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What does the research say about vegan nutrition?
You often hear people say that a vegan diet is good for the environment, for humans and other animals. Skeptics, on the other hand, warn of malnutrition. Both sides often refer to studies, which shows that the vegan diet brings advantages or disadvantages depending upon the individual point of view.
Which one is correct? How do scientific studies evaluate the vegan diet?
The Most Important Research on Vegan Nutrition
We have compiled some of the most relevant studies on vegan nutrition. Following our list, you will find a more detailed description of these and other studies, as well as tips on what you should keep in mind when evaluating reasearch on vegan nutrition and reading studies.
- The EPIC-Oxford (European Prospective Investigation into Cancer and Nutrition) Study: This large-scale cohort study has been examining people in Great Britain who are following a vegan, vegetarian or omnivorous diet since 1993. The aim is to investigate the relationship between diet and long-term health.
- The Adventist Health Study I and the Adventist Health Study II: Seventh-day Adventists in the USA and Canada have been studied here since 1958. The majority of them follow a vegetarian diet. The aim is to find out how diet and lifestyle influence the risk of developing various chronic diseases.
- The VeChi- and the VeChi-Youth Studies: These studies, led by Markus Keller, focus on how the diet and health status of vegan, vegetarian, and mixed-food children and adolescents in Germany differ.
- In “The Impact of Plant-Based Dietary Patterns on Cancer-Related Outcomes: A Rapid Review and Meta-Analysis” Spanish researchers investigated current literature to research the connection of plant-based diets and cancer.
- In the context of the ideal Planetary Health Diet developed by scientists, it is becoming clear that a plant-based diet has significant benefits for the health of the Earth and humanity in terms of sustainability.
The Interpretation of Research on Vegan Nutrition
With increasing popularity, interest in researching vegan nutrition is also growing. Nevertheless, the number of vegans is comparatively small. As a result, recruitment for studies on vegan nutrition is difficult, and the number of subjects is usually small and selected. This makes it difficult to draw meaningful conclusions.
In addition, the vegan diet has changed significantly in recent years. This is because both the choice of foods and the knowledge about potentially critical nutrients has grown considerably. Therefore, only limited conclusions can be drawn from the results of older studies to today’s vegan diets.
Two large-scale studies are particularly well known: the Adventist Health Studies I and II and the EPIC Oxford Study. However, they too must be evaluated according to their significance:
Adventists are a very health-conscious population overall. On the one hand, this means that differences between people on a mixed diet and vegans would possibly be even more pronounced compared to the average population, but on the other hand, Adventists who eat a vegan diet probably also have a health-promoting lifestyle in general. This limits transferability to the general population. The results indicated that the vegetarians studied have an overall healthier lifestyle and lower risk of disease than the non-vegetarians.
The EPIC Oxford study was conducted in the United Kingdom. Just like the Adventist Health Study, this is an epidemiological study, that means, an observational study. No causalities can be derived from it, only correlation. So if, for example, the vegans here suffered less frequently from a particular disease, that is initially an observation and does not necessarily mean that this is due to the vegan diet or a particular individual aspect of the vegan diet. Studies like this are therefore suitable for hypothesis generation for follow-up studies.
This applies to all observational studies. So-called randomized control studies are more likely to indicate causal relationships. In these studies, an intervention takes place, for example, people eat a vegan diet for a period of time and the change in certain parameters is compared with a control group that eats a mixed diet.
As you can see, research on vegan nutrition must always be seen in context and interpreted accordingly. An overview of relevant studies on vegan nutrition can be found below. We have linked them in the text, and you can also find them below in the literature references. With that, you have a practical overview of important studies about the vegan diet.
Reasons to Go Vegan
Studies show: People choose a vegan diet for a variety of reasons. In a recent forsa survey by the PHW Group, the majority of vegetarians and vegans surveyed said they give up meat because of sustainability and environmental protection. Just under half cite health aspects. Other reasons were motivation by third parties, taste and little desire for meat. 2 % do it out of habit and for 1 % the cost is a reason for the meat-free diet (PHW, 2021).
Among the children and adolescents surveyed in the VeChi-Youth study (study on vegan nutrition of children and adolescents in Germany), ecological reasons for vegan nutrition tend to be in the background. Ethics and animal welfare are the motive for the majority here, 13 % name parents and 11 % health reasons (Alexy et al., 2020).
It is obvious that ethics and animal welfare speak for a vegan diet. But what does the research say about health and ecological aspects of the vegan diet? What about the nutrientional status, are the concerns of many vegan critics, who are especially worried about pregnant women, children and athletes, justified?
Health Aspects
Studies examining the health effects of a vegan diet show that vegans often weigh less and are less often affected by obesity. Possible reasons for this are the lower average calorie density and higher fiber content of plant foods. But the overall more health-conscious lifestyle, independent of the vegan diet (for example higher physical activity levels), may also contribute to the results.
The lower prevalence of adiposity can also have a positive effect on the risk of various diseases, such as type 2 diabetes mellitus, coronary heart disease, dyslipidemias, hypertension, etc. Therefore, for the prevention and treatment of dyslipidemia and coronary heart disease, official institutions such as the European and American Societies of Cardiology recommend a diet rich in vegetables, fruits, legumes, nuts, and whole grains (Mach et al., 2019; Arnett et al., 2019).
High vegetable, fruit, and fiber intake may also reduce the risk of several cancers, according to the World Cancer Research Fund (Wild et al., 2021). In both the Adventist Health Study-2 and the EPIC-Oxford study, vegan diets were associated with lower cancer risk compared with mixed diets (Segovia-Siapco et al., 2018). However, according to a recent review, there are currently not enough conclusive studies to make concrete statements about specific cancers and vegan diets (Molina-Montes et al., 2020).
In rheumatoid arthritis, a review by Alwarith and colleagues shows improvements in symptoms with a plant-based diet (Alwarith et al., 2019). The authors attribute this to the elimination of many foods that can trigger symptoms. In a randomized controlled trial, a vegan diet also measured lower concentrations of leukocytes, neutrophils, monocytes, and platelets, which could explain a positive effect (Lederer et al., 2020).
The effects on the risk of osteoporosis are unclear; studies show inconsistent results here with vegan diets. In the seven studies with 2749 omnivorous, vegetarian and vegan subjects examined in a meta-analysis, the vegans often consumed less calcium than people on a mixed diet and in some cases, a higher prevalence of osteoporosis was observed in vegans. However, often no relevant negative effects of a vegan diet on bone density are shown (Ho-Pham et al., 2009). Currently, the body of research indicates that all nutrients most relevant to bone health (e.g., calcium, vitamin D, protein, magnesium) can be consumed in sufficient amounts via the vegan diet if it is thoughtfully designed (Mangels, 2014). That being underweight is a risk factor for low bone density may be one reason that vegans show lower bone density, especially in older studies. Therefore, it is important to pay special attention to nutrients relevant to bone health in a vegan diet and to consume sufficient energy overall. This is also pointed out by the small study of the (German) Federal Institute for Risk Assessment, on which we have published a statement. In addition to tips for healthy bones, you can find out more about the study design in this article (Menzel et al., 2021).
Overall, the dietary fiber, phytochemicals, vitamins and minerals abundant in plants, and the average low energy density, are associated with positive health effects. A vegan diet is usually rich in these ingredients and, if appropriately designed, can provide all essential nutrients. This is shown by the studies presented below.
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Vegan Nutrition and Nutrient Intake
Looking at the available studies, at first sight, nutrition societies come to somewhat different conclusions on the extent to which a vegan diet can meet nutrient needs: Both the Italian and American nutrition societies state in their position papers that a well-planned vegan diet can meet the nutrient needs of all groups of people, and thus it can be beneficial to health (Agnoli et al., 2017; Melina et al., 2016). In update to their position on vegan diets, the German Nutrition Society (DGE) does consider the study situation to be insufficient to recommend a vegan diet during pregnancy and lactation as well as for children and adolescents (Richter et al., 2020). Furthermore, in their original position paper from 2016, they emphasize for all population groups that a targeted food selection and, above all, supplementation with vitamin B12 are necessary (Richter et al., 2016).
Most vegans are now aware of this. This is shown, among other things, by a current German study in which most of the vegans studied took a vitamin B12 supplement. The study suggests a partial nutritional deficiency of iodine, iron and calcium in the vegan participants. On average, the vegans got all other nutrients examined (including vitamin B12 and vitamin D) in sufficient quantities, and their good blood lipid levels stood out (Weikert et al., 2020).
That a vegan diet can work for children and adolescents is shown by researchers who have developed a “meal planning system” for these age groups. Parents who follow it can ensure that their children consume all nutrients in quantities meeting the recommendations (Menal-Puey et al., 2019). Athletes, too, can consume all the nutrients important to them in sufficient amounts to build muscle or prevent muscle breakdown as they age (sarcopenia). A recent review and a new randomized control study with 19 vegans (Lim et al., 2021; Hevia-Larraín et al., 2021), among others, show that plant protein is probably in no way inferior to protein from animal sources in this respect, provided that athletes consume at least 1.6 g per kilogram of body weight.
Alexandra Foscolou and colleagues conclude in their review of observational studies that a plant-based high-protein diet is recommended for individuals over 50 years of age for healthy aging (Alexandra Foscolou et al., 2019). Other researchers note that adequate intake of protein and especially the important essential amino acid leucine may become problematic on a vegan diet. Those who pay attention to this may benefit from a vegan diet as they age (Lonnie et al., 2018).
Thus, the studies show that with a thoughtful vegan diet focusing on the potentially critical nutrients, through supplementation if necessary, individuals in all populations can likely meet their needs.
Studies on Ecological Aspects
The production of products of animal origin is very resource-intensive. Factors that contribute to this are, for example:
- animal feed cultivation, production, transport, storage, feeding
- animal husbandry, animal care
- animal transport
- slaughter
- product processing, transportation, packaging, disposal
These process steps come with greenhouse gas emissions, water pollution, eutrophication and forest clearing, among other things. Often, instead of animal feed, food for human consumption could be grown. Since plant-based food production eliminates most of these steps, it is on average more sustainable than animal-based products (Fresán and Sabaté, 2019).
Many studies indicate that a vegan diet is associated with significantly lower greenhouse gas emissions, land use and, in some cases, water consumption. This is reflected in recent reviews. Accordingly, a vegan diet causes only half the greenhouse gas emissions of an average mixed-food diet (Fresán and Sabaté, 2019: Chai et al., 2019; Springmann et al., 2016).
However, it is difficult to calculate the impact of a vegan diet on the environment. Because just as with the nutrient supply, the following applies: There is not one vegan diet. Depending on the composition, the consequences for the environment are different. A diet that is as seasonal and local as possible, with predominantly unprocessed foods, comes out ahead. Very resource-intensive, imported from far away and highly processed plant-based foods are of course not a good choice from an ecological point of view and can in some cases be more harmful than some of animal origin. Scientists conclude that a plant-based mixed-food diet that focuses on seasonal and local foods and contains few products of animal origin can be similarly ecologically beneficial to a vegan diet (Chai et al., 2019).
One difficulty in considering ecological impacts of a vegan diet: The model calculations are based on different assumptions regarding environmental impacts and resource expenditures. Depending on the methodology, the results therefore differ. However, it is also clear that a change in thinking away from the mass consumption of products of animal origin towards a plant-based diet is necessary to be able to counter climate change with all its fatal consequences. The Planetary Health Diet developed by the EAT-Lancet-Commission envisages that 70 to 80 % less meat would have to be consumed in Europe in order to ensure the health of humans and the planet in the future. You can read more about the recommendations in the accompanying Summary Report.
How to Read and Interpret Research
Science vs. Pseudoscience
Many claims circulate about vegan nutrition. “Gurus” preach the clear superiority of this nourishing diet and warn about the alleged serious health dangers of a mixed diet. But if one looks at the statements more exactly, it becomes clear that the scientific research often says something else. On the one hand, the convincing drastic statements attract gullible people who, as a result, become open to a vegan diet. The problem about this is: The absolute and unscientific statements, which ascribe dramatic disadvantages to food of animal origin, frighten off many omnivores and let vegans appear unsympathetic.
That is why it is even more important that there are vegans who can make well-founded arguments that a vegan diet can be good for the environment and health. But how do you recognize that studies on vegan nutrition are not interpreted in a scientifically sound way? Warning signs that indicate unscientific work or pseudoscience include:
- lack of description of the methods
- no explanation of the results
- no sources or substantiated evidence for statements
- absolute statements
- one-sided approach
- personal experience reports are classified as evidence
As a convinced vegan, you may tend to refer to studies that (apparently) show the superiority of this form of diet and ignore others that assign positive aspects to foods of animal origin or a mixed diet. This is called “confirmation bias” or “cherry picking”. Those who work unscientifically look specifically for studies that support their convictions and ignore those that contradict them. Proper scientists, on the other hand, consider the totality of relevant studies and derive positions from them, and are open to criticism and new findings (Lee and Hunsley, 2015).
If one is not familiar with the subject and/or with scientific work, it is hardly possible to understand and interpret scientific studies. This increases susceptibility to pseudoscience. Especially for this reason, you should not rely on media articles or videos reporting results of a study. The authors of these have often only read the abstract themselves and have not understood the study, or are looking for a good headline. As a result, the conclusion is often far from what the research really says.
What you can look for in the interpretation:
- number of subjects
- population (health status, age, gender, …)
- methodology (meaningful parameters, measurement method…)
- statistical methods
- relevance and significance for the practice.
Under the heading “Discussion”, the authors address the weaknesses and strengths of their study, which can help to interpret it. However, you should not rely on this alone. This part also usually refers to similar studies, which you can look at more closely and which can provide you with further insight.
Important: Studies are not easy to read, and it is difficult to interpret them correctly without appropriate studying. Even scientists need a lot of time to go through them in depth. There are books and guides that teach interested people how to read studies. If you want to argue based on studies, these are highly recommended. Taking apart studies can be really fun and lead to aha-moments!
In Table 1, you will see clues on how to distinguish pseudoscience from science (adapted from Lee and Hunsley, 2015).
Table 1: Differences Between Pseudoscience and Science
| Pseudoscience | Science |
| no “Peer Review“ (review by other scientists) | is peer-reviewed |
| tries to confirm existing opinions | questions existing assumptions |
| when questioning statements, demands evidence that they are false | proves statements, absolute statements are rare |
| makes ad-hoc hypotheses as a response to negative research findings | can substantiate statements with a broad level of knowledge |
| lacks connection with basic or applied science | basic knowledge as a prerequisite, simply understandable explanation derived from it |
| refers to anecdotal experience | relies on controlled studies |
Different Study Types
It is difficult to infer health effects of individual substances or foods from research around (vegan) nutrition. Even if diet could be closely controlled (in reality, self-reported food intake often differs from actual intake), this remains: Your body is a complex system and both bodily functions and food substances are interrelated. There are other influencing factors such as health status, age, weight and gender. Scientists can “calculate” these with the help of statistical methods, but there are also limits to that.
There are different types of scientific studies. They all have their justification, but have different significance and often allow only limited conclusions. Above, you have already learned about the terms “epidemiological studies” and “observational studies” and that they cannot show causalities.
In Figure 1 you can see the so-called levels of evidence for assessing the quality of evidence (EUPATI, 2015).
Table 2 lists the characteristics of these studies, as well as strengths and weaknesses (adapted from examine.com, 2021).
| Study Type | Design | Strengths | Weaknesses | |
| Study Summaries | Meta-Analysis | Summarizes data from all existing literature on a given topic and analyzes it statistically | Can provide greater statistical power to a research question | Time-consuming and advanced statistical knowledge necessary |
| Systematic Review | Expert overview of the available evidence on a given topic | Can provide guidance in areas of limited research | Design differences can make study comparison of studies difficult | |
| Experimental Studies | Randomized Controlled Trial (RCT) | Participants are randomly assigned to an intervention or a control group | Randomization can help eliminate population bias (sampling bias, purposeful composition of the test group) | Can be very expensive and resource intensive |
| non-randomized controlled trial | Participants are assigned to an intervention or a control group | Participants can be assigned to a treatment in a blinded manner | non-randomized | |
| Observational Studies | Cohort Study | A group is followed for a certain time and their habits and risk factors are recorded | May be easier to conduct than an RCT | Can take years to conduct |
| Case-Control Study | Comparison of the history of groups with and without a specific disease or health outcome | Helps identify potential risk factors | Often confounded by recall bias | |
| Case Report | Detailed account of individual cases | Helps identify new trends | Cannot be generalized | |
Table 2: Overview of Characteristics, Strengths and Weaknesses of Different Types of Studies (adapted from examine.com, 2021)
Even with meta-analyses and systematic reviews, it is not enough to read the abstract and/or the result. It is important to look more closely at which studies were included and excluded, for what reason, and how they were analyzed and interpreted.
As a rule, no absolute statements can be made, especially on the subject of nutrition. Research is subject to numerous difficulties, since diet and its effect on health are influenced by numerous factors. In this review, scientists address some aspects that need to be considered in so-called “evidence-based practice” in nutrition-related studies. These include:
- the assessment method of dietary intake
- the way the intervention is controlled
- which parameters are used
- which form of study is chosen and how it is interpreted
(Laville et al., 2017).
You see, referring to single studies on vegan diets alone is not enough to say that this form of nutrition is “clearly scientifically proven to be better.” The studies refered to must be well conducted, contextualized and interpreted correctly. Our overview can give you some initial insight.
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Our 10 Guidelines for Evaluating Statements in a Scientific Context
You probably have now realized that reading and interpreting scientific studies is a complex challenge. Putting these studies into a larger context is even more challenging. Large nutrition societies have their teams of scientists who specialize in the respective topics for that.
Some of the claims you come across in relation to diet seem reasonable and convincing at first, but lack practical confirmation. Even if there are several studies on similar topics, it is usually not possible to synthesize them, because of different target groups or different methodological approaches and analyses.
To help you deal with “expert opinions” that cause you doubt and confusion, we have developed ten guidelines.
- Consider the interests of the broadcaster: Who is making the claim and what motives, such as financial interest or power-seeking, might be behind it? Consider whether the statement is likely to arouse fears, and to what extent advertising for products, such as dietary supplements, plays a role. Consider whether there is any collaboration with companies that might benefit from the statements made. Find out if the person has any real scientific training in the field. Assess whether their approach is demeaning to others, how they deal with possible criticism, and whether their arguments are factual and sound.
- Review the actual study claims: Do your own research on the studies cited and see if their results are consistent with the expert’s claims. Pay particular attention to whether the results are both significant and relevant to everyday practice. Check to see if any results were omitted that might contradict the expert’s clear statements. Also look at the discussion section of the study: What are the strengths and weaknesses, and how do the results fit into the overall scientific context?
- Review the study design: Is what is being said based on single case studies, mechanistic studies, or animal studies? Has it only been tested on a small group of participants or participants with special characteristics? If so, it is important to remember that no general conclusions can be drawn.
- Analyze the study results: Results from different publications often cannot be combined directly. It is important to check them carefully: Are the conclusions really valid?
- Comparison with official nutritional societies or other renowned scientists in the field: What statements, recommendations, and viewpoints do they represent?
- Fact vs. Hypothesis: Have you found evidence of truly relevant effects or benefits, such as health or life expectancy? Or are these statements based on conjecture derived from individual parameters?
- Overwhelming citations: If numerous studies are cited which hardly can be thoroughly reviewed, making them difficult to refute or discuss in depth, this may hinder or even prevent critical review.
- Complex explanations and rationales: Be careful not to be fooled by extensive and rhetorically clever explanations and justifications in response to potential criticism!
- Absolute statements: Statements that leave no room for relativity raise doubts about their scientific validity, especially in the field of nutrition.
- Personal experiences: Statements that use personal or subjective experience as evidence are not consistent with the principles of scientific competence.
Conclusion
The question of whether the vegan diet is healthy or unhealthy, or better for the environment than a mixed diet, cannot simply be answered by studies with a yes or no answer. What the research on vegan nutrition does show, however, is that the issue must be viewed in a differentiated way: The effects depend to a large extent on how the diet is implemented. However, it is well documented that a large proportion of plant foods in the diet is associated with positive aspects for health and the environment.
When interpreting studies related to vegan diets, don’t be swayed, intimidated, or tempted into purchasing expensive supplements by statements that originate from non-scientific work. Our 10 guidelines can give you security.
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