Vegan Diet
Vegan diets exclude all animal products. Veganism has rapidly increased in popularity, although randomized trials are still few in number. Trials do show promise for a variety of outcomes, from weight loss to autoimmunity, although comparisons to other popular diets are rare. A vegan diet should be balanced and low in processed foods (e.g. a "Whole Food Plant-Based diet") for optimal results.
Last Updated:October 13, 2024
1.
Why do vegans avoid animal products?
Health, animal welfare, and environmental protection are the three most commonly stated motives for eating a vegan diet.[4]
However, the motives for eating vegan can vary quite a bit and also include economic, ethical, and hedonic reasons.[61]
Also, veganism may be politically motivated, aiming to bring societal change.[62]
The most common reason seems to be animal welfare, as evidenced by many surveys.[4][62] [61][63] For instance, a study from 2016[4] found that most vegans mention animal welfare (89.7%) as the most prominent motive, followed by health (69.3%) and environmental reasons (46.8%). Overall, protecting animals seems to be the key determinant of a vegan’s self-identification.
But there is also a political motivation to veganism. A survey from 2011 of 648 vegans in Switzerland found that a large majority of vegans report being politically active and motivated, aiming to induce societal change.[62]
There are also differences between a dietary vegan and an ethical vegan. While dietary vegans eat a vegan diet but may use animal products in other areas of their lives (e.g., clothing and toiletries), ethical vegans reject the commodification of animals altogether. Nevertheless, the distinction between these two groups is not as clear-cut as their definitions may imply. The concepts of dietary and ethical vegan often overlap.
2.
Do vegan diets cover all essential nutrients?
Contrary to common belief, a vegan diet can cover all essential nutrients — if it is well planned and supported by supplementation. The turmoil around vegan-related nutrient deficiencies revolves around a handful of manageable but critical micronutrients: the vitamins B2, B3, B12 and D, and the minerals zinc, calcium, iodine, and selenium. If properly designed, a vegan diet supported by supplementation and fortified foods can meet all these nutrients in sufficient amounts.
Following a vegan diet may increase the intake of certain nutrients while decreasing the intake of others.[13] This can have both positive and negative consequences.
On the one hand, vegan diets tend to be lower in calories, saturated fat, cholesterol, long-chain n–3 (omega-3) fatty acids, vitamin D, calcium, zinc, and vitamin B12.[13]
On the other hand, vegans tend to consume more iron, folic acid, vitamin C, vitamin E, magnesium, dietary fiber, and phytochemicals.[64]
While increasing healthful nutrients (e.g., vitamins, minerals, phytochemicals) and reducing those often associated with poor health outcomes (e.g., excess saturated fat) can certainly be beneficial, missing out on even one or two critical nutrients can be detrimental.
According to the current body of literature, however, there are two reasons why a vegan diet does not necessarily lead to nutrient deficiencies and its associated health problems.
First, nutritional deficiencies arise because of a generally poor diet, not because of the exclusion of animal products per se. Any diet, whether vegan, vegetarian, or omnivore, can lead to nutritional shortcomings if not properly designed.
The second reason why vegan diets are not necessarily dangerous is that suboptimal nutrient intakes can easily be overcome with a well-planned choice of plant-based foods rich in essential nutrients that are critical for vegans (i.e., those nutrients that are often found to be deficient in vegans).
Hence, individuals eating a vegan diet should be aware of the most common dietary deficiencies in order to prevent them and enjoy the health benefits of a vegan diet. The snag, of course, is that meeting all these nutrient requirements can be difficult. The table below lists the most important nutrients to consider in the context of a vegan diet, including their risk of deficiency and recommended plant sources to cover them.
A major exception is vitamin B12, which is not readily available in most plant-based foods (except fermented foods, which are, however, not sufficient to obtain healthy vitamin B12 levels).[65] This is why vegans need to supplement vitamin B12 for optimal health outcomes.
Overall, a well-planned vegan diet supported with vitamin B12 supplementation (and other potentially critical nutrients, depending on the specific diet) can meet all nutrient requirements.
| Nutrient | Risk of deficiency\* | Plant-based sources with the highest nutrient amounts | Do vegans need to supplement? | Comment |
|---|---|---|---|---|
High
|
Yes
|
While animal studies[68] indicate that “natural” vitamin B12 is better for restoring a low B12 status than supplementation, human observational studies[65] suggest that fermented and fortified foods may improve vitamin B12 levels slightly, but not enough to obtain optimal levels. Overall, a mixture of both could yield the greatest benefits.[65]
| ||
Vitamin D3
|
High
|
Yes, probably
|
Depends on exposure to the sun, distance from the equator, and skin pigmentation.
| |
Medium
|
No, not necessarily
|
Algae vary widely in iodine content, making overdosing a severe risk.**
| ||
Fish oil: EPA & DHA
|
Medium
|
Algal oil[74]
|
Yes, probably
| |
Low
|
Nuts (e.g., peanuts, cashews) and seeds (e.g., sesame, pumpkin, flax, and chia seeds)
|
No, not necessarily
|
Eating enough nuts and seeds will cover the recommended daily zinc intake.
| |
Medium
|
Brazil nuts***,[78] peas, grains, lentils, chickpeas
|
No, not necessarily
|
The selenium content of foods varies depending on the soil composition (0.1–10 μg of selenium per gram of soil) and country-specific fortification strategies.[79]
| |
Medium
|
Seeds (sesame, pumpkin, flax, hemp seeds, among others, nuts (almonds, hazelnuts), white beans, spinach, and lentils)
|
It depends
|
Iron absorption is influenced highly by other nutrients (e.g., improved by vitamin C and organic acids from fruits[80] and impeded by phytic acid[81] and polyphenols from coffee and tea).[82] Pregnant and nursing women have a 2–3 times higher iron need than non-pregnant and non-nursing women.[83] This increased iron need could be challenging to obtain via a vegan diet. | |
Low
|
Almonds, mushrooms, pumpkin seeds, oats, some fortified foods
|
No, not necessarily
| ||
Medium
|
Certain vegetables (sprouts, broccoli, kale, and bok choy), seeds (sesame, pumpkin, flax, hemp seeds, among others), nuts (almonds, hazelnuts), fortified plant beverages, and especially their fermented forms (e.g., soy yogurt)[86]
|
No, not necessarily
|
Consuming natural or fortified foods rich in calcium is sufficient to meet daily calcium intake requirements. Thus, supplementation is not necessarily required. There is evidence to suggest that plant beverages fortified with calcium are as bioavailable as cow’s milk.[87] | |
Protein
|
Low
|
Soy, peas, lentils, nuts, seeds, vegan protein powder
|
For most people, no. For athletes, it depends.
| |
low
|
Legumes, lentils, nuts (cashews, peanuts)
|
No
|
According to the WHO, humans need about 30 mg of lysine per kg of body weight. Thus, an average person needs about 1.5–2.5 grams of lysine per day. A well-planned vegan diet can meet this need. Lysine is relevant for building muscle protein, hormones, and enzymes. Also, lysine is vital for optimal calcium and iron absorption. |
* The risk of deficiency was rated as high, medium, or low based on a systematic review from 2021[13] combined with guidelines and position papers of leading nutrition societies.[90][91][92][93] ** Iodine content varies widely between different types of typical commercially available seaweeds and also within these types (e.g., Kombu/Kelp ranges between 500–11,000 mg per gram of dry weight). *** The natural content of selenium varies highly in brazil nuts,[78] especially from the Amazon region.[78] Depending on its origin, a single Brazil nut provides 11–288% of the daily selenium requirement for an adult man (70 μg).[78] This high natural variation of brazil nuts increases the risk of overdosing significantly.[94] That said, only a small handful of brazil nuts (10–20 grams) should be consumed to cover the daily selenium requirement.
3.
Is a vegan diet appropriate for every stage of life?
There is a lot of controversy around this topic.[95][96][90][91][92][93]
Currently, it remains unclear whether a vegan diet is appropriate for all stages of life, especially pregnancy and childhood. Professional bodies sometimes disagree specifically about these stages of life.
For example, some nutrition societies consider a well-planned vegan diet appropriate for all stages of life[90][91] while others disagree[93][97] or don’t take a stance[92] — even though all position papers rely on the same body of evidence.
Some nutritional societies are cautious when recommending a vegan diet because they are concerned about nutrient deficiencies, which can arise if a vegan diet is unbalanced. Thus, families eating a vegan diet should be cautious that all essential nutrients are met, either through diet or supplements.
3.1
Is a vegan diet appropriate for pregnant and lactating women?
A vegan diet that covers all critical nutrients can be safe for pregnant and lactating women, as well as their children. However, meeting all nutrient requirements can be more difficult on a vegan diet, compared to an omnivorous diet. Pregnant and lactating women should consult a dietician and pediatrician before beginning a vegan diet in order to learn about the potential negative health effects that can occur as a result of nutrient deficiencies in order to protect themselves and their babies.
It is especially important to get a sufficient amount of all essential nutrients during pregnancy in order to support healthy gestation and promote optimal fetal development.[98]
Since a vegan diet – if not well designed – can increase the risk of nutrient deficiencies,[13] pregnant women who consume a vegan diet must be aware of their dietary needs to ensure their intake of all these key nutrients.[99]
Otherwise, nutrient deficiencies during pregnancy can lead to negative health effects for both the mother and the infant, such as preterm delivery, low body weight, and even birth defects (e.g., fetal malformations and neurological disabilities).[99] Also, inadequate levels of essential nutrients during pregnancy can lead to chronic conditions later in life, such as obesity, diabetes, and CVD.[100]
In contrast to the negative effects stemming from malnutrition, there is evidence to suggest that a healthy vegan diet can offer positive maternal and pediatric benefits during pregnancy.[101][102] For example, a vegan diet is generally high in vegetables, fruits, and grains, which can benefit both the mother and the child. Potential maternal benefits include lower than average rates of mortality, preeclampsia, preterm birth, cesarean section, gestational diabetes, and postpartum depression. For infants, potential beneficial effects include a lower risk of pediatric asthma, eczema, and infant leukemia.
However, it bears repeating that these potential benefits hinge on getting enough of all essential nutrients, which, again, can be challenging in the context of a vegan diet. Thus, talking to a dietitian and pediatrician can be helpful. However, vegan women often perceive their pediatrician as skeptical or against a vegan diet,[103] which can be an impediment to effective communication.
Nevertheless, consulting a dietitian and talking to your primary care pediatricians about your decision to eat a vegan diet during pregnancy and lactation is warranted to ensure the safety of you and your baby.
3.2
Is a vegan diet appropriate for infants and children?
The current body of evidence is thin and thus many nutrition societies are reluctant to recommend a vegan diet for infants and children. However, emerging evidence suggests that a vegan diet can meet all nutrient requirements in children older than 1 year if all critical nutrient needs are met. However, there is currently no evidence supporting the safety of a vegan diet for infants. Overall, a vegan diet requires a strong awareness of critical nutrients and how to meet them. Parents should consult with a dietitian and pediatrician and do regular blood testing to monitor the nutritional status of their children.
Infancy and childhood are phases of rapid growth and development, which are accompanied by an increased nutrient need.[104][105][106]
Experiencing nutrient deficiencies during these crucial stages of life can lead to long-term negative health consequences.[107] This is for two reasons: Not only are nutrient requirements higher, but the potential harm of nutrient deficiencies is also higher at younger ages, while the body is still growing. In fact, the source, quantity, and quality of nutrients strongly influence not only weight and height, but also neurocognitive and psychomotor development.[96]
Consequently, some leading nutrition societies do not recommend a vegan diet for infants or children.[93][97] Generally, two main reasons are given for this position: First, the existing evidence seems insufficient to prove that vegan diets are safe for infants and children. Second, the higher risk of nutrient deficiencies may create an unnecessary risk that does not justify the potential benefits of a vegan diet.
That said, new evidence is emerging that challenges these statements. For example, the German VeChi study was a large cross-sectional study of 430 vegan, vegetarian, and omnivore children.[108][109] The study found that vegan children 1–3 years old had higher fiber and carbohydrate intakes but similar energy intakes as well as similar growth as vegetarian and omnivore children.[108] The researchers also reported that vegan children ages 6–18 had no higher risks of nutrient deficiencies than vegetarian or omnivore children.[109]
However, other evidence contradicts the findings of the VeChi study in other populations. For instance, a cross-sectional study of 187 Polish children reported that — even though a vegan diet was associated with a healthier cardiovascular risk profile — vegan children had an increased risk for nutritional deficiencies and had a lower height and bone mineral density than omnivorous children.[110] The most common deficiencies among vegan children were iron and vitamin B12. Also, vegan children in this study had lower HDL-C levels than omnivorous children.[110]
Another study in 40 Finnish children (median age: 3.5 years) found that they were deficient in vitamin A and were borderline sufficient in vitamin D. Also, vegan children had lower DHA levels and distinctly different bile acid biosynthesis and phospholipid balance than omnivorous children, but the clinical relevance of these differences is unclear.[34]
To conclude, there is no evidence that a vegan diet is appropriate for infants. However, there is evidence a vegan diet can be safe for children if all critical nutrient needs are met, ensuring normal growth and development. The snag, of course, is that meeting all nutrient requirements can be difficult. Most studies indicate that not all vegan parents are capable of ensuring appropriate intake of all critical nutrients for their children. Since children have higher nutrient needs, the costs of nutrient deficiencies can be serious, which may explain the reluctance of some nutrition societies to recommend a vegan diet for children. Hence, it is strongly recommended to ensure regular medical and expert dietetic supervision when weaning an infant onto a vegan diet.
4.
Can a vegan diet reduce the risk of heart disease?
A well-designed vegan diet may lower cardiovascular disease (CVD) risk to a small extent — with a caveat. On the one hand, vegan diets reliably lower the two most important CVD risk factors — lipids and blood pressure — with observational evidence linking vegan diets to lower CVD risk. On the other hand, the associations between vegan diets and CVD were found to vary between countries and specific CVD outcomes. More research is needed to explain these differences. Fortunately, several registered trials are on the way, so stay tuned for updates.
The major limitation to answering this question is that there are currently no clinical trials on how vegan diets affect outcomes in people with CVD (e.g., adverse events and mortality rates). This lack of clinical evidence is understandable, since these kinds of dietary intervention trials (i.e., well-controlled, long-term, and measuring hard outcomes) are difficult to conduct.
Instead, the evidence is limited to how vegan diets affect CVD outcomes and risk factors in high-risk populations (e.g., obesity, diabetes, and hypertension) over short periods of time (weeks to months). To date, most studies revolve around lipids and blood pressure (BP), two of the strongest CVD risk factors.[111]
Vegan diets may provide a slight benefit in terms of lipids and triglycerides. Two meta-analyses[32] found that a vegan diet, compared to a regular diet, improved lipid profiles in adults at risk of CVD, but only to a small extent and not for all measures. A vegan diet can reduce total cholesterol and low-density lipoprotein cholesterol (LDL-C) by 9–12 mg/dL, respectively, which is equivalent to a CVD relative risk reduction of 7%.[112] However, a vegan diet did not affect triglyceride levels (or even slightly increased them in one meta-analysis[32]) while decreasing high-density lipoprotein cholesterol (HDL-C) — the “good cholesterol” — by about 3 mg/dL. In sum, the current evidence suggests that vegan diets may reduce CVD risk to a small extent, mostly by improving two specific lipid risk factors: total cholesterol and LDL-C.
The effect of vegan diets on blood pressure has also been studied. While a Cochrane review from 2021[32] found no effect, three earlier meta-analyses[35][36][37] showed that a vegan diet can lower systolic and diastolic blood pressure by 2–4 mmHg, respectively, compared to a regular diet. These changes in blood pressure would be associated with a relative risk reduction for stroke mortality of 10–20%.[113] However, larger and longer studies are needed to substantiate these findings. The current body of evidence is not compelling enough to draw a generalized conclusion regarding the ability of vegan diets to effectively lower blood pressure.
Lastly, observational evidence on hard clinical outcomes also suggests that vegan diets slightly reduce CVD risk.[9] However, the association between vegan diets and CVD varies between countries[9] and specific CVD outcomes.[114] For instance, the EPIC-Oxford study[115] found that vegans and vegetarians (treated as one group due to the low number of vegans in the study) had a lower risk of ischaemic heart disease, but a higher risk of stroke than meat-eaters. Overall, more research is needed to explain these inconsistent findings.
5.
Can a vegan diet prevent cancer?
According to a large body of observational evidence, adopting a vegan diet may reduce the risk of developing cancer by 14–19%. However, there are several caveats with the current evidence:
- The evidence is inconclusive regarding specific cancer types.
- The evidence is based on small sample sizes of vegans.
- Correlation does not equal causation. Overall, larger cohort studies and ideally clinical trials are needed to shed more light on the relationship between vegan diets and cancer.
Adopting a vegan diet may reduce the risk of developing cancer to a small extent. This claim is primarily supported by a large body of observational evidence. A meta-analysis of 96 observational studies found that a vegan diet was associated with a 15% lower relative risk of developing cancer.[10] This finding is echoed by large prospective studies in which pooled analyses of vegan cohorts were performed. For instance, pooled analysis of the Oxford Vegetarian Study and EPIC-Oxford study showed that vegans had a 19% lower risk of developing cancer than omnivores, even after adjusting for various confounders (e.g., sex, BMI, smoking, alcohol consumption, and physical activity levels).[116] Similarly, vegans from the Adventists study had a risk-adjusted 14% lower cancer risk than meat-eaters, taking into account confounders such as BMI, race, education, smoking, alcohol consumption, and family history of cancer, among others.[117]
However, there are several caveats with the current research. First, the observational evidence is inconclusive regarding specific cancer types. Also, the situation is unclear for cancer survivors, where vegans show similar cancer mortality and recurrence rates as omnivores.[118] The reason for these inconclusive results is closely related to the second caveat: low sample sizes and low statistical power. For example, an analysis of the vegan cohort from the Adventists study found a reduction in breast cancer risk of 22%.[119] However, the statistical power was too low, leading to the data being compatible with a wide range of possibilities, including no correlation at all. Studies with larger cohorts are required to shed more light on the effect of vegan diets on specific cancer types.
The third caveat is, again: correlation does not equal causation. In other words, just because vegan diets (A) are linked to a lower cancer risk (B) does not necessarily mean that A causes B. The observed correlations could also be explained by confounding factors. For instance, the risk reduction of 19% found for the vegan cohort from the Oxford Vegetarian and EPIC-Oxford study was no longer statistically significant when the researchers adjusted for BMI, indicating that body weight may be a confounder. This makes sense because BMI is also associated with higher risk of cancer,[120] in that each 5-unit increase in BMI is associated with a 6% higher risk of total cancer in people with type 2 diabetes.[121]
In light of these caveats, it becomes evident that more evidence is needed. However, it’s important to note that conducting large-scale clinical trials for vegans to accurately assess cancer risk is very difficult. This is especially true when studying the incidence rates of rare cancer types in a relatively small population like vegans, who make up about 3% of the U.S. population, or about 10 million people. While observational studies have their limitations, they also provide the best evidence currently available.
6.
Is a vegan diet appropriate for type 2 diabetes?
A vegan diet may be an effective dietary intervention to prevent and treat type 2 diabetes (T2D), as supported by accumulating evidence from cohort and clinical studies. On the one hand, vegans show a 49–77% reduced risk of developing T2D in the first place. On the other, randomized controlled trials demonstrated that adopting a vegan diet improves glycemic control, β-cell function, and insulin resistance. A vegan diet may even be better than a conventional diabetes diet for glycemic control. Future studies comparing a vegan diet to a DASH or Mediterranean diet would be desirable and insightful.
6.1
Can a vegan diet prevent or treat type 2 diabetes?
Diabetes is widespread: 13% of U.S. adults have diabetes.[122] By 2050, this number is projected to climb to 21–33%,[123] mostly because of T2D.[124]
Adopting a healthy vegan diet could be a useful strategy to not only prevent T2D in the first place, but also to treat already existing T2D.[125] These claims are based both on cohort and clinical studies.
Large-scale cohort studies showed that vegans have a 49–62% reduced risk of developing T2D compared to people eating an omnivorous diet.[126][127] These benefits remained even after the researchers accounted for age, BMI, gender, smoking status, and other lifestyle factors. These findings suggest a protective role of vegan diets for T2D, but randomized controlled trials would be useful to prove causality.
Vegan diets may also be an effective dietary intervention for people with T2D to improve their treatment. In a number of randomized controlled trials, vegan diets were shown to improve several risk factors for T2D: body weight,[20] blood pressure,[35][36][37] lipid profiles,[32][128] glycemic status,[26][27][28][29] β-cell function[22], and insulin resistance.[30][31][30]
6.2
How effective is a vegan diet compared to other diabetes diets?
A vegan diet is not the only diet that can potentially treat and prevent T2D. Several diets, such as the Mediterranean diet, the ADA diet (recommended by the American Diabetes Association) diet, and the DASH diet (Dietary Approaches to Stop Hypertension) are backed by a large and compelling body of evidence. How does the vegan diet compare to these established diabetes diets?
It’s currently difficult to compare a vegan diet to a Mediterranean diet due to the very thin and partly contradictory evidence around this topic. There are two interesting studies worth discussing, though. First, in a network meta-analysis of 56 trials from 2018, researchers compared the efficacy of common dietary approaches recommended for glycemic control in the context of T2D. While the Mediterranean diet was found to most likely be the best at lowering HbA1c and fasting glucose, the vegan diet likely outperformed the low glycemic index, high protein, and low-fat diets. Overall, the researchers concluded that the Mediterranean diet to be the most effective option.[129]
In contrast, the second noteworthy study, a 16-week randomized crossover trial of 62 adults with overweight, compared a vegan and Mediterranean diet directly and found the vegan diet to be superior for reducing T2D risk factors.[130] Why do the findings of these two studies seem to contradict each other? One reason is that the network meta-analysis included no direct comparisons between the Mediterranean and vegan diets, which may have added uncertainty to its estimates; the direct RCT was the first of its kind comparing a vegan and Mediterranean diet directly. Another possible explanation is that the two studies measured different outcomes. While the network meta-analysis examined HbA1C and fasting glucose, the head-to-head RCT analyzed HOMA-IR, and predicted insulin sensitivity and oral glucose insulin sensitivity. Notably, the vegan diet group in the RCT also lost a notable amount of weight, probably caused by a small caloric deficit in the vegan diet group, which was not found in the Mediterranean diet group.
Overall, it’s unclear whether a vegan or a Mediterranean diet is superior for treating and preventing T2D due to a lack of RCTs comparing the two diets head-to-head. One thing is clear, though: both diets can be beneficial for T2D, but the evidence for the Mediterranean diet is probably a bit stronger at this point in time.
How does a vegan diet compare to conventional diabetes diets? Evidence is mounting that a vegan diet could be at least as good, if not more beneficial. There is growing evidence to support this claim:
A meta-analysis of six RCTs and 255 participants from 2014 found slightly better glycemic control for a vegan diet over a conventional diabetes diet.[28] Since then, this finding has been echoed by new evidence in more diverse populations. For instance, a 12-week RCT in 93 Korean adults with T2D found that vegan diets reduced HbA1c (glycated hemoglobin, which is a measure of long-term glycemic control) more effectively than a conventional diabetes diet.[26]
In conclusion, there is a growing body of evidence that a vegan diet is not only beneficial for treating and preventing T2D but could, in some cases, even surpass the benefits of traditional diabetes diets, such as the DASH and Mediterranean diet. However, it’s important to note that the classic diabetes diet still has a more solid body of evidence to support its use for diabetes management. This might change in the future, though, as new evidence is brought to light. Stay tuned for updates.
7.
Can a vegan diet help you lose weight?
Yes, a vegan diet is a useful tool to lose weight and shed some fat.
This claim is backed by a large amount of clinical evidence, summarized in a meta-analysis from 2016.[20] On average, a vegan diet led to an average weight loss of 2–3 kg after a study duration of 10–92 weeks.
Strikingly, a vegan diet can promote weight loss even without the burden of counting calories.
For instance, an 18-week randomized controlled trial of 319 participants with type 2 diabetes found that an unrestricted vegan diet led to a weight loss of 2.9 kg.[52] Among those in the vegan diet group who completed the study, weight loss was even greater (–4.3 kg).
These results raise the question: how can a vegan diet promote weight loss even when food intake was not restricted? Put differently, why did participants on a vegan diet lose weight even though they could eat as much as they want? Here are two possible explanations:
The first is that a vegan diet creates an unintentional caloric deficit. By now, numerous studies have shown that — even without being instructed to restrict their food intake — participants on a vegan diet consume fewer calories than those on a regular diet.[13]
This, in turn, can be explained by plant-based foods being more satiating than animal products,[131] given their lower caloric densities and higher dietary fiber content.[132]
The second reason why an unrestricted vegan diet can promote weight loss has to do with the digestive system, which does not treat all foods equally.
The energy it takes for your body to digest, absorb, and metabolize food is higher for plant foods than for animal products. This is called the thermic effect of food.[133] For instance, one randomized controlled trial found that a vegan diet can improve the thermic effect of foods by up to 14%.[30] As a result, a small chunk of the calories you consume is “lost to digestion.”
Calories from plants are also not as accessible as those from animals during digestion because of the food matrix.[134] In contrast to animal cells, which are encased in easily digestible lipid membranes, plant cells have robust cells walls made up of fiber (mostly polysaccharide polymers such as cellulose, hemicellulose, and pectin) that acts as an indigestible physical barrier.[135] Consequently, some of the calories you consume as structurally intact plant foods remain inaccessible to your digestive system — no matter how well you chew.[136]
To conclude, a vegan diet is a useful tool to lose weight because it creates a caloric deficit. This deficit can be achieved without counting calories because plant foods are more satiating, increase the thermic effect of food, and hide some of their calories from the digestive system.
8.
Do vegans have weaker bones and joints?
It’s possible, but not definite.
If sufficient amounts of calcium, protein, and vitamin D3 are consumed, vegans have the same fracture risk as non-vegans.[24]
However, vegans often have a reduced calcium intake[19] and bone mineral density (BMD) than omnivores,[23][56] which could explain their potentially increased fracture risk.[25]
These claims are based on observational evidence. A meta-analysis of 17 cross-sectional studies with 13,888 participants[23] found that vegans had a 3–4% lower BMD than omnivores. Also, meta-analyses report a 30%[24] and 43%[25] higher relative fracture risk for vegans, compared to omnivores. This sounds like a lot, but keep in mind that this comparison involves relative risks.
Here’s the good news, though: sufficient calcium intake can solve this problem. When the researchers of the EPIC-Oxford study only analyzed vegans with sufficient calcium intakes (more than 525 mg/day), the differences in fracture risk disappeared.[24]
Consequently, vegans should optimize their calcium intake toward the recommended daily intake of 700 mg to maintain bone health. In addition, a sufficient intake of protein, vitamin D3, and vitamin B12 is recommended to improve calcium absorption.[137][138]
One way to achieve this is by drinking calcium-fortified plant drinks (see sidebar), as well as eating more legumes and meat alternatives. The Adventist Health Study-2 reported that more frequent intakes of legumes and meat analogs reduced the risk of hip fracture, with a greater protective effect than that of meat.[139]
In summary: An unbalanced vegan diet will negatively affect bone health and increase risk of fracture, but these undesired effects can be avoided by ensuring a sufficient intake of calcium, protein, vitamin D3, and vitamin B12.[137][138]
9.
Is soy bad for you?
Soybeans are an excellent source of plant protein and contain a number of vitamins, minerals, antioxidants, and phytochemicals (especially isoflavones).
However, rumors have demonized soybeans and processed soy products for (allegedly) lowering men’s testosterone levels, impairing thyroid function, and raising the risk of Alzheimer’s disease.
How much truth is in these rumors?
The short answer is: not that much.
The long answer is: the supposedly negative health effects of soybeans seem to stem from exaggerated claims based on early preclinical and observational evidence,[142][143][144][145][146][147][148] which have later been disproved by a large meta-analyses of randomized controlled trials.[149][150][151][152][153][154]
9.1
Does soy decrease sexual hormones in men?
The idea that soy decreases men’s sexual hormones came from in vitro,[142] animal studies,[143] and small clinical trials using high concentrations of isolated isoflavones in the early 2000s.[144] These studies found that isoflavones can bind to estrogen receptors and thus it was hypothesized that they could negatively affect the sexual hormones of men.
However, this hypothesis is challenged by a vast trove of new clinical evidence finding no such effects. For instance, a meta-analysis from 2010 of 15 clinical trials[149] and a meta-analysis from 2021 of 42 clinical studies[150] examined whether soy intake affects the sexual hormones total and free testosterone, estradiol, estrone, and sex hormone-binding globulin in men. Both meta-analyses found that neither soy nor isoflavone intake affects male reproductive hormones, regardless of dose and study duration. Given the low heterogeneity among studies (meaning, the studies were all very similar in design and outcome), the findings of these meta-analyses are very robust.[150]
And so, after almost two decades, the tide has turned: the current clinical evidence clearly suggests that soy does not seem to affect men’s reproductive hormones.
9.2
Does soy impair thyroid function?
Cell culture and animal studies suggest that some compounds found in soy may affect thyroid function.[145][146]
Yet, human studies have shown little to no adverse effects, especially in people with healthy thyroid function.[151]
For instance, a meta-analysis of 18 studies investigated the link between soy consumption and thyroid function and found that soy had no effect on the thyroid hormones.[151]
However, the role of soy consumption in other populations is less clear. For example, women with subclinical hypothyroidism showed a 3-fold higher risk of developing overt hypothyroidism when supplementing soy phytoestrogens.[155] Interestingly, though, soy phytoestrogen supplementation also reduced insulin resistance, systemic inflammation, and blood pressure in this population.
More comprehensive studies are needed to investigate if soy consumption is safe for populations with existing diseases.
9.3
Does soy impair cognitive function and increase the risk of Alzheimer’s disease?
No, soy consumption does not seem to impair cognition nor increase the risk of developing Alzheimer’s disease. Even though early cohort studies from the 2000s found that soy products are linked to lower cognitive function, clinical trials from the last two decades found no such associations. Quite on the contrary, the newer clinical evidence even suggests that soy consumption, especially soy isoflavones, may increase cognitive function. More studies are needed to verify these findings since the form, dose, and duration of soy consumption varied widely between the studies.
The idea that soy consumption has a negative effect on brain aging and overall cognitive abilities first arose in the early 2000s, when observational evidence was published showing that the consumption of tofu is linked to lower cognitive abilities in midlife Japanese adults[147] and worse memory in elderly Indonesian adults.[148]
One possible explanation for these findings proposed at the time was that some soy products may contain increased amounts of aluminum,[156][157][158] which, in turn, is hypothesized to increase the risk of developing Alzheimer’s disease.[159][160] However, the link between aluminum and Alzheimer’s disease is still controversially discussed in the literature even 100 years after the hypothesis was first introduced.[161][162]
Be that as it may, clinical studies conducted in the last two decades have vastly challenged the notion that soy consumption causes cognitive impairment.
A 2007 review of eight clinical trials found four studies with positive results and four studies with no significant results in terms of the effect of soy isoflavones on cognitive abilities.[152]
Furthermore, a 2015 meta-analysis of 10 clinical trials, with just over 1,000 participants, concluded that dietary supplementation with soy isoflavones significantly improves cognitive abilities in postmenopausal women.[153]
Finally, a recent meta-analysis from 2020 of 16 clinical trials and 1,386 total participants found that soy isoflavones improved memory and cognitive function in midlife and older adults.[154]
However, a significant limitation is that the intake, duration, and form of administration varied widely among the studies. These influencing variables and confounding factors must be taken into account in future studies.
In summary, the current evidence seems sufficient to conclude that soy consumption does not lead to cognitive decline nor Alzheimer’s disease. In contrast, the current clinical evidence is leaning toward the hypothesis that soy consumption may even increase cognitive function. However, these clinical trials are mostly short-term and need to be confirmed by longer-term follow-up studies.
10.
How does a vegan diet affect athletic performance?
There is a small amount of evidence that suggests athletes following a well-planned vegan diet can achieve all their nutritional needs and enjoy similar strength and endurance exercise performance as athletes on an omnivorous diet. However, contrary to what some researchers may boldly claim, there is no evidence that a healthy vegan diet results in superior sports performance than a healthy omnivorous diet. Even though there are cross-sectional studies showing vegan athletes outperforming their omnivorous counterparts, these observational findings do not prove a cause-effect relationship. For this reason, randomized controlled trials are warranted to shed more light on this topic.
According to the documentary Game Changers, vegan diets lead to superior athletic performance compared to omnivore diets. Is this true?
The blunt answer is: The documentary overstates what little evidence there is on the matter.
The long answer is: There is currently no convincing evidence that vegan diets are in any way superior to omnivorous diets in the context of athletic performance, assuming that both diets are healthy and equal in calorie and protein intake.[88]
However, don’t mistake the ‘absence of evidence’ for the ‘evidence of absence’. Even though the evidence is limited, it deserves to be considered.
10.1
Can adopting a vegan diet influence sports performance or body composition?
There are two case studies that have investigated the effects of adopting a vegan diet on an athlete’s sports performance and body composition. One involved an elite male football player and had a duration of 14 weeks[163] and the other involved a powerlifter and had a duration of 6 weeks.[164]
Overall, both case studies showed that the athletes were able to meet their nutrition targets and maintain their sports performance on a vegan diet. Interestingly, while the football player experienced no large changes in body composition, the powerlifter reduced his fat mass by 24% and improved his lean mass by roughly 3%.
Even though the benefits the powerlifter experienced on a vegan diet look promising, there are two caveats to keep in mind.
First, it’s not possible to infer a direct cause-and-effect relationship from this case study.
Second, the athlete had considerable support in terms of personal nutrition consultations and the provision of meals and drinks to meet the challenges of transitioning to a vegan diet.
Besides these two caveats, the studies are sufficient proof-of-concept that, given the right support, planning, and training, vegan athletes can maintain — and even improve — their performance and body composition.
10.2
How do vegan athletes perform compared to their omnivorous counterparts?
There are three small-scale cross-sectional studies that compared vegan athletes with omnivorous athletes in the context of sports performance and may shed light on this question.
First, when researchers tested the maximal exercise capacity of 24 vegan and 26 omnivorous recreational runners, they found no significant differences between the two groups.[165]
Similarly, in a study of 9 vegan and 9 omnivorous healthy participants with similar physical activity, the researchers found no significant differences in sprint interval exercise performance.[166]
Finally, a study of 56 healthy young women found a superior sports performance for the vegan group. Even though body composition and physical activity levels were similar between the groups, women eating a vegan diet had a 7% higher estimated VO2 max and 38% higher submaximal endurance time to exhaustion than the omnivorous women.[167]
However, remember that observational studies do not prove a cause-effect relationship. Only randomized controlled trials can confirm causality. Or, even better, a meta-analysis of many randomized controlled trials.
However, some conclusions can be drawn from these case studies and cross-sectional evidence.
First, a well-planned vegan diet can achieve all nutritional needs and yield similar strength and endurance exercise performance as an omnivorous diet. Hence, a vegan diet can be a suitable alternative for ambitious athletes who don’t want to consume animal products.
Second, a vegan diet is not superior to an omnivorous diet in the context of sports performance. Even though two studies showed the beneficial effects of a vegan diet on sports performance and body composition, such observational evidence does not prove a causal relationship. Since only randomized controlled trials can do so, only more research can prove or refute this claim.
11.
Is a vegan diet healthy for the gut microbiome?
Gut health is challenging to study, in part because of the microbiome’s complexity. The effect of vegan diets on the gut microbiome is no exception. On the one hand, observational evidence has linked vegan diets to beneficial changes in gut microbiome composition, probably because vegan diets are high in fiber. On the other hand, the available evidence has significant limitations, such as low study quality and inconsistent findings across different studies. Moreover, few clinical trials have examined the causal link between vegan diets on gut health. Overall, it’s difficult to draw any conclusions at this point.
Transitioning to a vegan diet — as with every dietary change — will inevitably influence gut microbiota, which is highly dynamic and adaptable to even the smallest changes in diet. The question is: Will these changes make you healthier?
Some evidence suggests that a vegan diet may be healthy for your gut microbiome, potentially by improving the diverse ecosystem of beneficial bacteria in the gut. For instance, a systematic review of nine observational studies[41] found that eating a vegan diet is linked to an increased abundance of Bacteroidetes, one of the main phylums present in the human gut. Fueled by the promising results of cohort studies, clinical studies were conducted, confirming that vegan diets causally increase Bacteroidetes, as shown by a 16-week clinical trial in 168 adults.[42]
At first glance, these findings sound promising because a low abundance of Bacteroidetes is associated with various health problems, such as irritable bowel syndrome[168] and colorectal cancer[169] (the third most common cause of cancer mortality in the world[170]). Hence, it has been hypothesized that if vegan diets increase Bacteroidetes, it may also protect against other diseases associated with low Bacteroidetes by generally preventing gut dysbiosis.[171]
Even though these findings sound promising, the currently available evidence has significant limitations. Most importantly, the claim that vegan diets promote gut health by increasing Bacteroidetes is based chiefly on observational evidence. In other words, just because a vegan diet influences the abundance of certain bacteria in the gut does not mean that a vegan diet can prevent diseases that are distantly related to these gut bacteria. A causal impact of plant-based diets on diseases related to gut health has yet to be demonstrated.[18] Until then, any proposed pathways by which plant-based diets modulate the gut microbiome remain highly speculative.
To demonstrate such a causal relation, researchers first need to tackle another caveat lurking at the intersection of vegan diet and microbiome research: a lack of standardized and validated methods.[41] Without standardized procedures, the validity and reliability of the findings remain questionable. The lack of standardization could explain why most studies on how vegan diets influence the gut microbiome have been inconsistent. For instance, a recent systematic review of six cross-sectional studies from 2020[43] found no consistent association between a vegan diet and microbiota composition compared to a prudent omnivorous diet.
For all these reasons, it is currently difficult to associate a vegan diet and its changes on the gut microbiome with improved health status. Overall, the current body of evidence highlights the complexity of the gut microbiome and the need for more high-quality studies on this topic. Well-designed randomized controlled trials are needed to investigate the link between vegan diets and gut health.
12.
Do vegans live longer?
There is no definitive link between eating a vegan diet and life expectancy.
The limited amount of research that has focused on the relationship between vegan diets and longevity has provided mixed conclusions. A major limitation of this research is that most studies group vegetarians and vegans together, making it difficult to determine the exact effect of each diet on life expectancy.
In one study that analyzed vegans and vegetarians separately, a vegan diet was associated with an almost-significant 15% lower risk of all-cause mortality.[172]
Overall, more comprehensive studies are needed to shed more light on the association between veganism and longevity.
12.1
If vegan diets are healthier, then vegans should also live longer, right?
Vegan diets may indeed reduce the risk of cancer, heart disease, and type 2 diabetes, which could extend life expectancy.
However, research on the relationship between plant-based diets and longevity in prospective cohort studies has resulted in mixed conclusions.
While some studies found that vegan and vegetarian diets reduce the risk of dying from various diseases,[173] others found no association.[174][175]
A major caveat of these studies is that they group vegetarians and vegans together under the umbrella term of “plant-based diets,” which makes it difficult to determine the exact effect of each diet on life expectancy.
There is one prospective cohort study that did not pool vegans and vegetarians together, but analyzed both groups separately using the Adventist Health Study-2 cohort.[172] The researchers found that a vegan diet was associated with a 15% lower risk of all-cause mortality compared to omnivores, which was of borderline statistical significance. After adjusting for sex, the researchers found a significantly lower risk of dying for men (by 28%) but not for women, even though their diets did not differ in striking ways.[176]
This study was not the first one to uncover differences in the mortality rates of men and women. Previous studies analyzing the Adventist cohort have also indicated that men and women have different mortality risks (e.g., for ischemic heart disease).[177][178][179] The researchers speculated that the dietary factors that influence mortality may be different for men and women, which further complicates the picture.
To conclude, there is some evidence to suggest that a vegan diet may reduce mortality risk and thus extend life expectancy, but larger and longer trials are needed to confirm this relationship.
12.2
What mechanisms could explain the supposed longevity effect of a vegan diet?
Vegan diets are associated with many longevity-enhancing practices:[180]
- Calorie restriction[181]
- Protein restriction and avoiding certain amino acids (e.g., leucine and methionine)
- Gut microbiome-related improvements
- Risk reductions for life-shortening disease states
Even though some of the mechanisms explaining how these practices lead to increased longevity have been discovered, the biochemical details are not fully understood yet.[180]
For instance, one mechanism by which restriction of protein and the amino acids leucine and methionine may extend longevity is by inducing fibroblast growth factor 21 (FGF21). FGF21 is a potent longevity factor with many crucial properties that affect the aging process. For example, FGF21 regulates the interactions between energy metabolism and stress responses, two processes that are intimately intertwined in the complex regulation of longevity.[182]
However, the biochemical details of all the mechanisms through which FGF21 may promote longevity are far from clear.
Beyond the practices and mechanisms outlined above, it’s important to keep in mind that vegans live, on average, healthier lifestyles than omnivores, which could also expand their lifespan. For example, vegan diets are rich in nutritious foods like fruits, vegetables, whole grains, legumes, nuts, and seeds,[183] all of which may help vegans live longer.
However, it is important to acknowledge that some of these longevity-enhancing practices and mechanisms may not apply to older adults, and may even be harmful.
For instance, caloric or protein restriction is not recommended for older adults because it increases the risk of malnutrition and sarcopenia.[184] It’s crucial to eat enough protein and anabolic amino acids (e.g., leucine) in older age in order to sustain muscle mass.[185]
In summary, a handful of practices and mechanisms by which a vegan diet could promote longevity have been identified and are currently being researched. As such, more extensive studies are needed to pierce the complexity of this topic.
13.
Do vegans get enough protein?
Yes, vegans generally get enough protein through their diet. Even though an unbalanced vegan diet may sometimes lead to inadequate protein intake, studies find that most vegan diets provide enough protein above the recommended intake of 0.8–1.0 grams of protein per kilogram of body weight (approximately 50–80 grams for an average person). In addition, a vegan diet can cover all essential amino acid needs by including a variety of plant protein sources and using proper food processing like heating and extrusion, which increases the quality of plant protein by making it more digestible. Assuming that vegans consume a variety of plant proteins and cook them properly, plant protein can provide the same benefits as animal protein.
As a major source of nitrogen and amino acids, dietary protein is essential for the human body.[186]
However, not all amino acids are essential, and different protein sources vary widely in their content of these essential amino acids.[187]
For instance, some plant protein sources are low in lysine and the branched-chain amino acids (BCAAs) valine, leucine, and isoleucine (e.g., most cereals such as oats, rice, and wheat).[188] Also, some plant proteins, like peas and beans, may be slightly lower in the sulfur amino acid methionine.[189]
This raises the question of whether a vegan diet can provide all essential amino acids.
13.1
Can a vegan diet cover all essential amino acids?
The short answer is: Yes, a healthy vegan diet can cover all essential amino acids if it includes enough protein and complementary protein sources.[188] Alternatively, a variety of plant-based protein sources can be replaced with potato protein, one of the few plant-based protein sources that provides all essential amino acids.
The long answer is: It depends on the plant protein sources.
Since plant-based protein sources vary widely in their content and composition of essential amino acids, some could potentially be critical if a vegan consumes only certain types of plant proteins. The three amino acids showing the highest variation within plant protein sources are lysine, leucine, and methionine.[189]
For instance, vegans who get a high proportion of their protein from cereals may run short of the essential amino acid lysine.[188] However, studies show that consuming enough lysine may not be a problem for most vegans. For instance, in the EPIC-Oxford study[190] vegans consumed an average of 42 mg of lysine per kg of body weight, which is largely higher than the estimated average requirement of 30 mg/kg.[188]
Besides lysine, the essential amino acid leucine may be important for vegan athletes. An optimal intake of leucine (1–3 grams of leucine per meal, especially post-workout)[189] is recommended for optimal performance and recovery[88] because leucine acts as a molecular trigger to stimulate muscle protein synthesis.[191]
Athletes eating a vegan diet can get the recommended leucine intake from their diet if they choose the right plant protein sources for their meals. For instance, 15 grams of corn protein, 25 grams of potato protein, or 29 grams of either pea or soy protein will provide 2 grams of leucine.[189]
Another essential amino acid that vegans should be aware of is methionine. Methionine is typically lower in plant-based proteins (about 1.0%) compared to animal-based proteins (about 2.5%) and thus vegans may not meet their recommended requirements (about 1.6%).[189]
In addition, plant proteins that are high in methionine are often low in lysine, and vice versa. For instance, soy, microalgae, and pea contain little methionine but high amounts of lysine. In contrast, corn, hemp, and brown rice contain high amounts of methionine, but not lysine.[189]
That said, there are two ways to solve this problem: ingest more protein (if you only consume a few sources) or combine different plant protein sources with complementary amino acid profiles. For example, the low lysine or methionine content of corn, hemp, brown rice, soy, and pea protein can be overcome by eating two to four times more protein. Alternatively, combining corn, hemp, or brown rice (low in lysine but high in methionine) with soy, microalgae, or pea (high in lysine but low in methionine) will result in a more ‘complete’ amino acid profile.[189]
However, there is one plant-based protein source that can meet all essential amino acid requirements and may even be better than some animal proteins — the humble potato.
That may surprise you, but potatoes are a true all-rounder when it comes to protein. In fact, the essential amino acid content of potato protein (37%) is higher than egg (32%) and casein (34%).[189] Also, potato protein provides more lysine (8.3%) than egg (7.0%) and casein (8.3%).[189] And even though potato is high in lysine, it is one of the few plant-based sources that also provides sufficient amounts of methionine (1.6%).[189] Overall, 30 grams of pure potato protein contains as many essential amino acids as 25 grams of pure whey protein.[189]
However, one drawback with using potatoes as a protein source is, ironically, their low protein content. Depending on the preparation method, the protein content of most potato varieties ranges from 1 to 4%.[192] Thus, to obtain 30 grams of pure protein, one would have to eat 750–3,000 grams of potatoes.
That said, an attractive alternative to raw potatoes is potato protein isolate, which contains up to 80% protein.[189] Thus, a protein shake containing approximately 38 grams of potato protein isolate delivers 30 grams of high-quality plant protein alongside sufficient amounts of all essential amino acids.
To conclude, a well-balanced vegan diet can provide all essential amino acids. To make sure that a diet includes sufficient amounts of lysine and methionine (for the average person) and BCAAs such as leucine (for the vegan athlete), a variety of plant-based protein sources should be consumed. Alternatively, potato protein can also provide all essential amino acids, though eating a variety of different sources is nevertheless recommended to meet other nutrient requirements, such as vitamins and minerals.
However, this raises yet another question: If plant protein can provide all the essential amino acids, why is animal protein still considered to be superior in terms of overall protein quality?
13.2
Is plant protein less valuable than animal protein?
No, not necessarily. When compared food by food, animal protein is of higher quality (as measured, for example, by the biological value) than plant protein because of a generally better amino acid profile, bioavailability, and digestibility. However, when cooked and in the context of a well-planned vegan diet, plant protein is not necessarily of lower quality than animal protein. That’s because (as discussed in the previous question) combining different sources of plant protein can provide a sufficient intake of all essential amino acids and because food processing can improve the digestibility of plant protein, thus increasing its biological value. As a result, a diverse combination of properly cooked plant proteins can be as valuable of a protein source as animal protein.
It’s true that animal protein is of higher protein quality than plant protein.[89] But what does “better” mean, exactly?
There are many ways to measure protein quality (see link), but the most commonly used one is the biological value, defined as the percent of absorbed nitrogen from dietary protein that is actually incorporated into the body.
When comparing the biological value of different protein sources, it’s true that, on average, animal products have a higher biological value than plant protein sources.[89]
However, comparing the protein quality of single sources can be misleading because meals are rarely made up of just one type of food. Instead, different foods with different properties made with different processing techniques are combined into different meals eaten throughout the day.
This is why a meal should be evaluated as a whole in order to determine overall protein quality. Combining different complementary protein sources can significantly improve the overall protein quality of plant protein, in terms of the amino acid profile.[193]
For example, grains are low in lysine, but high in methionine, whereas legumes are high in lysine and low in methionine. In other words, the amino acid profile of grains and legumes complement each other, increasing the biological value of the consumed plant protein when eaten together.[88] Other popular examples of complementary protein sources include beans with rice and chickpeas with sesame paste (hummus).[194]
Another factor that influences protein quality is food processing, which can affect the nutritional quality of plant proteins in terms of protein digestibility (i.e., how well a specific dietary protein is digested and absorbed).
Food processing influences protein in two ways:[195]
First, food processing improves digestibility by disrupting the cell wall of plant cells and causing the denaturation of proteins. By disrupting the robust plant cell walls, nutrients from within the cells, like protein, become more available for digestion. When proteins denature, they may expose parts that make them even more accessible to proteolysis during digestion.[196]
Second, food processing makes proteins more digestible by inactivating certain plant compounds that would otherwise interfere with digestion. These compounds are also called antinutritional factors and include, for example, fiber, phytates, tannins, and lectins.[197][198] Inactivation of these antinutritional factors may significantly improve the quality of plant protein.[199]
In conclusion, a vegan diet can provide a comparable protein quality to an omnivorous diet, assuming that vegans combine a variety of complementary plant proteins to optimize the amino acid profile of the meal and use proper cooking techniques to optimize protein digestibility.
If prepared correctly, plant protein can be as nutritionally valuable as animal protein.