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Your diet is one of the first places to start if you’re looking to manage your health and weight. Focusing on whole foods from plant sources can reduce body weight, blood pressure and risk of heart disease, cancer and diabetes — and it can make your environmental impact more sustainable.

But how do we embrace plants in our diets if we’re so accustomed to including meat and dairy as primary nutrition sources?

We spoke with Dr. Reshma Shah, a physician, plant-based eating advocate, co-author of “Nourish: The Definitive Plant-Based Nutrition Guide for Families” and Stanford Healthy Living instructor, about simple ways to incorporate more plants into your diet and the benefits this can provide for both you and the planet.  

Focus on whole, minimally processed foods.

People use many different terms to describe a plant-based diet, including vegetarian, lacto-ovo vegetarian, pescatarian, and flexitarian to name a few. The most restrictive is veganism, which  excludes all animal products, including meat, eggs and dairy. 

While there are health benefits to adopting a vegan diet, highly processed foods with little to no nutritional value, like Oreos or French fries, could still be a legitimate part of a vegan diet.

In contrast, a whole-foods, plant-based (WFPB) diet: 

• Emphasizes whole, minimally processed foods
• Limits or avoids animal products
• Focuses on plant nutrients from vegetables, fruits, whole grains, legumes, seeds and nuts 
• Limits refined foods like added sugar, white flour and processed oils 

Recommendations from organizations including the U.S. Dietary Guidelines for Americans, World Health Organization, American Diabetes Association and American Cancer Society tout the benefits of plant-based whole foods and caution against high amounts of red and processed meats, saturated fats, highly refined foods and added sugar. 

The vast majority of what nutritional experts are saying reflects the mantra made famous by Michael Pollen in his book “The Omnivore’s Dilemma” — eat food, mostly plants, not too much . 

Eating a plant-based diet helps the environment.

According to a report by the U.S. Food and Agriculture Organization, “The meat industry has a marked impact on a general global scale on water, soils, extinction of plants and animals, and consumption of natural resources, and it has a strong impact on global warming.” 

The meat and dairy industries alone use one third of the Earth’s fresh water , with a single quarter-pound hamburger patty requiring 460 gallons of water — the equivalent of almost 30 showers — to produce.

Reducing your meat and dairy consumption, even by a little, can have big impacts. If everyone in the U.S. ate no meat or cheese just one day a week, it would have the same environmental impact as taking 7.6 million cars off the road.

Plant-based diets prevent animal cruelty. 

Ninety-four percent of Americans agree that animals raised for food deserve to be free from abuse and cruelty , yet 99% of those animals are raised in factory farms, many suffering unspeakable conditions . 

If you would like to lessen your meat and dairy consumption due to animal welfare concerns but aren’t ready to eliminate all animal products from your diet, then you can start by taking small steps, like going meatless one day a week or switching to soy, almond or oat milk. Shah admits that initially she was not ready to give up animal products entirely. 

“I think it is a process and recommend that people go at the pace that feels comfortable for them.” 

Plant-based diets include all nutrients — even protein.

According to the American Dietetic Association, “appropriately planned vegetarian diets, including total vegetarian or vegan diets, are healthful, nutritionally adequate, and may provide health benefits in the prevention and treatment of certain diseases. Well-planned vegetarian diets are appropriate for individuals during all stages of the life cycle, including pregnancy, lactation, infancy, childhood, adolescence, and for athletes.”

Shah says that there are a few key nutrients that strict vegans and vegetarians should keep in mind, including B12, iron, calcium, iodine, omega-3 fatty acids and vitamin D, but all of these can be obtained through plant-based foods, including fortified plant-based milks, fresh fruits and vegetables or supplemental vitamins, if needed. 

“I think the number one concern for people is that they won’t be able to get enough protein eating a plant-based diet. I also think that people widely overestimate the amount of protein they need.”

All plant foods contain the nine essential amino acids required to make up the proteins you need, and many vegetarian foods like soy, beans, nuts, seeds and non-dairy milk products have comparable amounts of protein to animal foods. 

“Ninety-seven percent of Americans meet their daily protein requirements, but only 4% of Americans meet their daily fiber requirements . I’ve never treated a patient for protein deficiency. If you eat a wide variety of foods and eat enough calories, protein should not be a concern.”

Savor the flavor of plant-based foods. 

Adopting a plant-based diet does not mean subsisting on boring, tasteless food. Shah enjoys incorporating flavorful, varied dishes from around the world, including Ethiopia, Thailand and her native India. 

To get started on your plant-forward journey:

• Start small: Start with adding a “Meatless Monday” to your meal plan and investigate one simple and delicious recipe to try each week. Once you have identified a few favorites, you can add them to your rotation and maybe go meatless one or two days a week. You can learn a few easy techniques to incorporate in many dishes, like roasting vegetables or blending quick and easy soups. 
• Change your plate proportions: Instead of giving up your meat-based protein completely, try to reduce the space it takes on your plate. Instead of a quarter-pound sirloin steak or a full serving of roasted chicken, try a vegetable-heavy stir-fry with a few slices of beef or a salad with chicken. Once your palate and mindset have adjusted to the smaller quantity of meat, try replacing it occasionally with plant-based proteins like tofu, seitan or beans.  
• Be prepared when dining out: If possible, try to examine the restaurant menu ahead of your meal, so you’ll arrive with a plan of what you can eat. Ask for the vegan options and don’t be afraid to request substitutions or omissions for your dish. Fortunately, with more people choosing a vegetarian lifestyle, many restaurants now provide tasty, meat-free options to their customers. 
• Share a dish: Bring a dish to share at a party or potluck; this will lessen your worries about food options. Let your host know ahead of time that you are planning on bringing a dish or, if that is not possible, be upfront and find out if any modifications can be made to accommodate your preferences. Often a simple solution can be found with a little advanced planning.
• Accommodate family members: It can be tricky when one family member is ready to commit to a new diet and lifestyle while others are not. Shah recommends approaching this situation compassionately and allowing for flexibility, if possible. Hopefully your family will be willing to support you even if they are not ready to make the same commitments. Communication is key, and Shah says that the conversation is over the minute someone feels judged, so try to look for points of compromise to reach an amicable solution. 
• Feeling satisfied: A diet of nothing but lettuce and vegetables will leave you feeling hungry and unfulfilled. Be sure to bulk up your meals with filling, fiber-rich whole grains, plant-based proteins and healthy fats. Plant-based meat substitutes like Beyond Beef, seitan and veggie burgers can also be a satisfying choice when you are craving your favorite meat-based comfort food.

Remember that small, consistent changes can add up to big benefits for your health and the planet. Treat yourself and others with compassion as you embrace this new lifestyle, and take time to enjoy the different flavors and textures you discover in your journey.

“It is a really delicious, healthful, sustainable and compassionate way of eating. It doesn’t have to be perfect. Just start simply, do what feels comfortable for you and your family, and don’t forget to celebrate the joy of eating and connection around food.” 

Dr. Reshma Shah will be teaching a plant-based online cooking class with Healthy Living this summer on Tuesday, July 13, from 4:00 – 5:30 p.m.

• https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6518108/
• https://www.sciencedirect.com/science/article/pii/S2212371713000024 
•  https://www.portland.gov/water/water-efficiency-programs/save-water-home 
•   https://water.usgs.gov/edu/activity-watercontent.php  
• https://www.ewg.org/meateatersguide/a-meat-eaters-guide-to-climate-change-health-what-you-eat-matters/reducing-your-footprint/)  
• https://www.aspca.org/about-us/press-releases/aspca-research-shows-americans-overwhelmingly-support-investigations-expose 
•  https://www.sentienceinstitute.org/us-factory-farming-estimates
•  https://pubmed.ncbi.nlm.nih.gov/19562864/ 
•   https://www.ars.usda.gov/ARSUserFiles/8040053 0/pdf/0102/usualintaketables2001-02.pdf

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• Review Article
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• Published: 12 September 2019

The effects of plant-based diets on the body and the brain: a systematic review

• Evelyn Medawar   ORCID: orcid.org/0000-0001-5011-8275 1 , 2 , 3 ,
• Sebastian Huhn 4 ,
• Arno Villringer 1 , 2 , 3 &
• A. Veronica Witte 1  

Translational Psychiatry volume  9 , Article number:  226 ( 2019 ) Cite this article

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• Human behaviour
• Molecular neuroscience
• Psychiatric disorders

Western societies notice an increasing interest in plant-based eating patterns such as vegetarian and vegan, yet potential effects on the body and brain are a matter of debate. Therefore, we systematically reviewed existing human interventional studies on putative effects of a plant-based diet on the metabolism and cognition, and what is known about the underlying mechanisms. Using the search terms “plant-based OR vegan OR vegetarian AND diet AND intervention” in PubMed filtered for clinical trials in humans retrieved 205 studies out of which 27, plus an additional search extending the selection to another five studies, were eligible for inclusion based on three independent ratings. We found robust evidence for short- to moderate-term beneficial effects of plant-based diets versus conventional diets (duration ≤ 24 months) on weight status, energy metabolism and systemic inflammation in healthy participants, obese and type-2 diabetes patients. Initial experimental studies proposed novel microbiome-related pathways, by which plant-based diets modulate the gut microbiome towards a favorable diversity of bacteria species, yet a functional “bottom up” signaling of plant-based diet-induced microbial changes remains highly speculative. In addition, little is known, based on interventional studies about cognitive effects linked to plant-based diets. Thus, a causal impact of plant-based diets on cognitive functions, mental and neurological health and respective underlying mechanisms has yet to be demonstrated. In sum, the increasing interest for plant-based diets raises the opportunity for developing novel preventive and therapeutic strategies against obesity, eating disorders and related comorbidities. Still, putative effects of plant-based diets on brain health and cognitive functions as well as the underlying mechanisms remain largely unexplored and new studies need to address these questions.

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Introduction.

Western societies notice an increasing interest in plant-based eating patterns such as avoiding meat or fish or fully excluding animal products (vegetarian or vegan, see Fig.  1 ). In 2015, around 0.4−3.4% US adults, 1−2% British adults, and 5−10% of German adults were reported to eat largely plant-based diets 1 , 2 , 3 , 4 , due to various reasons (reviewed in ref. 5 ). Likewise, the number of scientific publications on PubMed (Fig.  2 ) and the public popularity as depicted by Google Trends (Fig.  3 ) underscore the increased interest in plant-based diets. This increasing awareness calls for a better scientific understanding of how plant-based diets affect human health, in particular with regard to potentially relevant effects on mental health and cognitive functions.

From left to right: including all food items (omnivore), including all except for meat (pesco-vegetarian) or meat and fish (ovo-lacto-vegetarian) to including only plant-based items (vegan)

Frequency of publications on PubMed including the search terms “vegan” (in light green), vegetarian (in orange) and plant-based (dark green)—accessed on 19 April 2019

Note indicates technical improvements implemented by Google Trends. Data source: Google Trends . Search performed on 18 April 2019

A potential effect of plant-based diets on mortality rate remains controversial: large epidemiological studies like the Adventist studies ( n  = 22,000−96,000) show a link between plant-based diets, lower all-cause mortality and cardiovascular diseases 6 , 7 , while other studies like the EPIC-Oxford study and the “45 and Up Study” ( n  = 64,000−267,000) show none 8 , 9 . Yet, many, but not all, epidemiological and interventional human studies in the last decades have suggested that plant-based diets exert beneficial health effects with regard to obesity-related metabolic dysfunction, type 2 diabetes mellitus (T2DM) and chronic low-grade inflammation (e.g. refs. 6 , 7 , 10 , 11 , for reviews, see refs. 12 , 13 , 14 , 15 , 16 , 17 , 18 ). However, while a putative link between such metabolic alterations and brain health through pathways which might include diet-related neurotransmitter precursors, inflammatory pathways and the gut microbiome 19 becomes increasingly recognized, the notion that plant-based diets exert influence on mental health and cognitive functions appears less documented and controversial 20 , 21 , 22 , 23 , 24 . We therefore systematically reviewed the current evidence based on available controlled interventional trials, regarded as the gold standard to assess causality, on potential effects of plant-based diets on (a) metabolic factors including the microbiome and (b) neurological or psychiatric health and brain functions. In addition, we aimed to evaluate potential underlying mechanisms and related implications for cognition.

We performed a systematic PubMed search with the following search terms “plant-based OR vegan OR vegetarian AND diet AND intervention” with the filter “clinical trial” and “humans”, preregistered at PROSPERO (CRD42018111856; https://www.crd.york.ac.uk/PROSPERO/display_record.php?RecordID=111856 ) (Suppl. Fig.  1 ). PubMed was used as search engine because it was esteemed to yield the majority of relevant human clinical trials from a medical perspective. Exclusion criteria were insufficient design quality (such as lack of a control group), interventions without a plant-based or vegetarian or vegan diet condition, intervention with multiple factors (such as exercise and diet), and the exclusive report of main outcomes of no interest, such as dietary compliance, nutrient intake (such as vitamins or fiber intake), or nonmetabolic (i.e., not concerning glucose metabolism, lipid profile, gastrointestinal hormones or inflammatory markers) or non-neurological/psychiatric disease outcomes (e.g. cancer, caries).

Studies were independently rated for eligibility into the systematic review by three authors based on reading the abstract and, if needed, methods or other parts of the publication. If opinions differed, a consensus was reached through discussion of the individual study. This yielded 27 eligible out of 205 publications; see Table  1 for details. To increase the search radius for studies dealing with microbial and neurological/psychiatric outcomes, we deleted the search term “intervention”, which increased the number of studies by around one third, and checked for studies with “microbiome/microbiota”, “mental”, “cognitive/cognition” or “psychological/psychology” in the resulting records. Through this, we retrieved another five studies included in Table  1 . Further related studies were reviewed based on additional nonsystematic literature search.

Section I: Effects of plant-based diets on body and brain outcomes

Results based on interventional studies on metabolism, microbiota and brain function.

Overall, the vast majority of studies included in this systematic review reported a short-term beneficial effect of plant-based dietary interventions (study duration 3−24 months) on weight status, glucose, insulin and/or plasma lipids and inflammatory markers, whereas studies investigating whether plant-based diets affect microbial or neurological/psychiatric disease status and other brain functions were scarce and rather inconclusive (Table  1 ).

More specifically, 19 out of 32 studies dealing with T2DM and/or obese subjects and seven out of 32 dealing with healthy subjects observed a more pronounced weight loss and metabolic improvements, such as lowering of glycated hemoglobin (HbA1c)—a long-term marker for glucose levels—decreased serum levels of low-density (LDL) and high-density lipoproteins (HDL) and total cholesterol (TC), after a plant-based diet compared to an omnivore diet. This is largely in line with recent meta-analyses indicating beneficial metabolic changes after a plant-based diet 25 , 26 , 27 .

For example, Lee et al. found a significantly larger reduction of HbA1c and lower waist circumference after vegan compared to conventional dieting 28 . Jenkins et al. found a disease-attenuating effect in hyperlipidemic patients after 6 months adopting a low-carbohydrate plant-based diet compared to a high-carbohydrate lacto-ovo-vegetarian diet 29 , 30 . However, lower energy intake in the vegan dieters might have contributed to these effects. Yet, while a plant-based diet per se might lead to lower caloric intake, other studies observed nonsignificant trends toward higher effect sizes on metabolic parameters after a vegan diet, even when caloric intake was comparable: two studies in T2DM patients 31 , 32 compared calorie-unrestricted vegan or vegetarian to calorie-restricted conventional diets over periods of 6 months and 1.5 years, respectively, in moderate sample sizes ( n  ~ 75−99) with similar caloric intake achieved in both diet groups. Both studies indicated stronger effects of plant-based diets on disease status, such as reduced medication, improved weight status and increased glucose/insulin sensitivity, proposing a diabetes-preventive potential of plant-based diets. Further, a five-arm study comparing four types of plant-based diets (vegan, vegetarian, pesco-vegetarian, semi-vegetarian) to an omnivore diet (total n  = 63) in obese participants found the most pronounced effect on weight loss for a vegan diet (−7.5 ± 4.5% of total body weight) 33 . Here, inflammation markers conceptualized as the dietary inflammatory index were also found to be lower in vegan, vegetarian and pesco-vegetarian compared to semi-vegetarian overweight to obese dieters 33 .

Intriguingly, these results 28 , 29 , 30 , 31 , 32 , 33 cohesively suggest that although caloric intake was similar across groups, participants who had followed a vegan diet showed higher weight loss and improved metabolic status.

As a limitation, all of the reviewed intervention studies were carried out in moderate sample sizes and over a period of less than 2 years, disregarding that long-term success of dietary interventions stabilizes after 2−5 years only 34 . Future studies with larger sample sizes and tight control of dietary intake need to confirm these results.

Through our systematic review we retrieved only one study that added the gut microbiome as novel outcome for clinical trials investigating the effects of animal-based diets compared to plant-based diets. While the sample size was relatively low ( n  = 10, cross-over within subject design), it showed that changing animal- to plant based diet changed gut microbial activity towards a trade-off between carbohydrate and protein fermentation processes within only 5 days 35 . This is in line with another controlled-feeding study where microbial composition changes already occurred 24 h after changing diet (not exclusively plant-based) 36 . However, future studies incorporating larger sample sizes and a uniform analysis approach of microbial features need to further confirm the hypothesis that a plant-based diet ameliorates microbial diversity and health-related bacteria species.

Considering neurological or psychiatric diseases and brain functions, the systematic review yielded in six clinical trials of diverse clinical groups, i.e. migraine, multiple sclerosis, fibromyalgia and rheumatoid arthritis. Here, mild to moderate improvement, e.g. measured by antibody levels, symptom improvement or pain frequency, was reported in five out of six studies, sometimes accompanied by weight loss 37 , 38 , 39 , 40 (Table  1 ). However, given the pilot character of these studies, indicated by small sample sizes ( n  = 32−66), lack of randomization 37 , or that the plant-based diet was additionally free of gluten 40 , the evidence is largely anecdotal. One study in moderately obese women showed no effects on psychological outcomes 41 , two studies with obese and nonobese healthy adults indicated improvements in anxiety, stress and depressive symptom scores 23 , 24 . Taken together, the current evidence based on interventional trials regarding improvements of cognitive and emotional markers and in disease treatment for central nervous system disorders such as multiple sclerosis or fibromyalgia remains considerably fragmentary for plant-based diets.

Among observational studies, a recent large cross-sectional study showed a higher occurrence of depressive symptoms for vegetarian dieters compared to nonvegetarians 20 . Conversely, another observational study with a sample of about 80% women found a beneficial association between a vegan diet and mood disturbance 24 .

Overall, the relationship between mental health (i.e. depression) and restrictive eating patterns has been the focus of recent research 20 , 21 , 22 , 24 , 42 ; however, causal relationships remain uninvestigated due to the observational design.

Underlying mechanisms linking macronutrient intake to metabolic processes

On the one hand, nutrient sources as well as their intake ratios considerably differ between plant-based and omnivore diets (Suppl. Table  1 ), and on the other hand, dietary micro- and macromolecules as well as their metabolic substrates affect a diversity of physiological functions, pointing to complex interdependencies. Thus, it seems difficult to nail down the proposed beneficial effects of a plant-based diet on metabolic status to one specific component or characteristic, and it seems unlikely that the usually low amount of calories in plant-based diets could explain all observed effects. Rather, plant-based diets might act through multiple pathways, including better glycemic control 43 , lower inflammatory activity 44 and altered neurotransmitter metabolism via dietary intake 45 or intestinal activity 46 (Fig.  4 ).

BMI body-mass-index, HbA1c hemoglobin A1c, LDL-cholesterol low-density lipoprotein cholesterol, Trp tryptophan, Tyr tyrosine. Images from commons.wikimedia.org , “Brain human sagittal section” by Lynch 2006 and “Complete GI tract” by Häggström 2008, “Anatomy Figure Vector Clipart” by http://moziru.com

On the macronutrient level, plant-based diets feature different types of fatty acids (mono- and poly-unsaturated versus saturated and trans) and sugars (complex and unrefined versus simple and refined), which might both be important players for mediating beneficial health effects 18 . On the micronutrient level, the EPIC-Oxford study provided the largest sample of vegan dieters worldwide ( n (vegan) = 2396, n (total) = 65,429) and showed on the one hand lower intake of saturated fatty acids (SFA), retinol, vitamin B12 and D, calcium, zinc and protein, and on the other hand higher intake of fiber, magnesium, iron, folic acid, vitamin B1, C and E in vegan compared to omnivore dieters 47 . Other studies confirmed the variance of nutrient intake across dietary groups, i.e. omnivores, vegetarians and vegans, showing the occurrence of critical nutrients for each group 48 , 49 . Not only the amount of SFA but also its source and profile might be important factors regulating metabolic control (reviewed in ref. 14 ), for example through contributing to systemic hyperlipidemia and subsequent cardiovascular risk. Recently, it has been shown in a 4-week intervention trial that short-term dietary changes favoring a diet high in animal-based protein may lead to an increased risk for cardiovascular derangements mediated by higher levels of trimethylamine N-oxide (TMAO), which is a metabolite of gut bacteria-driven metabolic pathways 50 .

Secondly, high fiber intake from legumes, grains, vegetables and fruits is a prominent feature of plant-based diets (Table  1 ), which could induce beneficial metabolic processes like upregulated carbohydrate fermentation and downregulated protein fermentation 35 , improved gut hormonal-driven appetite regulation 51 , 52 , 53 , 54 , 55 , and might prevent chronic diseases such as obesity and T2DM by slowing down digestion and improving lipid control 56 . A comprehensive review including evidence from 185 prospective studies and 58 clinical trials concluded that risk reduction for a myriad of diseases (incl. CVD, T2DM, stroke incidence) was greatest for daily fiber intake between 25 and 29 g 57 . Precise evidence for underlying mechanisms is missing; however, more recently it has been suggested that high fiber intake induces changes on the microbial level leading to lower long-term weight gain 58 , a mechanism discussed below.

The reason for lower systemic inflammation in plant-based dieters could be due to the abundance of antiinflammatory molecule intake and/or avoidance of proinflammatory animal-derived molecules. Assessing systemic inflammation is particularly relevant for medical conditions such as obesity, where it has been proposed to increase the risk for cardiovascular disease 59 , 60 . In addition, higher C-reactive protein (CRP) and interleukin-6 (IL-6) levels have been linked with measures of brain microstructure, such as microstructural integrity and white matter lesions 61 , 62 , 63 and higher risk of dementia 64 , and recent studies point out that a diet-related low inflammatory index might also directly affect healthy brain ageing 65 , 66 .

Interventional studies that focus on plant- versus meat-based proteins or micronutrients and potential effects on the body and brain are lacking. A meta-analysis including seven RCTs and one cross-sectional studies on physical performance and dietary habits concluded that a vegetarian diet did not adversely influence physical performance compared to an omnivore diet 67 . An epidemiological study by Song et al. 11 estimated that statistically replacing 3% of animal protein, especially from red meat or eggs, with plant protein would significantly improve mortality rates. This beneficial effect might however not be explained by the protein source itself, but possibly by detrimental components found in meat (e.g. heme-iron or nitrosamines, antibiotics, see below).

Some studies further hypothesized that health benefits observed in a plant-based diet stem from higher levels of fruits and vegetables providing phytochemicals or vitamin C that might boost immune function and eventually prevent certain types of cancer 68 , 69 , 70 . A meta-analysis on the effect of phytochemical intake concluded a beneficial effect on CVD, cancer, overweight, body composition, glucose tolerance, digestion and mental health 71 . Looking further on the impact of micronutrients and single dietary compounds, there is room for speculation that molecules, that are commonly avoided in plant-based diets, might affect metabolic status and overall health, such as opioid-peptides derived from casein 72 , pre- and probiotics 73 , 74 , carry-over antibiotics found in animal products 75 , 76 or food-related carcinogenic toxins, such as dioxin found in eggs or nitrosamines found in red and processed meat 77 , 78 . Although conclusive evidence is missing, these findings propose indirect beneficial effects on health deriving from plant-based compared to animal-based foods, with a potential role for nonprotein substances in mediating those effects 18 . While data regarding chemical contaminant levels (such as crop pesticides, herbicides or heavy metals) in different food items are fragmentary only, certain potentially harmful compounds may be more (or less) frequently consumed in plant-based diets compared to more animal-based diets 79 . Whether these differences lead to systematic health effects need to be explored.

Taken together, the reviewed studies indicating effects of plant-based diets through macro- and micronutrient intake reveal both the potential of single ingredients or food groups (low SFA, high fiber) and the immense complexity of diet-related mechanisms for metabolic health. As proposed by several authors, benefits on health related to diet can probably not be viewed in isolation for the intake (or nonintake) of specific foods, but rather by additive or even synergistic effects between them (reviewed in refs. 12 , 80 ). Even if it remains a challenging task to design long-term RCTs that control macro- and micronutrient levels across dietary intervention groups, technological advancements such as more fine-tuned diagnostic measurements and automated self-monitoring tools, e.g. automatic food recognition systems 81 and urine-related measures of dietary intake 82 , could help to push the field forward.

Nutrients of particular interest in plant-based diets

As described above, plant-based diets have been shown to convey nutritional benefits 48 , 49 , in particular increased fiber, beta carotene, vitamin K and C, folate, magnesium, and potassium intake and an improved dietary health index 83 . However, a major criticism of plant-based diets is the risk of nutrient deficiencies for specific micronutrients, especially vitamin B12, a mainly animal-derived nutrient, which is missing entirely in vegan diets unless supplemented or provided in B12-fortified products, and which seems detrimental for neurological and cognitive health when intake is low. In the EPIC-Oxford study about 50% of the vegan dieters showed serum levels indicating vitamin B12 deficiency 84 . Along other risk factors such as age 85 , diet, and plant-based diets in particular, seem to be the main risk factor for vitamin B12 deficiency (reviewed in ref. 86 ), and therefore supplementing vitamin B12 for these risk groups is highly recommended 87 . Vitamin B12 is a crucial component involved in early brain development, in maintaining normal central nervous system function 88 and suggested to be neuroprotective, particularly for memory performance and hippocampal microstructure 89 . One hypothesis is that high levels of homocysteine, that is associated with vitamin B12 deficiency, might be harmful to the body. Vitamin B12 is the essential cofactor required for the conversion of homocysteine into nonharmful components and serves as a cofactor in different enzymatic reactions. A person suffering from vitamin B12 insufficiency accumulates homocysteine, lastly promoting the formation of plaques in arteries and thereby increasing atherothrombotic risk 90 , possibly facilitating symptoms in patients of Alzheimer’s disease 91 . A meta-analysis found that vitamin B12 deficiency was associated with stroke, Alzheimer’s disease, vascular dementia, Parkinson’s disease and in even lower concentrations with cognitive impairment 92 , supporting the claim of its high potential for disease prevention when avoided or treated 93 . Further investigations and longitudinal studies are needed, possibly measuring holotranscobalamin (the active form of vitamin B12) as a more specific and sensitive marker for vitamin B12 status 94 , to examine in how far nonsupplementing vegan dieters could be at risk for cardiovascular and cognitive impairment.

Similar health dangers can stem from iron deficiency, another commonly assumed risk for plant-based dieters and other risk groups such as young women. A meta-analysis on 24 studies proposes that although serum ferritin levels were lower in vegetarians on average, it is recommended to sustain an optimal ferritin level (neither too low nor too high), calling for well-monitored supplementation strategies 95 . Iron deficiency is not only dependent on iron intake as such but also on complimentary dietary factors influencing its bioavailability (discussed in ref. 95 ). The picture remains complex: on the one hand iron deficiency may lead to detrimental health effects, such as impairments in early brain development and cognitive functions in adults and in children carried by iron-deficient mothers 96 and a possible role for iron overload in the brain on cognitive impairment on the other hand 97 . One study showed that attention, memory and learning were impaired in iron-deficient compared to iron-sufficient women, which could be restored after a 4-month oral iron supplementation ( n  = 118) 98 . Iron deficiency-related impairments could be attributed to anemia as an underlying cause, possibly leading to fatigue, or an undersupply of blood to the brain or alterations in neurobiological and neuronal systems 99 provoking impaired cognitive functioning.

This leads to the general recommendation to monitor health status by frequent blood tests, to consult a dietician to live healthily on a plant-based diet and to consider supplements to avoid nutrient deficiencies or nutrient-overdose-related toxicity. All in all, organizations such as the Academy of Nutrition and Dietetics 100 and the German Nutrition Society do not judge iron as a major risk factor for plant-based dieters 101 .

Section II: Effects of diet on the gut microbiome

The link between diet and microbial diversity.

Another putative mechanistic pathway of how plant-based diets can affect health may involve the gut microbiome which has increasingly received scientific and popular interest, lastly not only through initiatives such as the Human Microbiome Project 102 . A common measure for characterizing the gut community is enterotyping, which is a way to stratify individuals according to their gut bacterial diversity, by calculating the ratio between bacterial genera, such as Prevotella and Bacteroides 103 . While interventional controlled trials are still scarce, this ratio has been shown to be conclusive for differentiating plant-based from animal-based microbial profiles 36 . Specifically, in a sample of 98 individuals, Wu et al. 36 found that a diet high in protein and animal fats was related to more Bacteroides, whereas a diet high in carbohydrates, representing a plant-based one, was associated with more Prevotella. Moreover, the authors showed that a change in diet to high-fat/low-fiber or to low-fat/high-fiber in ten individuals elicited a change in gut microbial enterotype with a time delay of 24 h only and remained stable over 10 days, however not being able to switch completely to another enterotype 36 . Another strictly controlled 30-day cross-over interventional study showed that a change in diet to either an exclusively animal-based or plant-based diet promoted gut microbiota diversity and genetic expression to change within 5 days 35 . Particularly, in response to adopting an animal-based diet, microbial diversity increased rapidly, even overshadowing individual microbial gene expression. Beyond large shifts in overall diet, already modest dietary modifications such as the daily consumption of 43 g of walnuts, were able to promote probiotic- and butyric acid-producing bacterial species in two RCTs, after 3 and 8 weeks respectively 104 , 105 , highlighting the high adaptability of the gut microbiome to dietary components. The Prevotella to Bacteroides ratio (P/B) has been shown to be involved in the success of dietary interventions targeting weight loss, with larger weight loss in high P/B compared to low P/B in a 6-month whole-grain diet compared to a conventional diet 106 . Only recently, other microbial communities, such as the salivary microbiome, have been shown to be different between omnivores and vegan dieters 107 , opening new avenues for research on adaptable mechanisms related to dietary intake.

A continuum in microbial diversity dependent on diet

Plant-based diets are supposed to be linked to a specific microbial profile, with a vegan profile being most different from an omnivore, but not always different from a vegetarian profile (reviewed in ref. 15 ). Some specifically vegan gut microbial characteristics have also been found in a small sample of six obese subjects after 1 month following a vegetarian diet, namely less pathobionts, more protective bacterial species improving lipid metabolism and a reduced level of intestinal inflammation 108 . Investigating long-term dietary patterns a study found a dose-dependent effect for altered gut microbiota in vegetarians and vegans compared to omnivores depending on the quantity of animal products 109 . The authors showed that gut microbial profiles of plant-based diets feature the same total number but lower counts of Bacteroides, Bifidobacterium, E. coli and Enterobacteriaceae compared to omnivores, with the biggest difference to vegans. Still today it remains unclear, what this shift in bacterial composition means in functional terms, prompting the field to develop more functional analyses.

In a 30-day intervention study, David et al. found that fermentation processes linked to fat and carbohydrate decomposition were related to the abundance of certain microbial species 35 . They found a strong correlation between fiber intake and Prevotella abundance in the microbial gut. More recently, Prevotella has been associated with plant-based diets 110 that are comparable to low-fat/high-fiber diets 111 and might be linked to the increased synthesis of short-chain fatty acids (SCFA) 112 . SCFAs are discussed as putative signaling molecules between the gut microbiome and the receptors, i.e. free fatty acid receptor 2 (FFA2) 51 , found in host cells across different tissues 113 and could therefore be one potential mechanism of microbiome−host communication.

The underlying mechanisms of nutrient decomposition by Prevotella and whether abundant Prevotella populations in the gut are beneficial for overall health remain unknown. Yet it seems possible that an increased fiber intake and therefore higher Prevotella abundance such as associated with plant-based diets is beneficial for regulating glycemic control and keeping inflammatory processes within normal levels, possibly due to reduced appetite and lower energy intake mediated by a higher fiber content 114 . Moreover, it has been brought forward that the microbiome might influence bodily homeostatic control, suggesting a role for the gut microbiota in whole-body control mechanisms on the systemic level. Novel strategies aim to develop gut-microbiota-based therapies to improve bodily states, e.g. glycemic control 115 , based on inducing microbial changes and thereby eliciting higher-level changes in homeostasis. While highly speculative, such strategies could in theory also exert changes on the brain level, which will be discussed next in the light of a bi-directional feedback between the gut and the brain.

Effects on cognition and behavior linking diet and cognition via the microbiome−gut−brain axis

While the number of interventional studies focusing on cognitive and mental health outcomes after adopting plant-based diets overall is very limited (see Section I above), one underlying mechanism of how plant-based diets may affect mood could involve signaling pathways on the microbiome−gut−brain axis 116 , 117 , 118 , 119 . A recent 4-week intervention RCT showed that probiotic administration compared to placebo and no intervention modulated brain activity during emotional decision-making and emotional recognition tasks 117 . In chronic depression it has been proposed that immunoglobulin A and M antibodies are synthesized by the host in response to gut commensals and are linked to depressive symptoms 120 . Whether the identified gram-negative bacteria might also play a role in plant-based diets remains to be explored. A meta-analysis on five studies concluded that probiotics may mediate an alleviating effect on depression symptomatic 121 —however, sample sizes remained rather small ( n  < 100) and no long-term effects were tested (up to 8 weeks).

Currently, several studies aim to identify microbial profiles in relation to disease and how microbial data can be used on a multimodal way to improve functional resolution, e.g. characterizing microbial profiles of individuals suffering from type-1 diabetes 122 . Yet, evidence for specific effects of diet on cognitive functions and behavior through changes in the microbiome remains scarce. A recent study indicated the possibility that our food choices determine the quantity and quality of neurotransmitter-precursor levels that we ingest, which in turn might influence behavior, as shown by lower fairness during a money-redistribution task, called the ultimatum game, after a high-carbohydrate/protein ratio breakfast than after a low-ratio breakfast 123 . Strang et al. found that precursor forms of serotonin and dopamine, measured in blood serum, predicted behavior in this task, and precursor concentrations were dependent on the nutrient profile of the consumed meal before the task. Also on a cross-sectional level tryptophan metabolites from fecal samples have been associated with amygdala-reward network functional connectivity 124 . On top of the dietary composition per se, the microbiota largely contributes to neurotransmitter precursor concentrations; thus, in addition to measuring neurotransmitter precursors in the serum, metabolomics on fecal samples would be helpful to further understand the functional role of the gut microbiota in neurotransmitter biosynthesis and regulation 125 .

Indicating the relevance of gut microbiota for cognition, a first human study assessing cognitive tests and brain imaging could distinguish obese from nonobese individuals using a microbial profile 126 . The authors found a specific microbiotic profile, particularly defined by Actinobacteria phylum abundance, that was associated with microstructural properties in the hypothalamus and in the caudate nucleus. Further, a preclinical study tested whether probiotics could enhance cognitive function in healthy subjects, showing small effects on improved memory performance and reduced stress levels 127 .

A recent study could show that microbial composition influences cerebral amyloidogenesis in a mouse model for Alzheimer’s disease 128 . Health status of the donor mouse seemingly mattered: fecal transplants from transgenic mice had a larger impact on amyloid beta proliferation in the brain compared to wild-type feces. Translational interpretations to humans should be done with caution if at all—yet the results remain elucidative for showing a link between the gut microbiome and brain metabolism.

The evidence for effects of strictly plant-based diets on cognition is very limited. For other plant-based diets such as the Mediterranean diet or DASH diet, there are more available studies that indicate protective effects on cardiovascular and brain health in the aging population (reviewed in refs. 129 , 130 ). Several attempts have been made to clarify potential underlying mechanisms, for example using supplementary plant polyphenols, fish/fish-oil consumption or whole dietary pattern change in RCTs 131 , 132 , 133 , 134 , 135 , 136 , 137 , yet results are not always equivocal and large-scale intervention studies have yet to be completed.

The overall findings of this paragraph add to the evidence that microbial diversity may be associated with brain health, although underlying mechanisms and candidate signaling molecules remain unknown.

Based on this systematic review of randomized clinical trials, there is an overall robust support for beneficial effects of a plant-based diet on metabolic measures in health and disease. However, the evidence for cognitive and mental effects of a plant-based diet is still inconclusive. Also, it is not clear whether putative effects are due to the diet per se, certain nutrients of the diet (or the avoidance of certain animal-based nutrients) or other factors associated with vegetarian/vegan diets. Evolving concepts argue that emotional distress and mental illnesses are linked to the role of microbiota in neurological function and can be potentially treated via microbial intervention strategies 19 . Moreover, it has been claimed that certain diseases, such as obesity, are caused by a specific microbial composition 138 , and that a balanced gut microbiome is related to healthy ageing 111 . In this light, it seems possible that a plant-based diet is able to influence brain function by still unclear underlying mechanisms of an altered microbial status and systemic metabolic alterations. However, to our knowledge there are no studies linking plant-based diets and cognitive abilities on a neural level, which are urgently needed, due to the hidden potential as a dietary therapeutic tool. Also, further studies are needed to disentangle motivational beliefs on a psychological level that lead to a change in diet from causal effects on the body and the brain mediated e.g., by metabolic alterations or a change in the gut microbiome.

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Acknowledgements

This work was supported by a scholarship (E.M.) by the German Federal Environmental Foundation and by the grants of the German Research Foundation contract grant number CRC 1052 “Obesity mechanisms” Project A1 (AV) and WI 3342/3-1 (A.V.W.).

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E.M., A.V. and A.V.W. designed research; E.M. conducted research; E.M., S.H. and A.V.W. analyzed data; E.M. and A.V.W. wrote the paper; E.M., A.V. and A.V.W. had primary responsibility for final content. All authors read and approved the final manuscript.

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Medawar, E., Huhn, S., Villringer, A. et al. The effects of plant-based diets on the body and the brain: a systematic review. Transl Psychiatry 9 , 226 (2019). https://doi.org/10.1038/s41398-019-0552-0

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Twin research indicates that a vegan diet improves cardiovascular health

A Stanford Medicine-led trial of identical twins comparing vegan and omnivore diets found that a vegan diet improves overall cardiovascular health.

November 30, 2023 - By Emily Moskal

Twin pairs Carolyn Sideco and Rosalyn Moorhouse, Aleksandra Shaichai and Mariya Foster, and Jean Jacquemet and Janet Hurt participated in a study examining the effect of a vegan versus omnivore diet on cardiovascular health. Lisa Kim

In a study with 22 pairs of identical twins, Stanford Medicine researchers and their colleagues have found that a vegan diet improves cardiovascular health in as little as eight weeks.

Although it’s well-known that eating less meat improves cardiovascular health, diet studies are often hampered by factors such as genetic differences, upbringing and lifestyle choices. By studying identical twins, however, the researchers were able to control for genetics and limit the other factors, as the twins grew up in the same households and reported similar lifestyles.

“Not only did this study provide a groundbreaking way to assert that a vegan diet is healthier than the conventional omnivore diet, but the twins were also a riot to work with,” said Christopher Gardner , PhD, the Rehnborg Farquhar Professor and a professor of medicine. “They dressed the same, they talked the same and they had a banter between them that you could have only if you spent an inordinate amount of time together.”

The study published Nov. 30 in JAMA Network Open . Gardner is the senior author. The study was co-first authored by Matthew Landry, PhD, a former Stanford Prevention Research Center postdoctoral scholar, now at the University of California, Irvine, and Catherine Ward , PhD, a post-doctoral scholar at the center.

Twin participants

The trial, conducted from May to July 2022, consisted of 22 pairs of identical twins for a total of 44 participants. The study authors selected healthy participants without cardiovascular disease from the Stanford Twin Registry — a database of fraternal and identical twins who have agreed to participate in research studies — and matched one twin from each pair with either a vegan or omnivore diet.

Both diets were healthy, replete with vegetables, legumes, fruits and whole grains and void of sugars and refined starches. The vegan diet was entirely plant-based, included no meat or animal products such as eggs or milk. The omnivore diet included chicken, fish, eggs, cheese, dairy and other animal-sourced foods.

During the first four weeks, a meal service delivered 21 meals per week — seven breakfasts, lunches and dinners. For the remaining four weeks, the participants prepared their own meals.

Christopher Gardner

A registered dietitian, or “diet whisperer,” according to Gardner, was on call to offer suggestions and answer questions regarding the diets during the duration of the study. The participants were interviewed about their dietary intake and kept a log of the food they ate.

Forty-three participants completed the study which, Gardner said, demonstrates how feasible it is to learn how to a prepare a healthy diet in four weeks.

“Our study used a generalizable diet that is accessible to anyone, because 21 out of the 22 vegans followed through with the diet,” said Gardner, who is a professor in the Stanford Prevention Research Center. “This suggests that anyone who chooses a vegan diet can improve their long-term health in two months, with the most change seen in the first month.”

Improving health

The authors found the most improvement over the first four weeks of the diet change. The participants with a vegan diet had significantly lower low-density lipoprotein cholesterol (LDL-C) levels, insulin and body weight — all of which are associated with improved cardiovascular health — than the omnivore participants.

At three time points — at the beginning of the trial, at four weeks and at eight weeks — researchers weighed the participants and drew their blood. The average baseline LDL-C level for the vegans was 110.7 mg/dL and 118.5 mg/dL for the omnivore participants; it dropped to 95.5 for vegans and 116.1 for omnivores at the end of the study. The optimal healthy LDL-C level is less than 100.

Because the participants already had healthy LDL-C levels, there was less room for improvement, Gardner said, speculating that participants who had higher baseline levels would show greater change.

The vegan participants also showed about a 20% drop in fasting insulin — higher insulin level is a risk factor for developing diabetes. The vegans also lost an average of 4.2 more pounds than the omnivores.

“Based on these results and thinking about longevity, most of us would benefit from going to a more plant-based diet,” Gardner said.

The vegan participants (and the omnivores to some extent) did the three most important things to improve cardiovascular health, according to Gardner: They cut back on saturated fats, increased dietary fiber and lost weight.

A global flair

Gardner emphasizes that although most people will probably not go vegan, a nudge in the plant-based direction could improve health. “A vegan diet can confer additional benefits such as increased gut bacteria and the reduction of telomere loss, which slows aging in the body,” Gardner said.

“What’s more important than going strictly vegan is including more plant-based foods into your diet,” said Gardner, who has been “mostly vegan” for the last 40 years. “Luckily, having fun with vegan multicultural foods like Indian masala, Asian stir-fry and African lentil-based dishes can be a great first step.”

Gardner is a member of the Stanford Cardiovascular Institute , the Wu Tsai Human Performance Alliance , the Maternal and Child Health Research Institute , and the Stanford Cancer Institute .

The study was funded by the Vogt Foundation; the Stanford Clinical and Translational Science Award; and the National Heart, Lung and Blood Institute.

About Stanford Medicine

Stanford Medicine is an integrated academic health system comprising the Stanford School of Medicine and adult and pediatric health care delivery systems. Together, they harness the full potential of biomedicine through collaborative research, education and clinical care for patients. For more information, please visit med.stanford.edu .

The majestic cell

How the smallest units of life determine our health

Veganism, aging and longevity: new insight into old concepts

Affiliations.

• 1 Department of Nutrition and Gerontology, German Institute for Human Nutrition Potsdam-Rehbrücke, Nuthetal.
• 2 Research Group on Geriatrics, Charité Universitätsmedizin Berlin, Freie Universität Berlin, Humboldt-Universität zu Berlin, and Berlin Institute of Health, Berlin.
• 3 Institute of Nutritional Science, University of Potsdam.
• 4 Department of Physiology and Energy Metabolism, German Institute for Human Nutrition Potsdam-Rehbrücke, Nuthetal, Germany.
• PMID: 31895244
• DOI: 10.1097/MCO.0000000000000625

Purpose of review: Plant-based diets are associated with better health and longevity. Veganism is a strict form of vegetarianism, which has gained increasing attention in recent years. This review will focus on studies addressing mortality and health-span in vegans and vegetarians and discuss possible longevity-enhancing mechanisms.

Recent findings: Studies in vegans are still limited. Epidemiologic studies consistently show lower disease rates, such as lower incidence of cancer and cardiovascular disease, but mortality rates are comparable with rates in vegetarians and occasional meat eaters. Reasons for following strict vegan diets differ, which may affect diet quality, and thus health and life-span. New insights into some characteristics of veganism, such as protein restriction or restriction in certain amino acids (leucine or methionine) show potentially life-span-enhancing potential. Veganism improves insulin resistance and dyslipidemia and associated abnormalities. Gut microbiota as mediator of dietary impact on host metabolism is more diverse in vegans and has been suggested to be a health-promoting factor. Vegan diets do not fulfill the requirements of children, pregnant women or old individuals who should receive adequate supplements.

Summary: There is substantial evidence that plant-based diets are associated with better health but not necessarily lower mortality rates. The exact mechanisms of health promotion by vegan diets are still not entirely clear but most likely multifactorial. Reasons for and quality of the vegan diet should be assessed in longevity studies.

Publication types

• Aging / physiology*
• Diet, Protein-Restricted / methods
• Diet, Protein-Restricted / mortality
• Diet, Vegan / methods
• Diet, Vegan / mortality*
• Diet, Vegetarian / methods
• Diet, Vegetarian / mortality*
• Gastrointestinal Microbiome / physiology
• Longevity / physiology*
• Nutritional Requirements / physiology*

Vegan diet can benefit both health and the environment

There is strong evidence that a plant-based diet is the optimal diet for living a long and healthy life, according to Harvard T.H. Chan School of Public Health nutrition expert Walter Willett .

In a January 7, 2019 interview on the NPR show “1A,” Willett, professor of epidemiology and nutrition, said that it’s not necessary to be 100% vegan in order to reap the benefits of a plant-based diet, which has been linked with lower risk of type 2 diabetes , heart disease , and overall mortality. Diets with modest amounts of dairy and fish, and even some poultry and meat, can also be healthy, as long as people steer clear of refined starches and sugar and focus on vegetables , fruits , nuts , seeds, and whole grains.

Willett also said that veganism is good for the planet. That’s because cattle grazing generates massive amounts of methane and carbon dioxide, both of which are potent greenhouse gases that contribute to climate change .

“I think if we really care about the world our children and grandchildren will inherit, we do need to shift toward [a vegan diet],” said Willett. “And the good news is that it’s not just our planet that will be more healthy, but we will be more healthy as well.”

Listen to the 1A interview: Planting A Seed: The Vegan Diet in 2019

Healthy plant-based diet linked with substantially lower type 2 diabetes risk ( Harvard Chan School release )

Vegetarian Recipes for a Healthy Eating Plate ( The Nutrition Source )

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The Effects of Vegetarian and Vegan Diets on Gut Microbiota

Aleksandra tomova, igor bukovsky, emilie rembert, willy yonas, jihad alwarith, neal d barnard, hana kahleova.

• Author information
• Article notes
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Edited by: Sergueï O. Fetissov, Université de Rouen, France

Reviewed by: Iolanda Cioffi, Azienda Ospedaliera Universitaria Federico II, Italy; Christine Ann Butts, The New Zealand Institute for Plant & Food Research Ltd, New Zealand

*Correspondence: Aleksandra Tomova [email protected]

This article was submitted to Clinical Nutrition, a section of the journal Frontiers in Nutrition

Received 2019 Jan 11; Accepted 2019 Mar 29; Collection date 2019.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

The difference in gut microbiota composition between individuals following vegan or vegetarian diets and those following omnivorous diets is well documented. A plant-based diet appears to be beneficial for human health by promoting the development of more diverse and stable microbial systems. Additionally, vegans and vegetarians have significantly higher counts of certain Bacteroidetes -related operational taxonomic units compared to omnivores. Fibers (that is, non-digestible carbohydrates, found exclusively in plants) most consistently increase lactic acid bacteria, such as Ruminococcus, E. rectale , and Roseburia , and reduce Clostridium and Enterococcus species. Polyphenols, also abundant in plant foods, increase Bifidobacterium and Lactobacillus , which provide anti-pathogenic and anti-inflammatory effects and cardiovascular protection. High fiber intake also encourages the growth of species that ferment fiber into metabolites as short-chain fatty acids (SCFAs), including acetate, propionate, and butyrate. The positive health effects of SCFAs are myriad, including improved immunity against pathogens, blood–brain barrier integrity, provision of energy substrates, and regulation of critical functions of the intestine. In conclusion, the available literature suggests that a vegetarian/vegan diet is effective in promoting a diverse ecosystem of beneficial bacteria to support both human gut microbiome and overall health. This review will focus on effects of different diets and nutrient contents, particularly plant-based diets, on the gut microbiota composition and production of microbial metabolites affecting the host health.

Keywords: gut microbiota, fiber, nutrition, plant-based diet, postbiotics

Introduction

Recent studies of the human microbiome have emerged as an area of popular interest. For decades, many investigations have elucidated the impact of the human gut microbiota on the physiology of the host, with new and unexpectedly broad implications for health and disease.

The human microbiota, defined as the total of all microbial taxa associated with human beings (bacteria, viruses, fungi, protozoa, archaea), consists of a newly estimated 3 × 10 13 (trillion) microbes harbored by each person ( 1 ). The term microbiome is often incorrectly used interchangeably with the term microbiota. However, microbiome refers to the catalog of these microbes and their genes. The human gut microbiome represents ~3.3 million non-redundant microbial genes, which outnumbers the human genome of some 21,000 genes in the ratio of ~150:1 ( 2 ). Interestingly, the diversity among the microbiomes of two different individuals is vast compared to their human genomic variation; humans are about 99.9% identical to each other in terms of their genome ( 3 ), but their gut microbiome can be up to 80–90% different ( 4 ).

Recent advancements in laboratory techniques have revealed functions of the human gut microbiota related to immunity and the gastrointestinal, brain, and cardiovascular systems. Research has also suggested a profound effect of the human gut microbiota on host cells and genes. This extensive interaction has suggested that the microbiome functions effectively as a separate “organ.”

Several studies have suggested that there are three basic bacterial enterotypes ( 5 ) (1) genus Prevotella (considered to be mostly anti-inflammatory and otherwise protective), (2) genus Bacteroides (more pro-inflammatory and possibly related to the heightened risk of metabolic syndrome and other pathological conditions), and (3) genus Ruminococcus (of which the biological significance is less evident) ( 6 ).

An imbalance of the gut microbiota has been linked with gastrointestinal conditions such as reflux, peptic ulcers, irritable bowel syndrome, non-alcoholic steatohepatitis, and inflammatory bowel disease. Additionally, some systemic conditions such as obesity, atherosclerosis, type 2 diabetes, cancer, Alzheimer's and Parkinson's disease, amyotrophic lateral sclerosis, autism spectrum disorder, atopy etc., also appear to be linked to unfavorable changes in gut microbiota composition ( 7 – 17 ). An accumulating body of evidence points to the gut microbiota as a mediator of dietary impact on the host metabolic status. Current research is focusing on the establishment of causal relationships in people and the development of therapeutic interventions such as personalized nutrition ( 18 ).

Dietary composition appears to have long-term and acute effects on the gut microbiota ecosystem ( 19 , 20 ). Different long-term dietary patterns, such as vegetarian/vegan vs. omnivorous diets, have significant influence on gut microbiota composition. The different gut microbiota content is shown to provide different food nutrients metabolites, termed postbiotics. For instance, SCFAs, phytoestrogens, or isothiocyanates are more linked with the plant-based food, while TMAO and secondary bile acids with the meat-based diet. These and other postbiotics take part in the metabolism of the host in different ways. This review will focus on some general as well as specific aspects of this dynamic field of research.

Gut Microbiota: General Aspects

In addition to bacteria, the gut is host to multiple kingdoms: archaea, viruses, and eukaryotes, including fungal species. The gut microbiota is represented by more than 1,000 microbial species, belonging primary to just two phyla: Bacteroidetes and Firmicutes ( 21 ). Based on human stool samples, overall, the genera Bacteroides, Prevotella, Bifidobacterium, Eubacterium, Clostridium, Streptococcus , and Enterobacteriaceae are most commonly found. It should be noted that stool samples provide reasonable estimations of the gut microbiota rather than a complete representation ( 22 ). This is because anaerobic species often attach to the gut mucosa, making it difficult to identify all bacterial species present in the large intestine. Also, it is probable that the biological significance of any genera or species is not given by its relative proportion in the whole ecosystem. Rather, its significance is observed through its metabolism/postbiotics and effects on the enteric nervous system, local immunity, brain, and genes.

Effect of Diet on Gut Microbiota Composition

The difference in gut microbiota composition between individuals consuming a vegan/vegetarian and an omnivorous diet is well documented. Research shows that vegetarian/vegan diets foster different microbiota when compared to omnivores, with only a marginal difference between vegans and vegetarians ( 23 ). Changes in microbiota composition might be due to differences in bacteria directly consumed through food, differences in substrates consumed, variations in transit time through the gastrointestinal system, pH, host secretion influenced by dietary patterns, and regulation of gene expression of the host himself and/or his/her microbiota ( 24 ).

A plant-based diet appears to be beneficial for human health by promoting the development of a more diverse gut microbial system, or even distribution of different species ( 25 , 26 ).

Diversity and Richness of the Gut Microbiota

The diversity of the microbiota appears to have an important association with BMI, obesity, and arterial compliance; and a majority of the research suggests that a plant-based diet fosters a greater microbial diversity. Klimenko et al. found a positive association between alpha-diversity, or local microbial richness, and long-term fruit and vegetable intake ( p < 0.05) ( 27 ). Likewise, Martinez et al. observed that adding whole-grain barley, brown rice, or a mixture of the first two to the diet of volunteers resulted in an increase in microbial diversity ( n = 28) ( 28 ). Klimenko et al. also found a negative association between alpha-diversity and BMI ( p < 0.05) ( 27 ).

However, a short-term dietary intervention advising increased fiber consumption resulted in a slight but significant decrease in diversity ( p < 0.001). The researchers suggest this reduction in diversity might be the result of a rapid dietary change resulting in a temporary disruption to the microbial composition. This hypothesis of transitory microbial “stress” also explains the slight but significant increase in Enterobacteriaceae as a result of the intervention ( p < 0.05). Enterobacteriaceae abundance is typically lower on a vegan diet vs. an omnivorous one ( P < 0.05) ( 29 ). This is likely due to the greater presence of butyrate-producing bacteria on a higher fiber diet, which can lower colonic pH, preventing the growth of pathogenic bacteria, such as Enterobacteriaceae ( 30 ).

Verdam et al. observed reduced microbial diversity in obese vs. non-obese study participants ( n = 28). The obese participants also displayed a reduction in the Bacteriodetes : Firmicutes ratio and an increase in Proteobacteria , a pro-inflammatory phylum. Likewise, an increase in C-reactive protein was observed ( p < 0.001) which inversely correlated with the Bacteriodetes : Firmicutes ratio ( p < 0.05). These observations suggest a pro-inflammatory effect of obesity-related microbiota ( 31 ). On the other hand, participants from the Adventist Health Study-2 (60,903) following a vegan diet displayed the lowest BMI values when compared with those following a vegetarian or omnivorous diet ( 32 ). These findings indicate that a vegan diet, associated with lower body weight, might benefit microbial diversity and protect against inflammation.

Menni et al. observed that carotid-femoral pulse wave velocity, a measure of arterial stiffness, was negatively associated with microbiome diversity ( p = 0.001) in women ( n = 617) ( 33 ). This correlation remained significant after adjusting for insulin resistance and visceral fat. Arterial stiffness is oftentimes caused by hyperglycemia or hyperinsulinemia ( 34 ) and is significantly correlated with inflammatory adipokine levels. The researchers suggest the association between arterial stiffness and microbial diversity can be explained partially by the role of the gut in modulating systemic inflammation. Thus, an increase in microbial diversity might improve systemic inflammation, thereby reducing arterial stiffness.

Additionally, vegans and vegetarians have a significantly greater richness (alpha diversity) compared to omnivores, specifically counts of certain Bacteroidetes -related operational taxonomic units (OTUs) ( 35 ). It seems likely that many health benefits of vegetarian/vegan diets are, in part, mediated by the gut microbiota—not only through the higher relative abundance of those OTUs that are currently considered to be protective ( Bacteroidetes, Prevotella, Roseburia , etc.), but also from postbiotic and epigenetic effects on various risk factors for chronic inflammation and chronic degenerative diseases ( 36 ).

Effects of Diet on the Bacteroidetes:Firmicutes Ratio

Despite significant inter-individual differences, a healthy adult intestinal microbiome is characterized by the dominance of these Bacteroidetes -related OTUs along with those of the Firmicutes phylum ( 37 , 38 ). Research has shown variability in these phyla concentrations to be heavily affected by diet, specifically the ratio between the two when comparing omnivorous diets of the type common in North America, vs. a vegetarian/vegan diet. One study compared the bacterial composition between Indian ( n = 11) and Chinese ( n = 5) adults ( 39 ). While both populations ate diets centered around carbohydrates and vegetables, the Chinese diet was heavier in animal fat and protein than the Indian diet of whole grains and plant-based vegetarian foods. The percentage of Bacteroidetes within the microbiomes of Indian participants was nearly four times greater than in the Chinese, 16.39% vs. 4.27%, respectively ( p = 0.001). The higher abundance of Bacteroidetes in Indians was hypothesized to be due to their lower consumption of animal products; indicating a diet lower in animal products to be associated with greater Bacteroidetes counts.

Another study compared the fecal microbiota of Italian children ( n = 15) vs. the fecal microbiota of children living in a rural western Africa, specifically in Burkina Faso ( n = 14) ( 40 ). The Italian children typically consumed a Western diet, high in animal protein, sugar, starch, and fat and low in fiber. The African children of Burkina Faso consumed a diet low in fat and animal protein and rich in starch, fiber, and plant protein. The abundance of Firmicutes was twice as much in the Italian children than in the Burkina Faso children (63.7 vs. 27.3%, respectively). The abundance of Bacteroidetes in the Italian children was less than half of that seen than in the Burkina Faso children (22.4 vs. 57.7%, respectively). A decrease in Firmicutes levels, usually occurring in favor of Bacteroidetes and Bifidobacteria , as seen in response to an increase in resistant starches, may be beneficial in preventing and treating obesity ( 41 ). While these correlations between diet and microbiota composition are observed among different populations, it is important to consider other factors that may play a role, such as ethnicity, host genotypes, environmental factors, etc.

Research has shown that the balance of Bacteroidetes and Firmicutes is an important marker for obesity and higher BMI. Specifically, a decreased Bacteroidetes : Firmicutes ratio has a strong negative correlation with BMI (r s = 0.59, P < 0.001) ( 31 ). A possible explanation for this correlation may be found in the observation that a 20% increase in Firmicutes and a corresponding decrease in Bacteroidetes abundance is associated with a 150 kcal/day increase in energy harvest, resulting in weight gain overtime. Therefore, an increased Bacteroidetes : Firmicutes ratio, as seen on a high fiber, plant-based diet, may result in weight loss by reducing the amount of energy extracted from the diet. Further research is needed to determine whether the increase in energy harvest due is due to the Bacteroidetes : Firmicutes ratio promoting adiposity or representing a host-mediated adaptive response to limit energy uptake ( 42 ).

Studies have also shown opposite trends in Firmicutes and Bacteroidetes . One study compared US children eating a Western diet to Bangladeshi children consuming a plant-based diet of rice, bread, and lentils. The Bacteroidetes : Firmicutes ratio was three times higher in the US children consuming the Western diet ( 43 ). This opposes the previous prediction of a Western diet resulting in a decreased Bacteroidetes : Firmicutes ratio. The researchers noted age and geographical differences as potential explanation for this departure from the expected ratio, as well as inter-subject variability. Another study asked participants to increase their fiber consumption and avoid Western diet foods. While prior studies would have suggested an increase in Bacteroidetes : Firmicutes , the ratio decreased (0.13 ± 0.2 to 0.03 ± 0.09, Wilcoxon paired test p < 0.0001, n = 430) ( 27 ). Another study analyzed the microbial composition of lean and obese subjects ( n = 98) and observed that, when compared to lean subjects, overweight and obese volunteers presented a higher ratio Bacteroidetes to Firmicutes ( P = 0.001 and P = 0.005, respectively) ( 44 ). Likewise, comparison of bacterial phyla did not show a significant difference in the Bacteroidetes to Firmicutes ratio between obese and lean volunteers ( n = 20) ( 45 ). These examples reflect the difficulties in broadly linking certain phyla to particular diets. The primary challenge in analyzing specific microbiota is the need to consider the state and interaction dynamic of microbes encompassing the whole microbiome.

Effects of Diet on Enterotypes

As mentioned above, there are three main enterotypes observed in human microbiomes: Prevotella, Bacteroides, and Ruminococcus . Prevotella , a genus of the Bacteroidetes phyla, appears to be significantly richer in response to a vegan diet. In the previously mentioned study by De Filippo et al., Prevotella was exclusively present in the children of Burkina Faso consuming a diet low in fat and animal protein and rich in starch, fiber, and plant protein when compared to children living in Italy consuming a Western diet, high in animal protein, sugar, starch, and fat, and low in fiber ( 40 ). Another study compared the diets of 178 elderly residents living in either the community or in long-term residential care ( 46 ). The community group was found to consume a low to medium fat, high fiber diet; while the residents in long-term care consumed a moderate to high in fat, and low fiber diet. The study found that those in the community, eating a profile more reflective of a plant-based diet, more frequently had gut communities of the Prevotella enterotype.

The study comparing Indian and Chinese adults shows similar results ( 39 ). As expected, the Indians who were consuming less animal products and more plant-based foods than the Chinese had a significantly greater percentage of Prevotella (13.07 vs. 0.58%, respectively). When the abundance of Prevotella was analyzed in Thai vegetarians vs. non-vegetarians, the vegetarians were found to have significantly higher numbers of Prevotella ( p = 0.005) ( 47 ). Other studies have shown vegan/vegetarian diets, high in plant-based foods, to be associated with high abundances of Prevotella ( 48 , 49 ). This suggests additional support for greater Prevotella presence in those whom consume less animal products and more plant-based food. While mice studies suggest Prevotella to improve glucose metabolism by improving glycogen storage ( 50 ), the current lack of any additional research makes Prevotella merely a genus to describe an overall ecosystem of human gut bacteria, primarily under a plant-based diet.

Bacteroides , another main enterotype and genus of the Bacteroidetes phyla, also appears to be affected by diet but in a different way to Prevotella . Bacteroides has been positively correlated with long-term diets rich in animal protein and saturated fat ( 20 , 27 ). This is likely due to their ability to tolerate bile, which is common in gut environments of those who consume animal products. In the study mentioned earlier comparing children in the US eating a Western diet vs. children in Bangladesh consuming a plant based diet, Bacteroides was the major genus in the US children's microbiota. High proportions of Bacteroides are found in the gut of humans consuming a Western diet and the opposite is found in those consuming a high fiber diet of fruits and legumes ( 27 , 37 , 43 , 47 , 48 ).

Ruminococcus is the third major enterotype and is associated with long term fruit and vegetable consumption. Species of this genus are specialized in degrading complex carbohydrates, such as cellulose and resistant starch, found in plant based foods ( 51 ). These microbes degrade dietary fibers, producing butyrate, which acts as an anti-inflammatory compound. Ruminococcus is positively associated with low BMI and negatively associated with poor lipid profile ( 27 ). Likewise, abundance of Rumminococcus has been linked to lower endotoxemia and lower arterial stiffness, a predictor of cardiovascular risk ( 33 ). Walnut consumption has been significantly associated with enrichment of Ruminococcus as well ( 38 ). However, Ruminococcus has also been linked to low dietary fiber consumption in college students. While these microbes degrade complex carbohydrates, they also break down the resistant starches found in refined grain products ( 52 ). Ruminococcus might also play a role in the conversion of animal-derived choline to trimethylamine (TMA) ( 53 ). Thus, the abundance of Ruminococcus is influenced by both animal and plant based diets.

Effects of Diet on Additional Bacteria

While Bacteroides can be pro-inflammatory and their concentration is associated with long term consumption of animal products, a study analyzing 11 vegetarians, 20 vegans, and 29 omnivores ( 49 ) found a discrepancy in this generalization. In addition to finding Clostridium clostridioforme and Faecalibacterium prausnitzii , both considered to be health protective, in higher relative abundance in the vegetarians/vegans compared to the omnivores, Bacteroides thetaiotaomicron was also observed in higher abundance in these groups. This discrepancy in categorizing bacteria abundance under a plant-based diet vs. animal-based diet is not uncommon. Clostridium cluster XIVa was found in lower ratio in the vegetarian/vegans, contrary to a study showing Clostridium cluster XIVa bacteria to be a major component of gut microbiota in vegetarian women ( 103 ). Therefore, while generalizations can be made, some genus subtypes will be outliers. This discrepancy in some bacterial phyla in response to diet has been acknowledged by previous review papers and has been attributed to various reasons, such as different microbiome profiling methodologies, different host genetics, body mass index, and red wine and aspartame consumption ( 54 , 55 ). These are all factors that have been shown to possibly modify our microbiota. Therefore, further studies are warranted in order to isolate their effects from those due to a plant based vs. omnivorous diet.

Taken together, dietary habits influence the composition of the intestinal microbiota. While dietary changes have a relatively fast impact ( 51 ) (within a week) on the microbial composition and consequently on its metabolites, these effects are modest and reversible ( 24 ). For example, changes of microbiota and immune parameters after a 3-month vegetarian diet are significant, but do not reflect the degree of change that occur with a long-term vegetarian diet ( 56 ).

How Plant Food Components Influence Gut Microbiota.

Nutrient bioavailability.

Consuming food nutrients with low bioavailability has recently been found to be important. Lower nutrient bioavailability, found in larger food particles, intact plant cell walls, and food without thermal treatment, means that more nutrients reach lower in the gastrointestinal system, thus enriching nutrient delivery to the gut microbiota ( 57 ). This helps support normal gut microbiota development and function ( 57 ). Modern westernized diets contain more ultra-processed foods and acellular nutrients, or nutrients not containing cells. These components are more easily absorbed in the small intestine, depriving the colon of important nutrients, which may alter the composition and metabolism of the gut microbiota ( 58 ). Acellular food, e.g., sugar, has been shown to induce inflammation in young infants, adolescents, women of child-bearing age, and older adults. Whole plant foods have protective effects, favoring the growth of beneficial fiber-degrading bacteria in the colon ( 58 ).

Carbohydrates

Unlike digestible carbohydrates, non-digestible carbohydrates, such as resistance starch, and some sugars, reach the large intestine where they can be fermented by the gut microbiota to provide energy or produce postbiotics. However, both digestible and non-digestible carbohydrates may influence the gut microbiota. Digestible carbohydrates from fruits (e.g., glucose, sucrose, and fructose) have been shown to reduce Bacteroides and Clostridia ( 54 ). Non-digestible carbohydrates most consistently increase lactic acid bacteria, Ruminococcus, E. rectale , and Roseburia , and reduce Clostridium and Enterococcus species ( 54 ). Both digestible and non-digestible carbohydrates have been shown to increase Bifidobacteria , genus of the Actinobacteria phylum.

A study compared the Bifidobacteria levels in response to a randomized, double-blind, crossover trail. Participants consumed both a standard enteral formula and a formula supplemented with fructooligiosaccharides (FOS) and fiber ( 59 ) as a sole source of nutrition for 14 days. FOS and fiber are both forms of carbohydrates found naturally and abundantly in plant foods–bananas, artichokes, onion, etc. While the volume of formula prescribed was based on individual energy expenditures, a benchmark of 2,000 calories of the FOS/fiber formula provided 10.2 g of FOS and 17.8 g of fiber. The average daily intake of fermentable non-digestible carbohydrates is estimated to be 10 g from inulin and FOS ( 60 ). This amount does not include meals and products supplemented with inulin and FOS, which typically add an additional 3–10 g/portion. Therefore, 10.2 g of FOS in the formula is realistic for human consumption. 17.8 g of fiber in the formula is also realistic for human consumption as the average US male and female intake is 18 g and 15 g, respectively 1 .

Bifidobacterium is a butyrate-producing genus known to play a protective role in the human gut barrier by providing defense against pathogens and diseases. When participants were given formulas with FOS and fiber, their Bifidobacteria increased from 5.1 to 26.6% ( P = 0.003) after 14 days. When formula was given without FOS and fiber, Bifidobacteria only increased from 3.3 to 8.6% ( P = 0.073). A negative correlation between baseline Bifidobacteria and magnitude of the bifidogenic effect was observed, indicating that those with lower initial amounts of Bifidobacteria benefit most from fructooligiosaccharides and fiber intake. In contrast, high consumption of cholesterol from animal products, was strongly associated with a lower abundance of Bifidobacteria (adj. p = 0.008).

While these studies suggest that Bifidobacterium increase in response to a fiber-rich, high carbohydrate diet, other studies have shown conflicting results. One important confounding factor may be alcohol intake, which has been strongly associated with a lower abundance of Bifidobacteria (adj. p = 0.006). Researchers comparing Bifidobacteria levels in vegans, vegetarians, and controls, found Bifidobacteria to be significantly lower ( p = 0.002) in vegan samples than in controls eating a standard omnivorous diet. No difference between vegans and vegetarians was observed ( 29 ). Another study observed higher Bifidobacteria levels in meat eaters compared to participants who switched to a vegetarian diet for 4 weeks after eating a mixed Western diet, high in fat and meat ( 58 ). The relative decrease of Bifidobacterium in vegetarians and vegans may be explained by a relative abundance of other protective bacteria species, such as Prevotella . Prevotella has been observed confers anti-inflammatory effects ( 40 ) and can decrease the growth of other bacteria by competing for fiber as an energy substrate ( 61 ).

A recent in vitro study elucidated the specific mechanism of action of carbohydrates, specifically selected dietary fibers, on gut microbiota. The study found the following fibers to have differing prebiotic effects: inulin, alpha-linked galacto-oligosaccharides, beta-linked galacto-oligosaccharides, xylo-oligosaccharides from corn cobs and high-fiber sugar cane, and beta-glucan from oats ( 62 ). Beta-glucan induced the growth of Prevotella and Roseburia with a concomitant increase in SCFA propionate production. Inulin and all oligosaccharides had a strong bifidogenic effect ( 62 ). This study also showed that all natural sugars, most notably non-digestible forms like inulin and oligosaccharides, increase SCFA levels ( 62 ). The prebiotic effects differ due to the type of bacteria that each fiber is broken down by. This is determined through bacterial specificity in which specific gene clusters within the bacterial genome dictate the saccharolytic enzymes that the bacteria can produce and, therefore, whether they can metabolize the prebiotic substrate ( 63 ). Non-digestible carbohydrates not only act as prebiotics by promoting the growth of beneficial microorganisms, but also reduce proinflammatory cytokine production, concentrations of serum triglycerides, total cholesterol, and LDL-cholesterol ( 54 ). Thus, non-digestible carbohydrates might confer protective effects against cardiovascular disease and central nervous system disorders.

A majority of the studies have noted that protein consumption correlates positively with microbial diversity ( 54 ). However, animal and plant-proteins influence the gut microbiota in different ways. For instance, individuals consuming a high animal protein diet, from beef which is also high in fat, displayed lower abundances of bacteria, such as Roseburia, Eubacterium rectale , and Ruminococcus bromii , that metabolize dietary plant polysaccharides ( 51 ). Populations of bacteria that increase in response to a high animal protein diet when compared to subjects consuming a meatless diet are typically bile-tolerant microorganisms, such as Bacteroides and Clostridia ( 64 ). Additionally, a high-protein diet typically limits carbohydrate intake, which may lead to a decrease in butyrate-producing bacteria, and thereby to a proinflammatory state and an increased risk of colorectal cancer ( 65 ).

Individuals consuming pea protein exhibit increases in beneficial Bifidobacterium and Lactobacillus and decreases in pathogenic Bacteroides fragilis and Clostridium perfringens and, consequently increases in intestinal SCFA levels ( 54 ). Likewise, plant-derived proteins have been associated with lower mortality than animal-derived proteins ( 54 ).

Current evidence suggests that both the quantity and the quality of consumed fat significantly impact the gut microbiota composition ( 65 ).

A plant-based diet is generally naturally low in fat, which favors beneficial Bifidobacteria in human gut microbiota. The fat that does come from a vegan/vegetarian diet is made up of predominantly mono and polyunsaturated fats, which increase the Bacteroidetes : Firmicutes ratio, and on the genera level, increase lactic acid bacteria, Bifidobacteria and Akkermansia muciniphila ( 54 ). Nuts, particularly walnuts, have been shown to improve the gut microbiota by increasing Ruminococcaceae and Bifidobacteria , and decreasing Clostridium sp. cluster XIVa species ( 38 ).

On the other hand, saturated fat, found almost exclusively in animal sources, increases Bilophila and Faecalibacterium prausnitzii , and decreases Bifidobacterium ( 54 ). Some studies report that this change activates inflammation (induces pro-inflammatory cytokines such as IL-1, IL-6, and TNF-α) and leads to metabolic disorders ( 66 ). High consumption of saturated and trans fat, predominately found in a Western diet, increases the risk of cardiovascular disease and reduces Bacteroidetes, Bacteroides, Prevotella, Lactobacillus ssp. and Bifidobacterium spp, and increases Firmicutes ( 40 , 67 ).

N-3 polyunsaturated fatty acids, have been found to result in either no change to the microbiota, or beneficial increases in Bifidobacterium, Adlercreutzia, Lactobacillus, Streptococcus, Desulfovibrio , and Verrucomicrobia ( Akkermansia muciniphila ) ( 54 , 67 ).

Polyphenols

Polyphenols, or naturally occurring plant metabolites ( 68 ), in plant foods increase Bifidobacterium and Lactobacillus abundance, which provide anti-pathogenic and anti-inflammatory effects and cardiovascular protection ( 54 ). Common polyphenol-rich foods include fruits, seeds, vegetables, tea, cocoa products, and wine. For example, polyphenol extracts from tea generate an increase in Bifidobacterium and Lactobacillus–Enterococcus spp., which then yields an increased SCFA production on human microbiota in vitro ( 69 ).

Influence of Microbiome Postbiotics on Human Health

Research on the gut-brain, gut-lung, and gut-liver axes highlights the importance of the microbiome on systemic human health. Studies note changes in central neural chemistry, inflammatory lung conditions, and non-alcoholic fatty liver disease pathogenesis with changes to microbial composition ( 70 – 72 ). The mechanism of communication among these organs stems from the microbial products and microbial metabolites of ingested nutrients. These products can be diet-independent (such as lipopolysaccharides, ribosomally synthesized post-translationally modified peptides etc.), but here we would like to describe a few examples of well-known diet-dependent metabolites, such as SCFA and others. Depending on the bacteria and location along the intestinal tract, different bioactive molecules can be produced from different prebiotics and nutrients ( 70 , 73 ). Microbial metabolites can have diverse positive health effects including local anti-inflammatory and immunomodulatory effects, and systemic anti-obesogenic, antihypertensive, hypocholesterolemic, anti-proliferative, and antioxidant effects ( 74 ). These postbiotic effects result from modulation of gene expression, metabolism, and intestinal functioning and depend on microbiota composition and substrates, largely dependent on diet.

Short-Chain Fatty Acids

SCFAs act as a substrate to maintain colonic epithelium, and are correlated with plant based food consumption ( 56 ). Maintenance of the intestinal barrier prevents endotoxemia and the subsequent inflammatory effects ( 75 , 76 ). SCFAs acetate, propionate, and butyrate, are mostly microbial metabolites of fermented fiber and other carbohydrates, although a small fraction can derive from proteins. The fecal levels of these metabolites (and the corresponding esters) positively correlate with the consumption of fruits, vegetables, and legumes. Thus, their levels significantly increase in people who begin a plant-based diet. Interestingly, an increase in SCFAs is observed when omnivores consume a Mediterranean diet rich in fruit, legumes and vegetables ( 77 ).

While specific gut microbes are predisposed for SCFA production, different bacteria are known to produce different SCFAs. For example, enteric bacteria, such as Akkermansia muciniphila, Bifidobacterium spp., Prevotella spp., and Bacteroides spp. produce acetate; Bacteroides spp. produce propionate; and Coprococcus produces butyrate ( 78 ). The most butyrate producing bacteria are in Clostridium Cluster XIVa, IV, and XVI. These species are positively correlated with consumption of plant foods, and produce SCFAs that yield several health benefits.

The protective role of acetate, propionate and butyrate against different types of disease, such as type 2 diabetes, inflammatory bowel disease, and immune diseases, is well documented. For example, it has been shown that SCFAs promote immunity against pathogens ( 78 ), and are important components for microglia function and maturation and control of the blood–brain barrier integrity ( 79 ). Other effect of SCFAs is to increase thermogenesis, preventing/treating obesity ( 80 , 81 ). SCFAs serve as energy substrates for colonocytes, as well as for the body generally. For example, propionate serves as a gluconeogenic substrate in the liver and in the intestine ( 78 ).

Microbial interactions with dietary polysaccharides and the resulting SCFAs are important energy and signaling molecules. It is becoming increasingly accepted that butyrate-producing bacteria and butyrate, per se , may be beneficial for human health ( 78 ). Butyrate has been shown to play a key role in gut physiology as a major carbon source for colonocytes. It helps regulate critical functions of the intestine, such as intestinal motility, mucus production, visceral sensitivity, the epithelial barrier, immune homeostasis, and the mucosal oxygen gradient ( 82 , 83 ). Thus, dietary fiber and carbohydrates can affect SCFA degradation while altering the abundance of the associated microbes. Taking together, diets rich in fiber might provide benefits to the intestine, as well as overall health.

Phytoestrogens

Phytoestrogens are plant-derived polyphenols that interact with estrogen receptors with either agonist or antagonist actions. A large majority of polyphenols are delivered to the gut, given their 1% bioavailability ( 57 ). The protective effects of plant polyphenols, particularly their anti-cancer, anti-inflammatory, and antioxidant effects, and their association with decreased risks of cardiovascular disease, obesity, diabetes, osteoporosis, and amyloid formation have been observed in humans ( 84 – 86 ). Increasing evidence shows that these effects are reached after bioactivation of the polyphenols by the gut microbiota ( 87 , 88 ). Even though plant polyphenols have protective effects on human health, especially in the bioactive form, there is still a possibility of adverse effects due to their complexity of action and potential inter-individual variability ( 89 ).

While not all types of microbes participating in polyphenol metabolism are yet known, it has recently been shown that Bifidobacterium, Lactobacillus sp., Coriobacteriaceae, Clostridium sp., Bacteroides , and Saccharomyces yeast, are involved in the process of converting polyphenols to equol, urolithins, and enterolignans ( 74 , 88 ). The qualitative and quantitative proportions of urolithins and equol produced correlate positively with the effects of phytoestrogens ( 88 ). Other bacterial species, such as Coriobacteriaceae and Eubacterium , are responsible for different polyphenol transformations ( 88 ).

The interaction of polyphenols and gut microbiota is bidirectional ( 90 , 91 ). The gut bacteria produce microbial metabolites from polyphenols, which in turn serve as prebiotics for the gut bacteria. These metabolites, particularly urolithins, promote the growth of Lactobacillus and Bifidobacterium ( 88 ).

Gut microbiota are crucial for adequate vitamin levels in the human body. Menaquinone, folate, cobalamin, and riboflavin (ie: vitamins K, B9, and B2) are produced by gut microbes ( 25 ). Different bacteria have biosynthetic properties for different vitamins, such as Bifidobacterium for vitamins K, B 12 , biotin, folate, thiamine, Bacillus subtilis and Escherichia coli for riboflavin ( 92 ), and Lactobacillus for cobalamin and other B vitamins ( 93 ). The pathway analysis of the predicted metagenomes showed an enrichment of folate biosynthesis in vegans compared with omnivores ( 77 ).

Isothiocyanates

Isothiocyanates are compounds from glucosinolates, mainly found in plants, like cruciferous vegetables. Escherichia coli , certain Bacteroides , some Enterococcus, Lactobacillus agilis , certain Peptostreptococcus spp. and Bifidobacterium spp. metabolize glucosinolates to isothiocyanates, secreting their own myrosinase enzyme ( 94 ). These metabolites have cytoprotective and anti-oxidative effects through regulation of gene expression relating toneoplastic, atherosclerotic, and neurodegenerative processes ( 25 ).

Aryl-Hydrocarbon Receptor Ligands

Intestinal aryl-hydrocarbon receptor ligands are predominantly diet derived from plant food, specifically cruciferous vegetables. Through aryl-hydrocarbon receptors, the ligands act to promote intestinal immune function and gut homeostasis ( 95 ). Since aryl-hydrocarbon receptor ligands are gut microbiota-derived, any impairment to the gut microbiota, such as from a high-fat diet, can decrease aryl-hydrocarbon receptor ligands. In turn, this can cause gut inflammation and permeability and promote the development of metabolic syndrome, which can be improved by supplementation with a Lactobacillus strain ( 96 ). Additionally, a decrease in aryl-hydrocarbon receptors or ligands compromises the maintenance of intraepithelial lymphocytes and the control of the microbial load and composition, resulting in heightened immune activation and epithelial damage ( 95 ).

Secondary Bile Acids and Coprostanol

A separate group of postbiotics are cholesterol metabolites. Several bacterial strains, isolated from intestine or feces, are described to convert dietary or synthesized de novo cholesterol into coprostanol ( 97 , 98 ), which is poorly absorbed by the human intestine. Thus, serum cholesterol in host is reduced, which decreases the risk of cardiovascular diseases. On the other hand, bile acids synthetized from cholesterol are converted by microbiota into secondary bile acids, found in different tissues and in feces. It is believed secondary bile acids are involved in the equilibrium of health/disease ( 73 , 97 ). For example, they are associated with inflammatory bowel disease, liver and colon cancer.

Trimethylamine N-Oxide (TMAO)

Trimethylamine N-Oxide is a microbial metabolite believed to be associated with cardiovascular and neurological disorders. Carnitine and choline are the precursors of TMAO and are primarily found in foods of animal origin (eggs, beef, pork), with lower amounts found in beans and fish ( 99 ). Several microbial genera, like L- Ruminococcus , have been linked to the intake of animal proteins and fat and have been associated with TMAO levels ( 77 ). In general, meat intake appears to proliferate species of Bacteroides, Alistipes, Ruminococcus, Clostridia , and Bilophila , and decease Bifidobacterium . Higher TMAO levels have also been observed with red meat intake, increasing risk for cardiovascular disease and inflammatory bowel disease ( 54 , 66 ). Vegetarians have a different gut microbiota composition than omnivores with a diminished capacity to produce trimethylamine (TMA), the precursor to TMAO. The plasma concentrations of TMAO appear to be similar in vegans and lacto-ovo-vegetarians ( 99 , 100 ).

Lowering TMAO levels may be achieved through greater adherence to the Mediterranean diet, particularly a vegetarian one rich in fruits and vegetables ( 77 , 100 ). Increased vegetable consumption reduces TMAO levels by reducing the enzymes responsible for converting TMA to TMAO and by remodeling the gut microbiota. The studies have shown TMAO production to decrease in vegetarians, which decreases their cardiovascular risk. To be objective, we have to mention a recent study, leaving a room for further analyses. Vegan fecal microbiota transplantation in metabolic syndrome patients resulted in significant changes in intestinal microbiota composition but failed to show changes in TMAO production. Authors explained that the 2-week follow-up was not a sufficient length of time to observe changes in TMAO production ( 101 ).

On average, twenty five percent of plasma metabolites are different between omnivores and vegans, suggesting a significant direct effect of diet on the host metabolome. No unique bacterial taxa have been significantly associated with individual metabolite levels after adjustment for multiple comparisons ( 102 ). These findings suggest that while inter-individual variability exists, dietary patterns significantly influence the microbial composition.

Current research indicates that diet is the essential factor for human gut microbiota composition, what in its turn is crucial for metabolizing nutrients into active for the host postbiotics. Up to date knowledge suggests that a plant-based diet may be an effective way to promote a diverse ecosystem of beneficial microbes that support overall health. Nonetheless, due to the complexity and inter-individual differences, further research is required to fully characterize the interactions between diet, the microbiome, and health outcomes.

Author Contributions

AT and IB contributed to conception and writing of the manuscript, ER, WY, JA, NB, and HK contributed and critically revised the manuscript. All authors approved the final manuscript.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

1 https://www.ars.usda.gov/ARSUserFiles/80400530/pdf/dbrief/12_fiber_intake_0910.pdf . (Accessed February 12, 2019)

Funding. This work was funded by PCRM and supported by the Grant Agency of Ministry of Education, Science, Research and Sport of the Slovak Republic VEGA 1/0286/18.

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The Best Vegan Butter

By Mace Dent Johnson

Mace Dent Johnson is a writer on the kitchen team. To test stand mixers, they baked 18 loaves of bread, 30 dozen cookies, and seven birthday cakes.

Butter is an elegant, single-ingredient food that humans have loved for millennia. Vegan butter is … not that. If your dinner roll needs a pat—and you can’t eat dairy—you find yourself at the mercy of the rambling ingredient lists and alien textures of vegan butter.

No vegan butter we’ve tasted replicates the subtleties and delights of the real deal . But after tasting and baking with 13 vegan butters, we’ve found a few favorites.

The research

The best vegan butter, a trader joe’s dupe for our favorite vegan butter, other vegan butters worth considering, the competition, what’s in vegan butter, a note on palm oil, how we picked and tested.

Melt Organic Cold Pressed Plant Butter

This simple vegan butter has a mellow flavor, a nice salt level, and a silky texture.

Buying Options

Melt Organic Cold Pressed Plant Butter is a smooth-textured, relatively neutral-tasting, allergen-free vegan butter. Most vegan butters we tasted were plagued by the same flavor and texture problems—bitter, acrid tastes and a loose and greasy or brittle and grainy consistency.   The Melt spread is not a perfect butter alternative—it won’t fool anyone into thinking it’s dairy—but its milder, simpler flavor won us over.

The Melt spread is sweet and slightly salty, and it has a more restrained level of faux butter flavor than most of the vegan butters we tasted. Some testers noted a slight smokiness and fishiness but still said it was one of the best-tasting of the bunch.

Out of the fridge, the Melt vegan butter had a silky, smooth texture that we enjoyed spreading on bread. Tasters said it was light, whereas other vegan butters were greasy and weighed down a bite of bread. But at room temperature, this butter was a little loose, with some tasters likening it to spreading sunscreen on bread. That looseness also contributed to frosting with the texture of cake batter and sugar cookies that spread a bit too much. But in flavor and texture, the resulting cookies were some of our favorites.

Trader Joe’s Organic Buttery Plant-Based Spread

The best vegan butter, this time at tjs.

Very similar to the Melt spread but for half the price, this plant-based spread also has a winning combination of silky smoothness, slightly salty sweetness, and as little vegetal twang as possible.

In our brand-concealed taste test, tasters detected similarities between Trader Joe’s Organic Buttery Plant-Based Spread (not to be confused with Trader Joe’s Dairy Free Buttery Spread , which comes in a block rather than a tub) and Melt Organic Cold Pressed Plant Butter, and we later noticed that the two have similar, though not entirely identical, ingredient lists. But at $4 for 13 ounces, this Trader Joe’s vegan butter costs about half as much as the Melt spread.

Like the Melt vegan butter, Trader Joe’s Organic Buttery Plant-Based Spread had only very light artificial popcorn-butter aftertaste and a saltiness that tasters found compelling and moreish. A couple of tasters picked out a slight smokiness or a vegetal aroma, while other tasters noted a sweet, slightly nutty, cookie-dough flavor.

Across the board, tasters said that this vegan butter was much more tolerable than others we tasted, with one remarking that they could see the butter disappearing readily into the background of a dish. Sometimes the best thing vegan butter can do is disappear.

Like the Melt vegan butter, this Trader Joe’s spread was smooth and spreadable out of the fridge, though not as dense and creamy as dairy butter. At room temperature, the Trader Joe’s spread was plagued by the same texture issues as the Melt spread, with a loose consistency not unlike mayonnaise or lotion. This vegan butter won’t produce stiff-peaked frosting or tall cookies, but it will spread onto a slice of bread without raising any eyebrows.

If you want the best butter substitute, regardless of the price (and if you can find it in stores near you): Monty’s Original Pure Plant-Based Cloud Butter is an impressive, high-end vegan butter. It has a somewhat cheesy dairy flavor and a rich, creamy texture. It won out in our sugar-cookies baking test—cookies we made with the Monty’s spread held some of their shape and maintained a chewy, slightly underdone center evocative of Levain-style cookies, while every other cookie spread thin and flat. If we could get an unlimited supply of any of these vegan butters, we would pick Monty’s. But this vegan butter is the most expensive we’ve tasted (about $13 for 5 ounces at this writing), and we can’t imagine splurging on it regularly. Plus, the Monty’s spread might not be easy to find in your area , and online orders have been sold out for over a month at this writing.

If you want a palm-oil-free option: Most tasters found Wayfare Dairy Free Salted & Whipped Butter to be sweet, slightly salty, and mild, free of the artificial butter flavor that overpowered other options, though vegetal in flavor. Our panel unanimously deemed its appearance challenging—the thick, grayish spread reminded multiple tasters of plaster. This vegan butter looks and tastes plant-based. But it spread well on bread, it was smooth, and it was palatable enough for most tasters.

If you’re making frosting or you want to know which of the widely available Country Crock spreads is the best: We preferred Country Crock Plant Butter with Olive Oil to the company’s avocado-oil option. Though the two were similar, the base flavor of the olive-oil spread did more to meld with the strong faux butter flavor that both options have. (The olive oil spread comes in sticks , too.)

The Country Crock olive-oil vegan butter was one of the easiest vegan butters for us to bake with, mixing and creaming more similarly to dairy butter than other options, though the cookies still had too much spread. It shone in our frosting test, yielding results with a texture somewhere between that of fluffy canned frosting and actually decent buttercream. And the flavor was fine, with no lingering offness that could ruin a cake.

On bread, the spread was grainy and a little oily but easily spreadable. On its own, it had a slightly sweet, baked-goods flavor, as if it already contained flour and sugar, so it was not our favorite option when we ate it with bread.

We didn’t hate Country Crock Plant Butter with Avocado Oil or its stick version , but we found it a bit gritty and overpowered by a potent faux butter flavor. Most tasters preferred Country Crock Plant Butter with Olive Oil .

One taster remarked that stuff like Earth Balance Original Buttery Spread gives all nondairy and vegan food a bad name. The rancid-oil flavor reminded us of eating a crayon or a slightly fishy lotion.

Earth Balance Soy Free Buttery Spread was our favorite among Earth Balance’s offerings, but it was still tinged with the bitter, vegetal flavor of vegetable oil, which came through in cookies and frosting.

Fresh out of the fridge, Miyoko’s Creamery Salted Oat Milk Butter was spreadable but gritty and had a plasticky flavor. At room temperature, it separated into an inedible mixture of greenish oil and sickly goo.

Miyoko’s Creamery Salted Plant Milk Butter was crumbly and bland at best, with hints of plastic-wrapped, freezer-burned vegan ice cream or rancid coconut.

Smart Balance Original Buttery Spread was firmer and smoother than many of the vegan butters we tasted, but the flavor was polarizing. For some tasters it evoked cultured butter or nostalgic, old-school margarine, while others thought it tasted of scallops and imitation crab. It made off-tasting cookies and frosting. But if you just want a widely available, margarine-like spread, it could do the trick.

Trader Joe’s Dairy Free Buttery Spread is almost identical in appearance and ingredients to the Violife spread we tested. Tasters found it slightly funky and overly fake-tasting.

Violife Salted Plant Butter had a bland, oat-y flavor and a slightly acrid aftertaste. Some tasters found the blandness a relief in comparison with more flagrantly unpalatable options—and it contains no palm oil, if you’re looking for another palm-oil-free option.

Each vegan butter maker uses a different formula to turn vegan oils into something that evokes churned cream butter. Most contain some blend of oils, from sources such as sunflower, soybean, palm, flaxseed, and canola. Some makers also use ingredients such as butter beans, cashews, or oats to try to add body for a creamy texture.

Vegan butters often also contain some amount of water, plus homogenizing or thickening agents like sunflower lecithin. In reading ingredient lists, we found that most vegan butters also contained some sort of unspecified “flavor,” with some vegan butters smacking of artificial popcorn-butter essence.

We tended to prefer salted options, even for baking—unsalted versions of spreads tended to have stronger off-flavors, especially industrial or plasticky tastes. Salt seems to help these spreads’ ingredients taste more like, well, food.

The ingredient lists of vegan butter can be hard to decipher and distinguish between. Across brands, or even different options within one brand’s line, many vegan butters seem to have nearly identical ingredients, in subtly different orders. As with vegan ice cream , if you notice that you like a certain vegan butter, it could help to check what its most prominent ingredients are, so you can try other vegan butters that are made with similar bases.

Many vegan butters available today contain palm oil. The production of palm oil is notoriously associated with environmental harm, including deforestation, loss of biodiversity, and harmful monoculture. Some brands, such as the maker of our top pick, Melt, say that their palm oil is from “sustainable ethical palm fruit” certified by the Roundtable on Sustainable Palm Oil (RSPO) , though some RSPO-certified companies have faced criticism for reported human-rights violations.

If you prefer to avoid palm oil, we have two palm-free options that are worth considering . If you are eating a vegan diet or eating less meat and dairy, you might also consider that you are lessening your participation in the meat and dairy farming industry, which has its own significant environmental impacts , and trading off that footprint for that of ingredients like palm.

I’m lactose intolerant, and I’ve been baking with and eating non-dairy butter for around eight years. I also wrote Wirecutter’s guide to the best vegan ice cream , and I tested vegan ice cream recipes while working on our guide to the best ice cream maker , so I’m well-versed in the vicissitudes of vegan dairy.

To determine what to test, I read up on vegan butter discourse and compiled a list of promising options that were relatively widely available in stores or for purchase online.

We aimed to include butters across a range of prices and as many options as possible that are free of common allergens and free of palm oil. We ruled out vegan butters that have water as their first ingredient, because those are not great for cooking or baking due to their lower fat content.

In a first round of brand-concealed tasting, a panel of tasters tried 12 different vegan butters. We ate each butter on its own off the spoon, spread on soft and mild potato bread, and spread on crusty sourdough. We did this round of tasting twice: once with cold butter from the fridge and once with room-temperature butter. We took note of how each butter changed as it melted, flagging any that separated or liquified.

Next, I baked sugar cookies with our favorites from the first round of testing, and we compared the appearance, texture, and flavor of the sugar cookies in another brand-concealed tasting. We took note of which cookies had too much spread, and which ones maintained a chewy interior more akin to that of dairy-butter cookies. We hoped for a pleasant buttery flavor but were satisfied in the end by any cookie free of the bitter, industrial flavors that can mar dairy-free baked goods.

Finally, I made a simple buttercream frosting with the vegan butters from the baking round. We tasted the frosting on its own and on yellow sheet cake. We wanted a frosting with stiff peaks, a fluffy, smooth texture, no acrid or vegetal flavors, and enough longevity to remain on a cake at room temperature without wilting.

This article was edited by Marguerite Preston and Marilyn Ong.

Meet your guide

Mace Dent Johnson

Mace Dent Johnson is a staff writer on the kitchen team at Wirecutter. Their background is in creative writing and academic research, and they are always thinking about food.

Further reading

The Best Vegan Ice Cream

by Mace Dent Johnson

We tried 35 pints of vegan ice cream from 16 ice cream brands. We loved nine of them, including flavors from Jeni’s, Van Leeuwen, and Trader Joe’s.

The Best Pizza Stone and Baking Steel

by Lesley Stockton

We think the FibraMent-D Baking Stone is the best all-purpose stone for making crisp pizza, crusty bread, and golden pastries.

The Best Ice Cream Maker

We’ve tested 21 ice cream makers, and our favorite is the Cuisinart ICE-21 . It’s a great tool for beginners and pros alike.

Most Gift Baskets Are Terrible. These Are Great.

by Gabriella Gershenson, Anna Perling, and Wirecutter Staff

These gift baskets offer the best combination of taste, presentation, and value we’ve found, and we think they’ll delight recipients year-round.