zhang Bo et al. [ 13 ] divided the exercise training into low intensity (25% ~ 44% VO 2 max ) and moderate intensity (45% ~ 59% VO 2 max ) based on the subjects' peak oxygen uptake (VO 2 max ). There was a gradual transition from low to moderate intensity during the intervention. This. (2) Exercise frequency: The intervention frequency of each study varied from 2 to 3 times per week, while the duration of intervention was typically set based on the subjects' tolerance level and training objectives. (3) Duration of exercise intervention: There are a total of 13 studies in which the intervention period spans 12 weeks [ 11 , 12 , 13 , 15 , 16 , 17 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 29 ], two studies have an intervention duration of 24 weeks [ 14 , 19 ], three studies have an intervention duration of 12 months [ 10 , 18 , 28 ], and two studies have an intervention duration of 6 months [ 19 , 23 ].

Outcome indicators and measurement tools

Outcome indicators included functional tests, body composition, strength, laboratory examination, cardiopulmonary function, and strength, and functional evaluations. The principal physical activity assessments comprised a walk test, sit-to-stand test, functional reach, balance test, SPPB score, and physical activity recall. The measurement of body composition included weight, body mass index, body fat, and lean body mass. Skeletal muscle mass was assessed using dual-energy X-ray absorptiometry, anthropometry, and muscle ultrasound. Laboratory examination indicators included inflammatory markers, renal profile, and body fat. Cardiopulmonary function indicators examined forced expiratory volume in one second (FEV-1), vital capacity, forced vital capacity (FVC), maximum inspiratory pressure (MIP), and peak oxygen uptake (VO 2 max ). In terms of patient-reported outcomes, the primary measurement tools encompassed the Quality-of-Life Questionnaire, Pain Scale, Leicester Uremic Symptom Score, Sarcopenia Quality of Life Scale, Activities of Daily Living Scale, Barthel Index, Kidney Disease Quality of Life Scale, Dialysis Patient Quality of Life Scale, and Revised Piper Fatigue Scale.

Effects of Exercise Intervention

(1) Effects of resistance exercise intervention: The exercise intervention schemes for muscle atrophy in CKD patients in the literature included different levels of resistance exercise. A total of ten studies have documented that resistance training protocols yielded a positive impact on several aspects of the subjects' physical function [ 10 , 11 , 13 , 16 , 29 ], muscle strength [ 10 , 13 , 16 , 22 , 26 , 27 , 28 , 29 ], daily living ability [ 10 , 13 , 27 ] and renal function [ 19 , 22 ]. Danielle and colleagues [ 12 ] found that a 12-week program of high-intensity progressive resistance exercise training (PRET) for hemodialysis patients led to a statistically and clinically significant increase in thigh muscle volume, with a mean difference of 193 cm 3 (95% CI: 63 to 324 cm 3 ; P  = 0.007) compared to the control group. However, it did not significantly enhance the subjects' physical activity capacity or overall quality of life. (2) Aerobic exercise intervention effect: Eleven studies [ 11 , 13 , 15 , 16 , 22 , 23 , 24 , 25 , 26 , 27 , 28 ] reported the effects of different levels of aerobic exercise on subjects. Since low-intensity aerobic exercise was difficult to improve muscle strength [ 30 ] significantly, it was mostly used as an exercise intervention program for the control group [ 11 , 13 , 16 , 22 , 23 , 26 , 27 , 28 ]. Furthermore, other studies showed positive impacts of aerobic exercise training interventions on the subjects. For example, Wu Qian et al. [ 29 ] found significant improvements in the intervention group's handgrip strength, daily walking speed, and physical activity level compared to the control group, with the revised Piper fatigue scale (RPFS) scores also being significantly lower (indicating less fatigue) in the intervention group. These differences were statistically significant ( P  < 0.05). (3) Effects of aerobic exercise combined with resistance exercise intervention: Four literatures [ 13 , 15 , 22 , 24 ] incorporated a combination of aerobic exercise and resistance training in their physical exercise intervention program. Emma et al. [ 24 ] showed that combined resistance and aerobic training were more beneficial than aerobic training alone. Zhang Bo et al. [ 13 ] demonstrated that after 12 weeks of low and medium intensity aerobic combined with resistance exercise, elderly patients with chronic kidney disease complicated with sarcopenia showed significant improvements in appendicular skeletal muscle mass index (ASMI), grip strength index, 6-m walking speed, peak oxygen uptake (VO 2 max ), one-leg standing time, and reach in seat distance, with p-values indicating statistical significance ( P  < 0.05) for these improvements. This indicates that the exercise intervention was effective in enhancing muscle mass, muscle strength, and motor function without adversely affecting renal function.

This study demonstrates that exercise intervention for muscle atrophy in individuals with CKD includes different modalities, mostly resistance training and a combination of aerobic exercise. Guiding patients regarding resistance training can facilitate the anabolism and metabolism of skeletal muscle, enhancing muscle quality, improving muscle strength, and effectively enhancing patients' overall quality of life [ 31 ]. Exercise intervention schemes for CKD patients are often carried out in various forms of exercise combinations. Researchers often use aerobic exercise as an important auxiliary intervention method that benefits cardiovascular health. It improves the heart and lung function of the subjects, enhances their endurance, and facilitates the development of resistance training.

There are significant differences among CKD patients at different stages, so we should comprehensively evaluate the physiological function and tolerance of CKD patients and develop individualized exercise intervention programs for patients [ 7 ]. This study showed that the exercise intervention program includes detailed pre-exercise examinations, such as preliminarily assessing the subjects’ muscle strength and cardiopulmonary function status, determining whether they can tolerate exercise intervention, and developing personalized exercise prescriptions. Jonathan [ 15 ] and colleagues completed detailed medical examinations before the intervention, measuring the subjects' cardiopulmonary function and upper and lower limb strength. Results indicated that the design of the exercise intervention program was personalized. This study showed that the exercise form of CKD patients is not a single repeat of a certain type of exercise but a combination of various exercise methods that can achieve the best effect. Despite numerous RCTs reported that resistance exercise could significantly improve muscle strength in patients with CKD [ 32 ], a meta-analysis [ 33 ] has revealed that the use of progressive resistance exercise as the sole intervention did not result in a significant improvement in the 6-min walk test, which serves as a proxy indicator of cardiopulmonary function. It indicates that resistance training alone makes it difficult to improve the cardiopulmonary function of subjects significantly. As resistance exercise training necessitates a minimum level of cardiopulmonary function as a foundation, it also imposes specific cardiopulmonary function requirements on the subjects. Low-intensity, long-duration aerobic exercise has been shown to enhance the cardiopulmonary function and exercise capacity of patients with chronic kidney disease [ 34 ]. Therefore, clinical researchers frequently combine two forms of exercise to enhance patients' quality of life by enhancing their cardiopulmonary function and muscle strength. It demonstrates the comprehensive nature of exercise intervention program design. Universality: For patients with non-maintenance dialysis CKD, regular visits to the hospital for dialysis treatment means that they cannot receive the supervision of clinical researchers throughout the process. They also face problems such as a lack of exercise equipment and professional guidance. Emma et al. [ 24 ] found that patients considered frequent visits to the hospital as a major obstacle to participating in the study. So, future research needs to design simple and feasible home training programs for different patient situations and provide feedback when patients come to the hospital for dialysis. Luca’s [ 23 ] exercise training program was designed and organized by the hospital and ultimately completed by patients at home. It improved patients' physical function significantly after six months of intervention with low-intensity exercise.

The results of this study show that exercise intervention can improve physical function, activity ability, muscle atrophy, and quality of life in patients with CKD. This viewpoint is also supported by Vanden et al.'s systematic review of exercise intervention in CKD patients [ 35 ]. Furthermore, a cross-sectional study [ 36 ] also suggested that clinical medical staff should actively implement exercise interventions for CKD patients to address adverse outcomes such as muscle atrophy and decreased quality of life caused by long-term catabolism and dialysis treatment. Although exercise intervention positively impacts CKD patients, the current studies are mainly small-sample randomized controlled trials with low quality. Therefore, larger-sample randomized controlled trials are needed to evaluate the impact of exercise intervention on muscle atrophy in CKD patients more accurately. Notably, the frequency and duration of exercise intervention are not positively correlated with the intervention effect. Excessive exercise intensity may lead to lactic acidosis in patients, and severe cases may cause acute renal failure secondary to rhabdomyolysis [ 37 ]. Therefore, exercise frequency must be determined by the patient's condition; high-intensity training should not be pursued indiscriminately. Furthermore, cardiac function in CKD patients must be matched with the intensity of exercise. A systematic review have shown that high-intensity interval exercise with passive recovery leads to a greater increase in cardiac troponin T levels compared to moderate-intensity continuous exercise [ 38 ]. To minimize cardiovascular risk during exercise, exercise prescriptions should be tailored based on individual circumstances.A unified standard regarding the exercise frequency and intervention cycle of CKD patients does not exist. The guidelines indicate that the optimal resistance exercise frequency is 2 − 3 times per week, with an interval of 24 − 48 h each time, which is consistent with the results shown in this study [ 39 ]. In the literature included in this study, only seven articles [ 12 , 16 , 17 , 19 , 25 , 26 , 29 ] mentioned the safety of exercise intervention, and 3 of them [ 12 , 17 , 19 ] reported adverse events. Danielle et al. [ 12 ] reported muscle soreness and laceration of the back wound, indicating that there is a lack of comprehensive reports regarding the safety of exercise intervention trials in CKD patients. Some researchers failed to fully consider the safety of exercise intervention based on individual differences among subjects.

Integrating insights from the studies on exercise's impact on CKD, sarcopenia in hemodialysis patients, and diabetes management through physical activity, the future perspective emphasizes a holistic approach to CKD treatment. This approach advocates for the incorporation of tailored exercise programs to address the multifaceted challenges of CKD. A study elucidates how diverse exercise modalities can significantly impact blood glucose levels, emphasizing the importance of exercise in the management of diabetes, particularly in patients with CKD. It underscores the role of physical activity in enhancing insulin sensitivity and glycemic control, offering a non-pharmacological strategy to mitigate diabetes-related complications in CKD patients [ 40 ]. Additionally, exercise has beneficial effects on mitochondrial function and diabetes management. The research emphasizes that various exercise modalities, such as resistance and endurance training, can enhance mitochondrial density, dynamics, and oxidative capacity in muscle tissues. These improvements aid in boosting glucose metabolism and insulin sensitivity, which are crucial for diabetes control. The findings indicate that structured exercise programs are an indispensable component in addressing mitochondrial dysfunction and managing diabetes, especially in populations with chronic conditions like CKD [ 41 ]. Moreover, research has identified the efficacy of exercise in alleviating fatigue among hemodialysis patients, highlighting the potential of structured exercise programs to enhance energy levels and overall health in this demographic, regular physical activity could be an essential component of comprehensive care for hemodialysis patients, thus improving their quality of life and physical function [ 42 ]. The envisioned future includes rigorous clinical trials to further validate exercise as a key adjunct therapy, potentially leading to standardized exercise recommendations that enhance patient outcomes and quality of life in CKD management.

Strengths and limitations

The strengths of this scoping review include an extensive and repeated search of the literature to capture all relevant articles and limit emissions while strictly adhering to the PRISMA reporting guidelines. Unpublished dissertations are more likely to report uncertain or negative results, which may produce a reverse publication bias and be excluded from this study. The literature included in this study lacks large-sample, multi-center, randomized controlled studies, and the methodological rigor and data quality of the included studies have not been systematically evaluated.

Patients with CKD face many complex issues that seriously affect their quality of life. For them, long-term dialysis treatment has produced negligible side effects, and exercise therapy is needed to improve their quality of life. Exercise intervention can improve the muscle strength, physical function, and quality of life of patients with chronic nephritis muscle atrophy, which is an effective intervention method. We are still waiting to find its potential positive effects on CKD patients. In future nursing work, we should evaluate the physiological function status of CKD patients, formulate safe and effective exercise intervention plans, and intervene early in their muscle atrophy symptoms to promote their rehabilitation and improve their clinical outcomes.

Availability of data and materials

All data generated or analysed during this study are included in supplementary information files.

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The literature review and inclusion criteria were collaboratively developed by all authors, while Jiawei Yin and Xiaotu Zhang conducted the systematic literature search and screening. Data extraction from relevant literature was performed solely by Jiawei Yin. Following data extraction, Jiawei Yin composed the manuscript. Zilin Wang managed the data. Yuqing Song, Zihan Qu, and Xuefeng Sun undertook data collection and analysis, as well as provided suggestions. The final draft was reviewed and approved by Hongshi Zhang.

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Yin, J., Zhang, X., Wang, Z. et al. Application of exercise therapy in patients with chronic kidney disease-induced muscle atrophy: a scoping review. BMC Sports Sci Med Rehabil 16 , 100 (2024). https://doi.org/10.1186/s13102-024-00876-8

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Article Contents

Introduction, acknowledgments, supporting information, mapping the evidence of novel plant-based foods: a systematic review of nutritional, health, and environmental impacts in high-income countries.

• Article contents
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Sarah Nájera Espinosa, Genevieve Hadida, Anne Jelmar Sietsma, Carmelia Alae-Carew, Grace Turner, Rosemary Green, Silvia Pastorino, Roberto Picetti, Pauline Scheelbeek, Mapping the evidence of novel plant-based foods: a systematic review of nutritional, health, and environmental impacts in high-income countries, Nutrition Reviews , 2024;, nuae031, https://doi.org/10.1093/nutrit/nuae031

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Shifting from current dietary patterns to diets rich in plant-based (PB) foods and lower in animal-based foods (ABFs) is generally regarded as a suitable strategy to improve nutritional health and reduce environmental impacts. Despite the recent growth in supply of and demand for novel plant-based foods (NPBFs), a comprehensive overview is lacking.

This review provides a synthesis of available evidence, highlights challenges, and informs public health and environmental strategies for purposeful political decision-making by systematically searching, analyzing, and summarizing the available literature.

Five peer-reviewed databases and grey literature sources were rigorously searched for publications.

Study characteristics meeting the inclusion criteria regarding NPBF nutrient composition and health and environmental outcomes in high-income countries were extracted.

Fifty-seven peer-reviewed and 36 grey literature sources were identified; these were published in 2016–2022. NPBFs typically have substantially lower environmental impacts than ABFs, but the nutritional contents are complex and vary considerably across brands, product type, and main primary ingredient. In the limited evidence on the health impacts, shifts from ABFs to PB meats were associated with positive health outcomes. However, results were mixed for PB drinks, with links to micronutrient deficiencies.

If carefully selected, certain NPBFs have the potential to be healthier and nutrient-rich alternatives to ABFs and typically have smaller environmental footprints. More disaggregated categorization of various types of NPBFs would be a helpful step in guiding consumers and key stakeholders to make informed decisions. To enable informed policymaking on the inclusion of NPBFs in dietary transitions as part of a wider net-zero and health strategy, future priorities should include nutritional food standards, labelling, and subdivisions or categorizations of NPBFs, as well as short- and long-term health studies evaluating dietary shifts from ABFs to NPBFs and standardized environmental impact assessments, ideally from independent funders.

The fragile interconnection between food systems and the environment is increasingly evident. 1–3 While current agricultural practices are damaging the environment, environmental change is putting food supplies at risk of disruption if timely adaptation strategies are not used. 4–8 This relationship exists at a time when food systems are already struggling to provide healthy diets for all, with many populations experiencing a coexistence of undernutrition and obesity. 1 , 3

Structural changes in food systems are critical to both safeguard people’s health and accomplish the climate adaptation and mitigation commitments mentioned in The United Nations Framework Convention on Climate Change 9 and the United Nations’ Sustainable Development Goals. 10 While production-side strategies can contribute toward climate mitigation, substantial opportunities for further emission reductions and acceleration toward net-zero targets can be achieved through dietary changes and the resulting lower demand for foods with a large environmental footprint.

In food-secure and high-income settings, a shift from “conventional diets” (which typically contain high amounts of animal-based foods [ABFs]) to predominantly plant-based (PB) diets could improve population and planetary health. 2 , 11 Dietary change has many obstacles, with diets influenced by many factors 12 , 13 that act as barriers to increasing consumption of minimally processed PB foods (eg, legumes, vegetables). If common barriers are removed, such as the need for additional cooking skills, major changes in taste and appearance of commonly consumed dishes, and fear of social stigma, 14 , 15 novel plant-based foods (NPBFs), products designed to mimic and replace ABFs to allow easy incorporation into habitual diets (eg, vegan and vegetarian meat and dairy) (see Box 1 ), may offer an easier option to facilitate this shift.

In recent years, the NPBF landscape has expanded rapidly. Several new types of NPBFs (eg, PB drinks, yogurts, eggs, meats) were introduced to the market, and trends showed increasing sales, volume, and investment growth across many countries. 16–21 In 2023, data suggested a possible slowdown, especially for PB meats, with some consumers criticizing their cost and taste, 22 and some NPBF manufacturers reporting net losses. 23 , 24 However, sales of supermarkets’ own-label PB meat alternatives have seen growth, 23 alongside consistent increases in sales of PB dairy and eggs 25 (see Supplementary file 1, section 1.1, in the Supporting Information online for detailed information on costs).

According to a global survey focusing on individuals following vegan or vegetarian diets most or all of the time, 22.0% of consumers reported adhering to a meat-free diet, and there is growing interest in embracing PB eating, with approximately 42.0% of consumers anticipating that PB foods will replace most meat within a decade. 26 With consumption of NPBFs in the United Kingdom doubling between 2008 and 2019, particularly among women and younger generations, and the fact that in 2022, 60.0% of US households purchased at least 1 type of NPBF, verification of any health and sustainability claims in marketed products is of vital importance. 22 , 27 , 28 Currently, various NPBFs are advertised as potential dietary “game changers,” with claims that they would play an important and positive role in sustainability and health, 29 , 30 and, thus, could play a pivotal role in the so-called consumption corridors. 31 However, because of their novelty, some consumers question these positive claims. 32 Although NPBFs are generally regarded as a low-carbon alternative to ABFs, their nutrient and health profiles remain largely unknown and are often criticized. This is primarily related to concerns regarding micronutrient and protein content, along with higher content of saturated fats and sodium in comparison to ABFs, and level of processing. 33 , 34

Previous reviews have primarily focused on single aspects of NPBFs 17 , 19 , 22 , 25 , 29 , 34–46 or ingredients of NPBFs 39 , 47–50 ; a few recent reviews explored the positive health and environmental outcomes of consuming selected NPBFs. 51–53 However, research quantifying the potential impacts of NPBFs is still in its infancy, and an overview that is both systematic and comprehensive, comprising health, nutrient, and environmental outcomes from peer-reviewed and grey literature of different types of NPBFs, does not yet exist, to our knowledge. This lack makes it difficult for policy makers and consumers to assess the trade-offs between nutrient composition and the environmental and health impacts of NPBFs, and hinders the potential inclusion of NPBFs in sustainable and healthy dietary recommendations.

To synthesize available evidence, highlight challenges, inform public health and environmental strategies, and inform purposeful political decision-making, we aimed, in this study, to systematically search, analyze, and summarize the available grey and peer-reviewed literature on the nutrient composition, environmental footprints, and health effects of NPBFs sold and consumed in high-income countries, and to quantify and summarize their reported results.

The full-study protocol we followed is published elsewhere (see Nájera Espinosa et al 54 and Supplementary file 1, section 2, in the Supporting Information online for more details on the methods). Briefly, a systematic search was performed to identify peer-reviewed journal articles and grey literature that contained data on the nutrient composition, health impacts, and environmental impacts of NPBFs. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were followed. 55

Peer-reviewed literature

Five scientific databases were systematically searched (MEDLINE, Embase, Global Health, GreenFILE, and the Web of Science Core Collection) on August 29, 2021; we conducted an updated search on June 29, 2022. The search was limited to articles published and accepted after January 2016 until June 29, 2022, because of the substantial growth in supply and demand of NPBFs in the past 7 years. 16–19 In addition to database searching, citation lists from identified systematic literature reviews were handsearched (see Supplementary file 1, section 2.6, in the Supporting Information online for the full search strategy). After the quality criteria were applied (described in Supplementary file 1 , Table S1 in the Supporting Information online ), titles were manually and triple screened. Abstracts were manually double screened after application of a supervised machine-learning algorithm (ie, a support vector machine 56 ) through Scikit Learn 57 that ranked and highlighted likely relevant articles (ie, conducted priority screening). This approach is described elsewhere in detail (see Supplementary file 1, section 2.1, in the Supporting Information online ). 58 Full texts were manually screened by 2 authors and data were also double extracted.

Grey literature

To capture grey literature in a systematic way, a manual search was conducted on Google (see Supplementary file 1, section 2.3 , and Table S3 in the Supporting Information online ). Text from the webpages was then scraped and a state-of-the-art, pretrained language model from Hugging Face 59 was used to create a summary of each web link. Results were exported into a comma-separated value, or CSV, file. Additionally, a manual search in Google of relevant websites from the top NPBF producers in the United Kingdom and United States was conducted. 60–63 And literature from relevant websites that promote NPBFs, such as the Good Food Institute and Green Queen, were searched and screened manually (see Supplementary file 1, section 2.3 , and Tables S4 and S5 in the Supporting Information online ).

Data analysis, categorization, and key definitions: nutrient, health, and environmental outcomes

The PICO (population, intervention, comparison, and outcome) criteria are defined in Table 1 (see Supplementary file 1, Table S1 in the Supporting Information online for a detailed list of the inclusion and exclusion criteria). Main study characteristics and nutrient, health, and environmental outcomes were extracted (see Supplementary files 1 and 3 in the Supporting Information online for more details).

PICO criteria for inclusion of studies

PB drinks and milk reported in 100 ml of product.

NPBFs and their ABF counterparts were categorized into food groups on the basis of their primary ingredient ( Table 2 ). See Supplementary file 1, sections 2.4 and 2.5, in the Supporting Information online for more details on the selection of nutrients, data analysis assumptions, and ABF baseline comparators). The following terms for each NPBF type are used in this review:

Food groups for novel plant-based foods and animal-based foods and their respective reported main primary ingredient

For the purposes of this review, peanuts were included in the Nuts and Seeds group because they are typically consumed as such.

Blended or mixed products, if reported, the first ingredient was taken as the primary ingredient. For example, soy & almond PB drinks were labelled as legumes.

If a product did not report any ingredients, they were categorised as unknown.

PB meat products or alternatives: include different types of PB meats (eg, PB chicken, sausages, mincemeat), categories (eg, mycoprotein, legumes), and brands

PB drink products or alternatives: include different PB drink categories (eg, legumes, nuts, seeds) and brands

PB yogurt products or alternatives: include different PB yogurt categories (eg, legumes, coconut) and brands

PB cheese products or alternatives: include different types of PB cheese categories (eg, coconut, nuts, seeds) and brands

PB egg products or alternatives: include different types of PB egg categories and brands

Mention of PB products (without further specification) refers to all the listed product subcategories mentioned, except for PB eggs.

Assessment of robustness and relevance

A modified version of the Critical Appraisal Skills Program checklist for randomized controlled trials 64 was adapted to assess robustness and relevance of the studies in the full-text reviewing stage. The modifications involved the exclusion of the randomization, blinding, and cost-effectiveness criteria on the Critical Appraisal Skills Program checklist, and funding source was added as a criterion. Studies were assessed by 4 reviewers (G.H., R.P., S.P., and S.N.E.). Studies were assessed as follows: (1) clear description of the study design, (2) appropriate comparison group, (3) clear description of the methods, (4) rigorous and clearly described analysis, (5) funding source, and (6) precision of measure of effect. Studies with a minimum score of 1 were included, and sensitivity analysis was performed by funding source (see Supplementary file 1, section 2.2, in the Supporting Information online for more details).

Fruit, vegetable, legume, and nut content in novel plant-based foods

In addition to the review component, a cross-sectional analysis was conducted to examine the total fruit, vegetable, legume, and nut content (percentage estimate) of each type of NPBF sold in the United Kingdom. For this, a time-stamped data set of observations from UK supermarkets generated by FoodDB in October 2021 was used. Details are described elsewhere 65 and in Supplementary file 1, section 2.7, in the Supporting Information online . Detailed data at the global level are not available to date; hence, this part of the analysis is limited to the United Kingdom only.

Sensitivity analysis

A common concern about studies on the health impacts and environmental sustainability of NPBFs is that they can be funded by the industry that produces them; hence, we conducted a sensitivity analysis by funding source. Furthermore, given that relative improvements in health and environmental sustainability depend on the baseline comparator used ( Supplementary file 1, section 2.2, in the Supporting Information online ), the sensitivity analysis based on the main primary ingredient of a given NPBF and its respective ABF comparator was also performed. The Wilcoxon test for sensitivity analysis with a significance level set at P  ≤ 0.05 was used.

Systematic search results

A total of 49 563 peer-reviewed and 891 grey literature records were identified from the initial search. After unique literature sources were screened, 57 peer-reviewed articles and 36 grey literature studies met the inclusion criteria ( Figure 1 ). Supplementary 1, section 2, in the Supporting Information online provides further details on the screening process. The study characteristics that were extracted included basic study details (eg, authors, year, type of study, country, number of participants, follow-up period), relevant macro- and micronutrient content (eg, those related to common deficiencies, such as iron, calcium, vitamin B 12 ), health and health proxy data (eg, obesity, micronutrient status, risk factors related to noncommunicable diseases), and environmental variables (eg, carbon, water, and land-use data).

Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) flowchart of systematic review process reporting nutrient composition, and environmental and health outcomes of novel plant-based products in high-income countries . Abbreviations : IPCC, Intergovernmental Panel on Climate Change.

Nutrient composition of novel plant-based foods

The nutrient content of NPBFs was the most frequently studied outcome (n = 56 studies). Nutrient data were typically collected through supermarket cross-sectional surveys or manufacturers’ websites. PB meat alternatives (n = 35) and PB drink alternatives (n = 19) were most frequently reported; fewer studies researched PB cheese (n = 5) and yogurt alternatives (n = 4). No studies were found that assessed PB egg alternatives. The nutritional profile of NPBFs varied greatly by manufacturing process, including the main base ingredient (eg, soy, almond); the processing techniques, time, and temperature applied; and the type of product manufactured (ie, PB drinks, PB meats). 39 , 40 , 66 , 67

Energy density, saturated fat, fiber, sugar, sodium, and micronutrient content of plant-based meat alternatives

The 35 publications evaluating PB meat alternatives reported on 508 PB meat products with 66 ABF comparators. Where the median values for meat comparators were reported to be 221.0 kcal/100 g (interquartile range [IQR], 186.6–246.7), 5.7 g/100 g saturated fat (IQR, 3.2–7.1), and very low fiber (<0.1 g/100 g; IQR, 0.0–0.5), most meat-alternative groups were reported to have lower energy density, lower saturated-fat content, and more fiber ( Figure 2 and Supplementary file 2: Table S1 for detailed macronutrient information disaggregated by main ingredient). Mycoprotein-based meat alternatives were reported to be the least energy dense, with a median energy value of 123.0 kcal/100 g (IQR, 94.0–198.5; with ABFs, P value of difference [ P d ] < 0.001), whereas meat alternatives based on cereals and grain had the highest energy density of all PB meats (226.0 kcal/100 g [IQR, 189.8–268.5]; P d < 0.360), with values very similar to those of meat and poultry. Mycoprotein-based meats were also reported to be lowest in saturated fat (0.8 g/100 g [IQR, 0.5–1.3]; P d < 0.001), whereas nut- and seed-based meats had the highest saturated fat content (1.4 g/100 g [IQR, 1.1–1.7]; P d = 0.003) of all PB meats, which still was significantly lower than saturated fat content in meat and poultry. Finally, mycoprotein-based meat was reported to contain the highest fiber content (median, 6.0 g/100 g [IQR, 5.2–7.1]; P d  < 0.001), whereas cereal- and grain-based meats had the lowest fiber content of all PB meats (3.1 g/100 g [IQR, 2.3–3.9]; P d < 0.001), which still was significantly higher than in meat and poultry.

Macronutrient, sodium, and energy content in plant-based meat and drink alternatives in their respective food group based on main primary ingredient   (ie, predominant or core food item on the ingredient list) compared with meat and poultry, and dairy, respectively . Data were limited to raw products only. Abbreviation : M, median of each category.

Meat and poultry contained a median of 0.5 g/100 g total sugar (IQR, 0.0–0.9) and 426.7 mg/100 g sodium content (IQR, 101.0–672.8). All PB meats contained more total sugar but had similar levels of sodium in comparison with meat and poultry. Mycoprotein-based meats had the lowest total sugar content of all PB meats (median, 0.8 g/100 g [IQR: 0.5–1.8]; P d < 0.001], and nut- and seed-based meats contained the highest total sugar amount (median, 4.2 g/100 g [IQR, 2.3–6.6]; P d = 0.002); both showed strong evidence of being higher in total sugar content than meat and poultry. This is equivalent to 0.4 g and 3.4 g of total sugar/80.0 g serving size, or, if these sugars are considered free, 1.6% and 13.4% of the maximum recommended approximately 25.0 g average daily sugar intake. 68 Finally, the median sodium values for all PB meat groups did not show strong evidence of a difference from meat and poultry, except for legume-based meats (median, 520.0 mg/100 g [IQR, 400.0–636.0]; P d = 0.011). This is equivalent to 416.0 mg of sodium (or 1.0 g of salt) per 80.0 g serving size, or 20.8% of the maximum recommended 5.0 g average daily salt intake. Moreover, there were extreme outliers, with some PB meats reported to contain more than 1400.00 mg sodium (equivalent to 2.8 g salt) per 80.0 g; thus, consumption of 1 portion of this PB meat alternatives is more than half the recommended maximum daily intake of salt. 69

Only a few studies (n = 9) evaluated micronutrient data; these reported on 250 PB meat products and 24 ABF comparators. Micronutrient content ranged vastly across all groups: whereas some products would provide substantial contributions to average daily requirements, others were much less nutritious ( Table 3 and Supplementary file 2: Table S2 ). 69–83 For example, the median iron content for cereal- and grain-based PB meats (5.4 mg/100 g [IQR, 4.2–5.4]) was higher than the median of meat and poultry (1.3 mg/100 g [IQR, 1.1–1.6]). On the contrary, vitamin B 12 levels were lower for PB meat alternatives (medians ranged from 0.1 μg/100 g [IQR: 0.0–0.9] to 0.3 μg/100 g [IQR: 0.3–0.3]) as compared with 1.2 μg/100 g (IQR: 0.6–1.6) in meat and poultry. However, certain individual products had a comparable or higher vitamin B 12 content than their ABF comparator.

Summarized micronutrient values for PB meat and drinks and animal-based foods a

Values are compared with global average daily requirements (see Supplementary file 2 in the Supporting Information online for detailed information containing all disaggregated numbers by main ingredient of each novel plant-based food and animal-based foods). The table only reports micronutrients commonly found in meat and dairy. PB products also provided other micronutrients not commonly found in meat and dairy (ie, calcium in PB meats).

Abbreviations : ADR, average daily requirement; max, maximum; min, minimum; IQR, interquartile range; PB, plant-based.

No studies reported nutrient data from organic products. Although protein levels were not the main focus of this study, protein results are reported in Supplementary file 1: Figure S2 and Supplementary file 2: Table S1 , and show that, particularly, legume- and mycoprotein-based PB meats typically match meat and poultry in protein content.

Energy density, saturated fat, fiber, sugar, sodium, and micronutrient content of plant-based drinks

The 19 studies evaluating PB drinks reported on 397 PB drinks (unflavored and unsweetened) and 52 dairy milk products. Where dairy milk comparators were reported to contain median values of 50.1 kcal/100 mL energy density (IQR, 39.3–63.0), 1.1 g/100 mL saturated fat (IQR, 0.9–2.2), and no fiber (0.0 g/100 mL; IQR, 0.0–0.0), most PB drink groups were reported to have lower energy density, lower saturated fat content, and more fiber ( Figure 2 and Supplementary file 2: Table S1 ). Coconut-based drinks were reported to be the least energy dense (median energy value, 20.0 kcal/100 g [IQR: 19.0–33.7]; P d < 0.001), whereas drinks based on cereals and grains had the highest energy density of all PB drinks (median, 59.0 kcal/100 mL [IQR: 43.0–57.0]; P d = 0.566) but not significantly higher than dairy milks. PB drinks made of cereals and grains, fruits and vegetables, and nuts and seeds were reported to be lowest in saturated fat (median, 0.2 g/100 mL; IQRs, 0.1–0.2, 0.2–0.2, and 0.1–0.3, respectively; P d < 0.001), whereas coconut-based drinks had the highest saturated fat content (median, 1.1 g/100 mL; IQR, 0.9–1.7; P d = 0.952) of all PB drinks, but this was not significantly different than dairy milks. All PB drinks contained more fiber than dairy milks; however, only the drinks based on cereals and grains, legumes, and nuts and seeds were significantly higher in fiber when compared with dairy milks (for cereals and grains, and for legumes: median, 0.5 g/100 mL [IQRs, 0.2–0.8 and 0.2–0.6, respectively]; and for nuts and seeds, 0.3 g/100 mL [IQR, 0.3–0.5]; P d < 0.001).

Dairy milks contained a median of 4.7 g/100 mL total sugar (IQR, 4.3–5.0) and 39.1 mg/100 mL sodium (IQR, 33.6–43.3). Most PB drinks contained less total sugar than did dairy milks, but they had similar levels of sodium. However, the total sugar content was only significantly lower for coconut (median, 1.9 g/100 mL; IQR, 1.5–2.5), legumes (median, 1.9 g/100 mL; IQR, 0.5–2.6), and nut- and seed-based drinks (median, 2.4 g/100 mL; IQR, 0.2–3.3) when compared with dairy milks ( P d < 0.001). This is equivalent to 3.8 g and 4.8 g of total sugar/200.0 mL serving size, or, if these sugars are considered free, 15.2% and 19.2% of the maximum recommended 25.0 g average daily sugar intake. 68 The only PB drink group that was statistically different in sodium content compared with dairy milks was the group based on nuts and seeds (median, 47.2 mg/100 mL [IQR, 34.0–60.0]; P d = 0.032). This is equivalent to 94.4 mg of sodium (0.2 g of salt) per 200.0 mL serving size, or 4.0% of the maximum recommended 5.0 g average daily salt intake. 69 However, there were also some extreme outliers, some of which reported containing more than 3 times this amount of sodium per 200.0 mL, the equivalent of approximately 12.0% of the daily World Health Organization recommendation. 69

A few studies (n = 16) evaluated micronutrient data of PB drinks, reporting on 249 PB alternative products and 37 ABF comparators. Iodine was only reported in PB drinks, not in other types of PB products. Like PB meat alternatives, micronutrient content ranged vastly across all groups: some products contributed to the average daily requirement, whereas others were much less nutritious ( Table 3 and Supplementary file 2: Table S2 ). For example, the median calcium content for all PB drink categories was 120.0 mg/100 mL (IQRs as follows: cereals and grains, 120.0–120.0; coconut, 120.0–120.0; fruits and vegetables, 120.0–120.0; legumes, 120.0–120.0; nuts and seeds, 114.5–120.0) as compared with 116.7 mg/100 mL (IQR, 109.3–124.0) for dairy milks. However, none of the PB products (median, 0.0 μg/100 mL; IQR, 0.0–1.4) matched the iodine content of dairy milks (median, 24.9 μg/100 mL; IQR, 20.0–36.5).

Only 4 studies (evaluating 29 PB drinks and 11 dairy milk products) reported nutrient data from organic PB products. All evaluated different nutrients, hence no further pooling of results was possible for organic products as a subgroup. Protein results are reported in Supplementary file 1: Figure S2 and Supplementary file 2: Table S1 in the Supporting Information online, which show that, particularly, legume-based PB drinks typically match dairy milk in protein content.

Energy density, saturated fat, fiber, sugar, sodium and micronutrient content of plant-based yogurt alternatives

The 4 studies on PB yogurt alternatives evaluated 191 PB yogurt products with 90 dairy-based comparator products (unflavored and unsweetened). The overall nutritional composition of PB yogurts appears to show some variation by main primary ingredient (see Supplementary file 2 in the Supporting Information online ); however, formal disaggregated assessment of PB yogurts by primary ingredient was not possible, because that information was often not reported by authors. At an aggregate level, PB yogurts typically contained less saturated fat and sodium but had a higher energy density and higher total sugar and fiber content.

Only 2 studies evaluated micronutrient data of PB yogurts (excluding sodium) and, therefore, no further pooling of results was possible. No studies reported nutrient data from organic PB yogurts. Protein results are reported in Supplementary file 2: Table S1 in the Supporting Information online . Only the sample of a legume-based PB yogurts came close to matching dairy yogurt in protein content.

Energy density, saturated fat, fiber, sugar, sodium and micronutrient content of plant-based cheese alternatives

The 5 studies evaluating PB cheese alternatives reported on 163 PB cheese products with 143 dairy-based comparator products. PB cheese alternatives were the least nutritionally diverse foods. Where the primary ingredient of PB cheeses was known, this was mostly coconut oil ( Supplementary file 2 in the Supporting Information online ); however, like PB yogurts, the main ingredient was often not reported by authors.

The cheese comparators were reported to contain median values of 284.0 kcal/100 g energy density (IQR, 108.0–330.1), 14.0 g/100 g saturated fat (IQR, 11.0–17.3), and no fiber (0.0 g/100 g; IQR, 0.0–0.0). Most PB cheese subgroups were reported to have higher energy densities and higher saturated fat and fiber content. PB cheese based on nuts and seeds had the highest energy density (328.0 kcal/100 g [IQR, 306.0–328.0]; P d = 0.334]), whereas coconut oil-based cheese had the highest saturated fat content (21.0 g/100 g [IQR, 19.7–22.0]; P d < 0.001]), a significant difference, with 50.0% more than dairy cheese. Unlike PB drinks, PB meat, and PB yogurt alternatives, not all PB cheese contained fiber. Nut- and seed-based cheese had the highest fiber content (median, 2.5 g/100 g [IQR, 2.4–2.7]; P d < 0.001). Although the median fiber content of PB cheese made from coconut oil was 0.0 g/100 g (IQR, 0.0–1.7; P d = 0.011), some products did contain up to 5.9 g/100 g and, therefore, strong evidence was found that both PB cheese based on nuts and seeds and on coconut oil had significantly higher fiber content than did dairy cheese.

Most PB cheese contained less sugar and sodium than did dairy cheese, which had a median of 2.0 g/100 g (IQR, 0.5–5.0) and 720.0 mg/100 g (IQR, 560.0–1000.0), respectively, across the identified studies. In general, PB cheese alternatives had either no or minimal total sugar content. Finally, coconut oil–based cheese had the highest sodium content across all PB cheese (median, 714.0 mg/100 g [IQR, 600.0–880.0]; P d = 0.897), but this was similar to dairy cheese. PB cheese made of nuts and seeds had the lowest median sodium content (240.0 mg/100 g [IQR, 200.0–240.0]; P d = 0.001), which would equal 48.0 mg of sodium (0.1 g of salt) per 20.0 g serving size, or 2.0% of the recommended maximum daily salt intake 69 ; hence, this type of PB cheese had a large reduction in sodium compared with dairy cheese.

The micronutrient content of PB cheese was evaluated by only 2 studies. Only 1 product made of nuts and seeds was fortified with calcium, whereas coconut-based PB cheese was typically fortified with vitamin B 12 (median, 2.5 μg/100 g; IQR, 2.5–2.5). For dairy cheeses, these medians were 815.0 mg/100 g (IQR, 463.0–930.0) for calcium and 2.5 μg/100 g (IQR, 1.8–2.5) for vitamin B 12 .

No studies reported nutrient data from organic products. Protein results are reported in Supplementary file 2: Table S1 in the Supporting Information online. Nut- and seed-based cheese typically had the highest protein content, though it did not match the protein content of dairy cheese.

Health impacts and risk factors of novel plant-based foods

Eleven peer-reviewed studies were included in this review, 9 of which evaluated PB meat alternatives and 3 evaluated PB drinks ( Table 4 ) 84–94 (see Supplementary file 1, section 3.3, in the Supporting Information online for further details on the health outcomes). No health studies were found that evaluated consumption of PB cheese, yogurt, or egg alternatives; links between NPBFs and mental health outcomes; nor any grey literature evaluating any health outcomes.

Summary of the evidence on the health impacts and risks of novel plant-based foods

Health impacts and risk factors of plant-based meat alternatives

Studies of PB meats (n = 9) showed positive health outcomes when individuals switched from consuming ABFs. Three studies on mycoprotein consumption by both healthy and overweight adults found a positive association with lower glycemic markers, 84 reduced energy intake, 84 , 85 and insulin release. 85 Moreover, mycoprotein consumption was hypothesized to have a beneficial impact on the plasma lipidome. 86

Four studies with healthy adults evaluated PB meat alternatives consumption (other than mycoprotein). When considering the same caloric intake, consumption of PB meats was associated with a lower risk of cardiovascular disease than was consumption of ABFs, mostly by reducing fasting serum levels of trimethylamine- N -oxide, and low-density lipoprotein cholesterol concentrations, compared with ABF consumption. 87 Furthermore, consumption of PB meats was associated with a reduction in body weight as compared with meat consumers. 87 , 88 Lysine-enriched PB meat as a substitute for ABFs was reported to increase muscle protein synthesis rates, which is a biological process of building new protein cells via amino acids. 89 Last, the replacement of 4 meat-containing meals per week with PB meat alternatives elicited positive changes in the gut microbiome, with changes in the presence of butyrate-producing pathways and increased taxa. 90

Health impacts and risk factors of plant-based drinks

Studies assessing PB drinks (n = 3) only focused on almond and soy drinks. The main focus and health outcomes of these studies varied. Sun et al 91 researched the reduction in glycemic response in young adults consuming soy drink or bovine milk together with white bread. These authors found that both products had a similar glycemic response through different biological pathways. Dineva et al 92 assessed micronutrient content in PB drinks and found significantly lower iodine intake and urinary iodine concentration in people consuming only PB drinks, 93 highlighting the need for appropriate fortification as more people transition to eat more NPBFs. Finally, Shen et al 93 evaluated the impact of PB drinks on dental health and found that a soy drink with added sugar caused enamel demineralization, compared with dairy milk, which promoted remineralization.

Environmental impacts of novel plant-based foods

A total of 53 studies evaluated at least 1 environmental outcome, using the life cycle assessment method, evaluating 209 PB products and 91 ABFs as comparators. Most studies used life cycle assessment inventories, and some relied on data providers (n = 32) to calculate environmental footprints. System boundaries varied across studies, with the majority evaluating category impacts from cradle-to-retail (see Supplementary file 3 in the Supporting Information online ). Studies mainly assessed the effect of substituting ABFs with NPBFs on greenhouse gas emissions (GHGE) (n = 50), followed by blue-water footprint (WF) (n = 39) and land use (LU) (n = 17) ( Figure 3 and Supplementary file 1: Table S11 in the Supporting Information online ). Although methods, assumptions, and inventory data varied from 1 study to another, most studies consistently reported percentage reductions in GHGE and LU for the production of NPBFs as compared with ABFs. Wider differences were observed in blue WF.

Reduction of environmental impacts by respective funding source . Calculated as a percentage difference between each novel plant-based (PB) product (by product type and food group based on main primary ingredient [ie, predominant or core food item on the ingredient list]) in comparison with their respective reported baseline (eg, dairy milk and cheese, meat and poultry). See Supplementary file 3 in the Supporting Information online for detailed information on the baseline used for each reference. Data were limited to raw products only. Studies reporting data on cooked PB products also found reductions in environmental impacts.

Environmental footprints of plant-based meat alternatives replacing meat and poultry

The 34 publications evaluating PB meat alternatives reported on 135 PB meat products with 53 ABF comparators. The percentage difference showed reductions of more than 70% in GHGE, LU, and WF for most products when shifting from ABFs to PB meat alternatives. GHGE reductions across PB meat groups, based on primary ingredients, were similar, with the largest reduction in GHGE seen for nut- and seed-based meats, with a median value of –94.2% (IQR, –94.4 to –93.4), whereas PB meats based on legumes had the smallest reduction (–86.1%; IQR, –88.6 to –77.5). Only 2 of 134 PB products had higher levels of GHGE than their ABF comparator. For LU, mycoprotein (median, 89.0%; IQR, –92.3 to –76.5) and nut- and seed-based meats (median, 89.5%; IQR, –90.0 to –89.0) had the largest reduction. Alternatively, legume-based meats had the smallest LU reductions (median, –71.2%; IQR, –84.7 to –47.6). Only 3 of 55 products had higher LU than their ABF comparator. Finally, the largest reduction of WF was observed in PB meats made of cereals and grains (median, –92.6%; IQR, –94.1 to –92.0), and the smallest was observed with products made of mycoprotein (median, –73.7%; IQR, –84.4 to –55.2). Nine of 51 products had a higher WF than their respective ABF counterparts. Specifically, when certain individual legume- and mycoprotein-based meats were compared with chicken, PB meat alternatives reported requiring between 2.7% and 339.0% more water, with the largest difference observed in a Swedish chicken comparator to mycoprotein-based meats. This variation was attributed to differences between feed types, rearing systems, and farm efficiency across countries. 74 Comparisons were also made between the upper limit footprint of mycoprotein-based items and the average or lower limit footprint of the ABF. Moreover, there were extreme outliers, with some PB meats reporting a water percentage difference of 8006.9%. The authors attributed this to soybeans’ substantial water demand during processing and lower yield per soybean. 74

Environmental footprints of plant-based drinks alternatives replacing dairy milk

The 21 publications evaluating PB drinks reported on 51 PB drink products with 13 ABF comparators. PB drinks also were associated with reductions in GHGE and LU when shifting from dairy milk to PB drinks. Fruit- and vegetable-based drinks had the largest reduction of GHGE (median, –90.2%; IQR, –90.8 to –90.2]), whereas PB drinks based on cereals and grains had the smallest reduction (median, –76.9%; IQR, –88.8 to –56.0). Only 2 products of 36 had an increase of GHGE when comparing soy- (40.0%) and almond-based (18.9%) drinks with dairy milk (equivalent to 0.3, 0.4, and 0.3 kg CO 2 eq/100 g, respectively). 95 Wider differences were observed on the LU percentage difference; however, reductions were found for all products (n = 13 PB drinks).

Cereal- and grain-based drinks had the largest reduction (median, –86.4%; IQR, –92.7 to –76.0), whereas legume-based drinks had the smallest LU reductions (median, –56.6%; IQR, –75.5 to –38.8). The magnitude of change in the percentage difference for WF varied considerably, although, these data were less frequently reported by authors (n = 11 PB drinks). Cereal- and grain-based drinks had the largest reduction (median, –85.0%; IQR, –88.7 to –71.0), whereas legume-based drinks had the smallest WF reductions (median, –67.6%; IQR, –73.9 to –42.2). Nut- and seed-based drinks presented contradictory evidence. For example, Grant and Hicks 95 observed that almond drinks (9241.9%) required considerably more water than soy (–35.6%) and dairy milks (equivalent to 109.3, 0.8, and 1.2 L/100 g, respectively); whereas Ritchie 96 found that an almond drink required half the amount of water (–40.87%) than dairy milk (equivalent to 37.2 and 62.8 L/100 g, respectively). Data were limited to these 2 products; hence, no further pooling of results was possible.

Environmental footprints of plant-based yogurt alternatives replacing dairy yogurt

The 2 publications evaluating 2 PB yogurt alternatives compared to 2 dairy yogurts. They reported GHGE reductions ranging between –64.7% and –52.9%. Analysis of LU and WF was not possible due to lack of a baseline, differences in methods, and system boundaries.

Environmental footprints of plant-based cheese alternatives replacing dairy cheese

The 2 publications evaluating PB cheese alternatives reported on 21 PB cheese products with 23 ABF comparators. Data on the environmental impacts were particularly from coconut oil–based cheese alternatives (n = 20). All coconut oil–based cheese alternatives had a large reduction in amounts of GHGE and LU (GHGE: median, –75.4% [IQR, –77.4 to –59.3]; LU: median, –83.1% [IQR, –83.8 to –80.6]). A smaller reduction was observed in WF (median, –45.1%; IQR, –52.0 to 38.5), with a higher WF being reported than for the ABF comparator for only 3 products.

Health effects and environmental impacts of novel plant-based foods

Studies that simultaneously assessed both health and environmental outcomes and/or nutrient profiles of NPBFs were pooled ( Figure 4 ). Only 1 study reported environmental outcomes together with diet-related health effects of PB meat alternatives, and this study found that free access to NPBFs was associated with greater weight loss and reduced dietary carbon and LU, as compared with a control arm. 88 From 93 references, 20 studies assessed the environmental outcome and nutrient content of NPBFs; only 6 studies evaluated the health effects and nutrient content of NPBFs (see Supplementary file 1: Table S9 in the Supporting Information online ).

Reduction of environmental outcomes and their associated nutrient outcomes of novel plant-based foods (NPBFs) compared with baseline (eg, dairy milk and cheese, meat and poultry), expressed in percentage difference . The y -axis shows the increase or decrease of the nutrient content (energy, fiber, sodium, and saturated fat) in comparison with baseline; and the x -axis shows the reduction (or increase) of the environmental categories. Three environmental categories are reported: greenhouse gas emissions (circles), land use (triangles), and blue-water use (squares). Three NPBFs are reported: plant-based (PB) cheese alternatives (brown), PB meat alternatives (purple), and PB drinks (orange). PB yogurts were not included due to the limited amount of data. See Supplementary file 2 in the Supporting Information online for detailed information on the baseline used for each reference. Data were limited to raw products only.

When compared with ABF counterparts, data suggest NPBFs are overwhelmingly associated with smaller environmental footprints. Data on nutritional profiles of NPBF were mixed: nutritional profiles for some NPBF groups were better aligned with healthy diets, but not for others. Clear co-benefits were observed for fiber intake from NPBFs. However, for the other nutrients, the picture was much more mixed due to the variability in content arising from differences in the main primary ingredients and the type of NPBFs.

Fruit, vegetable, legume, and nut content of novel plant-based foods

The percentage of fruit, vegetable, legume, and nut content in each NPBF in the United Kingdom was estimated as a case study ( Figure 5 ). Most NPBFs had at least 1 fruit, vegetable, legume, or nut, ranging from 0.0% to 100.0% of their weight. Overall, median content was low, with a few exceptions. PB meat alternatives had the highest content of vegetables and legumes, and PB cheese alternatives had the lowest content ( Supplementary file 1: Figure S5 and Supplementary file 2: Table S5 in the Supporting Information online ).

Estimated fruit, vegetable, legume, and nut content (%) in each novel plant-based foods product from time-stamped data from UK supermarkets . Panels show (a) plant-based (PB) drink alternatives; (b) PB meat alternatives; (c) PB cheese alternatives; and (d) PB yogurt alternatives.

Assessment of robustness and relevance of the included studies

For results on the assessment of robustness and relevance of the included studies see Supplementary file 1 : Table S12 in the Supporting Information online in section 3.6 .

Sensitivity analysis of funding sources of nutrient composition studies

Almost half of the nutrition studies included (n = 26; 46.4%) were funded by academic funders; 44.6% (n = 25) were fully funded or partially funded by industry; and 10.0% (n = 5) did not state their funding source. NPBF manufacturers were the support for the majority of industry-funded studies (n = 21; 37.5%), followed by the livestock industry (n = 3; 5.4%), and both (n = 1; 1.8%). The sensitivity analysis of the percentage difference for all the nutrients associated with the burden of disease, except total sugar, revealed that studies funded by industry were more likely to find differences than those funded by academia, with the former typically reporting more positive results on lower energy and saturated fat ( Table 5 and see Supplementary file 1: Table S13 in the Supporting Information online for sensitivity analysis on studies partially funded by the industry). However, the direction across all studies was the same: reductions in energy and saturated fat content, and increases in fiber, total sugar, and sodium content.

Sensitivity analysis, based on funding source, of the percentage difference between novel plant-based foods vs animal-based foods in nutrient content and environmental impacts a

The funding source of 6 articles were unknown, so they were excluded from this analysis. The superscript b and c indicate the direction and dimension of the association.

Industry-funded studies show a more positive impact on health and environmental outcomes of their PB products (vs animal sourced foods) as compared with academically funded studies.

Industry-funded studies show a less positive impact on health and environmental outcomes of their PB products (vs animal-sourced foods) as compared with academically funded studies.

Abbreviations : ABF, animal-based food; IQR, interquartile range; NPBF, novel plant-based food.

Sensitivity analysis of funding sources of health studies

Only 2 health studies were funded by academia; the rest of the studies were either partially or wholly funded by industry (n = 9). Most industry-funded studies were from NPBF manufacturers (n = 8); 1 study was partially funded by Dairy Australia.

Sensitivity analysis of funding sources of environmental studies

Compared with nutritional studies, a greater percentage of environmental studies were by industry researchers, particularly from NPBF manufacturers (67.9%). Approximately 71.7% of studies (n = 38) were fully funded or partially funded by industry; 26.4% (n = 14) were supported by academic funders; and 1.9% (n = 1) did not state their funding source. Of the industry-funded studies, only 2 (3.8%) were funded by the livestock industry. The sensitivity analysis revealed that the percentage differences were significantly larger between academic and industry funders in terms of GHGE and LU. Studies funded by industry typically reported more positive results on LU than did studies funded by academic funders, and the opposite was observed for GHGE. Like nutrient studies, the direction (decreases in GHGE, LU, and WF) was the same regardless of the funding source ( Table 5 , and see Supplementary file 1: Table S13 in the Supporting Information online for the sensitivity analysis of studies partially funded by the industry).

Research findings

We reviewed evidence from high-income countries that was published in peer-reviewed and grey literature within the past 7 years on nutrient content, and environmental and health outcomes of consuming NPBFs. Most NPBFs typically have much lower environmental impacts compared with ABFs, particularly with respect to GHGE and, to a lesser extent, to LU and WF. The nutrient content of NPBFs is highly variable in comparison to the nutrient profiles of ABFs. Although several individual NPBFs had positive health and environmental outcomes, co-benefits identified were not universal across all NPBFs and several trade-offs were identified. The main primary ingredient, type of product, processing techniques, and brand were all important determinants of health, and nutritional and environmental outcomes, findings that show the need for further subcategorization of NPBFs to better educate consumers and enable them to take informed decisions regarding the healthiness and sustainability of their diets and (potential) dietary changes.

Research in context

If carefully selected, certain NPBFs (particularly certain PB drinks and meat alternatives) could be an effective part of interventions to achieve net-zero and health targets in high-income countries. By applying a combination of strategies, enhanced uptake of these foods could improve the nutritional quality of diets, improve health, and contribute to tackling climate change impacts.

At the macronutrient level, NPBFs are generally the healthier option, given their higher fiber content and typically lower saturated fat and calorie contents, which could be advantageous in high-income (often obesogenic) settings. Certain types of NPBFs, particularly mycoprotein and legume-based meats, often also contain a substantial amount of fruit, vegetables, legumes, and/or nuts, which are food groups that are typically underconsumed in high-income settings. Composition of legume and fruit and vegetable-based drinks, were also typically consistent with healthier diets in high-income food secure settings, including low energy density, low total sugar, high fiber and low saturated fat content. Caution is recommended in the selection of these products if they were to be part of dietary recommendations, or standard institutional procurement for example, as certain NPBFs can also have higher levels of total sugar, sodium, and saturated fats in comparison to their respective ABF. This is particularly true for certain cereal and grain-based drinks, and coconut-based cheese and yogurts. Although the specific type of oil used in each NPBF product was not analyzed, coconut oil, which is high in saturated fatty acids, is often the ingredient that increases saturated fat levels in NPBFs to levels similar to its ABF counterparts. 51 , 75 Indeed, coconut oil-based cheese had approximately 50% more saturated fat than dairy cheese, and typically contained the least amount of fruit, vegetables, legumes or nuts, with the majority being absent.

In line with other evidence, 39 , 97 , 98 fortified NPBFs, in some cases, can be nutritionally comparable to their respective ABFs. Some individual NPBFs contained even higher concentrations of iron, vitamin B 12 , and calcium, whereas others did not. However, micronutrient assessment was difficult because not all included studies reported micronutrients. This could be because either NPBFs were unfortified or the information simply was not reported. Especially when nutrient information is gathered from supermarket websites for individual studies, micronutrient data are generally not reported.

The highly varying nutrient content across and within all PB products and categories may cause consumer confusion when individuals are looking for healthy and environmentally friendly alternatives to ABFs. Clearer front-of-package labelling of certain nutrients and information campaigns could reduce such confusion and better enable the consumer to make informed decisions about food purchases. 99 Potential development of rules and regulations on the food standards of NPBFs could also be a step forward in having a larger range of “healthy” NPBFs, because such regulations could potentially encourage reformulation of NPBFs, including the reduction of sodium, total sugar, and saturated fat content, and increased micronutrients. From a technological perspective, this is certainly possible. For example, new biotechnological techniques have been developed that enable companies to reduce sugar content and improve palatability, nutrient profile, and digestibility of PB drinks. 67 , 100–103 Some processing techniques can also decrease levels of anti-nutrients and polyphenols, which commonly are associated with low mineral and vitamin bioavailability, 35 , 98 , 101 , 104–107 and increase protein yield. 101 Given that specific raw materials, isolated proteins, processing levels, and fortification methods, often used in NPBFs, as compared with ABF nutrient profiles, are still debated in the scientific community, further research on the nutrient content and health risks related to bioavailability, bioaccessibility, and byproduct formation during industrial processes will reveal whether there are differences in terms of health impacts of “natural” vs more “isolated” nutrients. 30 , 108 , 109 More research into the metabolic profiles of NPBFs is imperative, particularly in light of a recent study identifying differences in the abundance of profiled metabolites between beef and PB burgers, despite their labelled nutritional similarities. 110 Instead of continuing the debate between the superiority of ABFs vs NPBFs, or vice versa, acknowledging and embracing their complementary differences can contribute to a less polarized dietary transition. This is especially relevant because emerging evidence has suggested that people who consume NPBFs also tend to purchase ABFs. 111

From the limited evidence on health, the inclusion of NPBFs into diets appears to typically have beneficial health effects, particularly the consumption of PB meat alternatives. The positive health effects mostly relate to better weight management and associated reduced risk of noncommunicable diseases in high-income (and often obesogenic) countries. This is aligned with a recently published meta-analysis that found positive outcomes on total cholesterol, low-density lipoprotein cholesterol and triglycerides when consuming PB meat alternatives as replacements for meat. 51 Furthermore, a few older studies also found positive health outcomes when assessing consumption of mycoprotein-based foods (eg, drinks, cookies, milkshakes, crisps) 112–115 and soy protein with isoflavones, 50 compared with consumption of dairy milk and/or meat products.

Previous evidence revealed that NPBFs are often regarded as healthier alternatives to ABFs 116 ; hence, it could be hypothesized that people may consume NPBFs in larger quantities than they would otherwise have done when eating ABFs. This may have negative health implications, especially if consumed regularly. Establishing a clear division in PB foods classifications, including ultraprocessed and less processed PB alternative foods, could enable better assessment of short- and long-term health impacts of NPBFs if they were to be consumed at an even larger scale. 116

Ultraprocessed foods have been associated with many diet-related diseases because these foods are generally energy dense and hyperpalatable. 117 , 118 Almost all NPBFs fall, technically, within this category; however, in this review, we found that the nutritional composition of some NPBFs aligns well with healthy dietary recommendations, such as having a high fiber content, low energy density, and low saturated fat content. Additionally, 1 of the included studies 90 also found positive associations with the gut microbiome when substituting meat in certain meals with PB meat alternatives. To get a better overview of the overall effect of NPBFs on health, more information and detailed analyses are needed regarding level of processing and gastrointestinal fate.

Consistent evidence was found regarding environmental outcomes, similar to previous research. 52 , 53 , 108 , 119–121 Most NPBFs had smaller environmental footprints than their ABF counterparts, with median reductions reported of up to 94.3%, 89.5%, and 92.6% for GHGE, LU, and WF, respectively. Nevertheless, some PB products had greater environmental impacts than their ABF counterparts, with some extreme outliers particularly in terms of WF. Although evidence was rather consistent, and the direction of effect appears to be clear, care should be taken not to overinterpret the exact numerical results: environmental impact calculations are notoriously context dependent and sensitive to methodological and data choices. This makes it impossible to come up with a summary figure that is representative for all products, produced in all countries; generally, however, there is a broad body of evidence demonstrating a reduction in GHGE, LU, and WF for a wide range of PB products in a wide variety of contexts compared with their ABF equivalents.

To improve the strategic use of NPBFs to achieve more sustainable food systems, life cycle assessments of these products should incorporate the full range of environmental impact categories, as well as sociocultural, economic, and health impacts with harmonized methods and assumptions across studies.

This study revealed an evidence gap for health impacts of NPBFs, including mental and dental health, and other risks associated with micronutrient deficiencies. There is also a lack of health studies on PB yogurts, PB cheese, and PB egg alternatives. Research on the health effects of PB drinks has been conducted with only certain products, “generally soy and almond drinks,” but there is a gap in knowledge about other PB drinks, such as those made from oat, potato, and hazelnut, among others. Furthermore, some concerns have been raised about the carbohydrate content in some PB drinks. A study by Jeske et al 122 revealed that the presence of β-glucan in many oat-based drinks causes a moderate glycemic index, despite the high carbohydrate content. In fact, Dhankhar 104 associated the consumption of oat drinks with high β-glucan levels with a reduction in cholesterol levels in study participants. However, this evidence needs to be updated to reflect the potential benefits of different types of PB drinks and current market brands. Although dairy products contain naturally occurring sugars from lactose, it is difficult to determine the breakdown of “natural” vs added sugars in NPBFs from the available literature. More research is also required on dental health to assess the potential risks of increased dental cavities due to lower calcium bioavailability, and the effects of free sugar content, pH levels, and buffering capacity in NPBFs.

Additional research is needed to provide more nutrient environmental and health evidence for PB yogurts, cheese, and egg alternatives. Last, although this review assessment focused on 3 environmental outcomes, evidence on other environmental impacts, including biodiversity loss and socioeconomic implications, is scarce. Across the 3 themes assessed in this review, better standardization and clear reporting of results in NPBF studies in the future would facilitate updates of this review.

Relevance for policy and practice

Minimally processed PB foods are still considered the gold standard for healthier and more sustainable diets. However, shifts from ABFs to PB whole foods remain problematic because, despite all the scientific knowledge about healthy eating, dietary change toward minimally processed PB foods has not been achieved. This review revealed that NPBFs can be healthier and more environmentally friendly alternatives to ABF consumption, if carefully selected. Although behavioral aspects are embedded in this transition, NPBFs could offer a convenient, novel, and potentially more realistic option to facilitate dietary transitions at large scale, diversifying diets, and increasing consumption of fruits, vegetables, legumes, and nuts without the need for significant individual dietary habits.

For potential promotion of the inclusion of NPBFs as part of public procurement or embedding them into food-based dietary guidelines, some of the consideration regarding varying healthiness of specific types of NPBFs and the need for further subclassifications needs to be carefully addressed. Furthermore, affordability is a concern because NPBFs often are more expensive than their ABF counterparts. Although comprehensively synthesizing price data was outside of the scope of this study, in the United Kingdom, the Food Foundation found that PB drinks are, on average, 50.0% more expensive than dairy milk. 71

Active promotion of NPBFs would require more detailed analysis of consumer behavior: current consumption of NPBFs is generally higher among younger generations, women, White populations, and those with higher education and incomes. 28 Better understanding of main drivers and barriers of consumption of NPBFs would allow targeted promotion to widen this consumer group. 71 NPBFs could play an additional role in reducing the prevalence of micronutrient deficiencies, especially given their reformulation and fortification potential. For example, in Finland, a mass fortification strategy of vitamin D across dairy and nondairy products has shown positive health outcomes over the past decade. 123 Finally, formalization, standardization, and accountability of environmental labelling could help consumers making informed decisions and avoid misinformation.

Strengths and limitations

To our knowledge, this is the first systematic review assessing the published peer-reviewed and grey literature evidence from studies that evaluated nutrient, and health and environmental impacts or benefits of NPBFs. A strict and comprehensive search string was developed to assess the full breadth of studies and reports, and machine-learning models were used to filter the large number of studies and systematically present all the available evidence on various NPBFs.

This study only covered the past 7 years to assess the current evidence, and an exhaustive cross-check of references was not performed, which possibly introduces reporting bias for missed relevant studies from previous years. However, it was assumed that only a small amount of additional findings had been missed, given the recent emergence of the variety and types of these novel products. Second, only 3 environmental impact categories were examined: carbon footprint, LU, and blue-water consumption. However, the heterogeneity of study designs, from system boundaries to geographical location, agricultural inputs, and methods used to calculate environmental footprints, made the review process too time consuming to expand on other environmental impacts in this particular study. Reliable reporting of environmental impacts of novel ingredients used in NPBFs, including added minerals and vitamins for fortification purposes, are generally missing in many studies. All the data reported by authors were collected and each study was compared individually against its own baseline (ie, the ABF comparator provided by author). Given the large spectrum of methods to determine environmental footprints, this could have introduced some bias; however, the alternative (using a standardized comparator) would equally have its limitations (eg, this would not be representative for all farming systems and products). Third, products and nutrients were assessed individually. Although the nutrient content gives some guidance on probable health risks, in reality, people consume diets in which individual compounds interact, influencing unknown biological pathways. Fourth, several studies that did not specifically report on the proportion and type of NPBF in (self-)reported PB diets had to be excluded. For those studies, it was impossible, therefore, to assess the effect on health and environment of NBPFs alone vs all PB foods together (ie, whole foods, NPBFs, other PB foods such as tofu and tempeh) and complicated any efforts to calculate dietary shifts. Finally, most studies did not report the precision of measures of effect (n = 68), making it difficult to pool and synthesize results across the 3 themes assessed in this review.

Food systems and diets need to change to meet environmental and health targets. This comprehensive systematic review presents a holistic approach to summarize the evidence on the nutrient, health, and environmental impacts of NPBF consumption. Although PB whole foods remain the preferred option on health grounds, some NPBFs have potential for being a useful steppingstone in the process of food system and dietary transformation, functioning as a healthy and environmentally friendly alternative to ABFs, if carefully selected. Reformulation and fortification could further enhance NPBFs as a viable and effective food group that could accelerate the dietary transition toward sustainable and healthy diets. However, given the great variability in nutritional composition of individual NPBFs, widespread promotion of such products should be introduced and addressed with caution. Given that NPBFs are already important in the food system and consumption is expected to continue to increase, a few steps are urgently required to guide consumers and enable them to make informed decisions regarding their diets. These include a further subdivision or categorization of NPBFs, which currently fall mainly in the ultraprocessed (hence, “unhealthy”) food category. Furthermore, standardized and verifiable environmental assessments of NPBFs are needed to compare foods with regard to their environmental footprints. Finally, more research on the short- and longer-term health effects of NPBFs is urgently required to facilitate informed decision-making on the inclusion of NPBFs as part of a wider net-zero and health strategy.

Gratitude is extended to the authors who responded to inquiries and generously shared their individual data. Additionally, sincere appreciation is expressed to the FoodDB team for sharing their time-stamped data set of observations from UK supermarkets to estimate the total fruit, vegetable, legume, and nut content of foods.

Author contributions. S.N.E. contributed to conceptualization of the study, methodology, investigation, data curation, formal analysis, coding and analysis of machine learning, and writing the original draft of the article. G.H. contributed to the formal analysis (screening process), data validation, and review and editing of the manuscript. A.J.S. led the coding and analysis of machine learning, and reviewed and edited the manuscript. C.A.-C. and G.T. contributed to the literature screening process, and reviewed and edited the manuscript. R.G. reviewed and edited the manuscript. S.P. and R.P. contributed to data validation, and reviewed and edited the manuscript. P.S. contributed to conceptualization of the study, methodology, reviewed and edited the manuscript, supervised the work, acquired funding, and contributed to project administration.

Funding. This work was supported by a research grant from the National Institute for Health Research, Health Protection Research Unit PhD Studentship in Environmental Change and Health (grant NIHR200909) and the af Jochnick Foundation.

The funders had no role in the conception, design, performance, and approval of this work.

Declaration of interest. The authors have no relevant interests to declare.

Data availability. Source code for this work is available online (DOI: 10.5281/zenodo.7116157).

The following supporting information is available through the online version of this article at the publisher’s website:

Supplementary file 1 .

Supplementary file 2 .

Supplementary file 3 .

Box 1 Key Definitions

Novel plant-based foods (NPBFs): Acknowledging differences in terminology for NPBFs, for the purpose of this review, the term novel plant-based foods is used to describe plant-based (PB) drinks and PB meat, cheese, eggs, and yogurt alternatives that are of plant or fungal origin and designed to directly replace or mimic animal-based foods. This definition includes fungi-based foods (ie, mycoprotein) that biologically do not belong to the plant kingdom but are typically “designed” similarly to NPBFs as a direct replacement for animal-based foods. Here, the term excludes tofu, tempeh, and seitan because although these might be novel to some high-income settings, they have been part of traditional Asian diets for centuries and, hence, are not subject to the same challenges and evidence gap as NPBFs.

Ultraprocessed: Foods that have undergone a series of industrial techniques and processes

Minimally processed plant-based foods: Plant-based whole foods such as nuts, seeds, cereals, and legumes

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Crossover from Linear to Quadratic Electro-optic Behavior in BaTiO 3 and ( Ba , Sr ) TiO 3 Solid Solution

Sergey prosandeev, charles paillard, and l. bellaiche, phys. rev. lett. 132 , 196901 – published 7 may 2024.

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We derive a numerical method based on coupled density functional theory and effective Hamiltonian schemes to calculate the linear and quadratic electro-optic response of ferroelectrics at finite temperature and in different frequency ranges. By applying the developed method to BaTiO 3 , we successfully resolve apparent discrepancies in the experimental literature that reported a linear or quadratic electro-optic response when visible or terahertz radiation was employed to measure the optical index, respectively. We further demonstrate that (and explain why), in the case of the Ba 1 − x Sr x TiO 3 disordered solid solutions, structural phase transitions not only lead to larger linear electro-optic constants, as previously demonstrated in the literature, but also significantly enhance the quadratic electro-optic constants.

• Received 16 November 2023
• Revised 5 March 2024
• Accepted 5 April 2024

DOI: https://doi.org/10.1103/PhysRevLett.132.196901

© 2024 American Physical Society

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• 1 Smart Functional Materials Center, Department of Physics and Institute for Nanoscience and Engineering, University of Arkansas, Fayetteville, Arkansas 72701, USA
• 2 Université Paris-Saclay, CentraleSupélec, CNRS, Laboratoire SPMS, 91190 Gif-sur-Yvette, France
• * [email protected]
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• ‡ [email protected]

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Vol. 132, Iss. 19 — 10 May 2024

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(a) Contributions to the linear EO constant r 33 σ in BTO at ℏ ω = 4     meV and 1.55 eV. (b) The change of the refractive index Δ n for BTO at ℏ ω = 4     meV (in blue) and ℏ ω = 1.55     eV (in orange).

(a) Calculated linear EO constants versus composition x in bulk BST; open symbols denote experimental values [ 4 ]. (b) Computed quadratic EO constant R 333 versus composition x . (c) Expected change of refractive index for electric fields along the polar direction. (d) Crossover field at which the linear and quadratic contributions to the change of refractive index are equal. All data were calculated at 300 K and for ℏ ω = 1.55     eV .

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