Diet is inextricably linked to human health. Poor diets, low in unprocessed, plant-based foods, cause more deaths globally than any other risk factor, with cardiovascular disease, cancers, and type 2 diabetes as the leading causes of diet-related deaths. Unhealthy diets also carry a wide range of negative environmental impacts. Animal-based foods contribute more than plant-based foods to global environmental change through their impact on climate, land and freshwater use, and biodiversity. Consequently, there is increased interest in diets with higher fractions of plant-based foods that decrease both risk of disease and negative environmental impacts.
The Gut Microbiome: A Key Player in Human Health
The gut microbiome plays an integral role in human health that can be modified by diet. Fermentation of otherwise indigestible plant polysaccharides by gut microbes contributes to a healthy, non-inflamed gut barrier and maintenance of gut homeostasis through the production of short-chain fatty acids (SCFAs) and immune system crosstalk. Plants contain polyphenols, the products of plant secondary metabolism, that are known to promote beneficial bacteria that prevent inflammation, enhance the gut barrier, and hinder potential pathogens.
By contrast, a diet rich in animal foods leads to increased protein fermentation, which may result in a leaky mucosa, local and systemic inflammation, and reduced production of SCFAs. The breakdown of certain animal proteins is linked to the synthesis of gut microbial trimethylamine (TMA), which is oxidized in the liver to trimethylamine N-oxide (TMAO). TMAO has been implicated in various (cardio)vascular diseases and is a potential contributing factor in colorectal cancer.
Investigating the Gut Microbiome: A Large-Scale Study
To elucidate how prolonged dietary preferences affect the structure and function of the human gut microbiome at both the global and single-species level, a study capitalized on three cohorts from the ZOE PREDICT program from the United Kingdom (P1, P3 UK22A) and from the United States (P3 US22A). The study further included two additional, publicly available cohorts comprising Italian participants (Tarallo et al. (2022) and De Filippis et al. (2019)). Each participant of the five cohorts reported their nutritional habits as being either ‘omnivore’ (including meat, dairy, and vegetables), ‘vegetarian’ (excluding meat), or ‘vegan’ (excluding both meat, dairy, and other animal products) and donated stool samples that underwent shotgun metagenomic sequencing. In total, 656 vegans, 1,088 vegetarians, and 19,817 omnivores were considered. In addition to participants’ overall dietary habits, the ZOE PREDICT cohorts included data on habitual consumption of over 150 single foods per individual, obtained from validated quantitative food frequency questionnaires (FFQs).
Plant-Based Diet Index (hPDI)
To quantify the consumption of plant-based foods, the plant-based diet index (hPDI) was considered, which gives higher scores to healthy plant foods and reverse scores to less healthy plant and animal foods. Within each of the three PREDICT cohorts, hPDI significantly differed between diet patterns as expected, with significantly higher hPDI in vegans compared with vegetarians and similarly for vegetarians compared with omnivores.
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Gut Microbial Diversity and Composition Across Diet Patterns
Gut microbial richness differed significantly according to diet patterns in the PREDICT cohorts, with a lower observed richness in vegans and vegetarians compared with omnivores, but no significant differences between vegans and vegetarians. This highlights that alpha diversity might correlate with diet patterns that are potentially more diverse.
Overall gut microbial composition also differed significantly according to diet pattern, with the variation in beta diversity explained by diet pattern aligning with previous studies. In addition, diet patterns were highly distinguishable based on quantitative gut microbial profiles when using machine learning classifiers. The highest predictability was obtained when separating vegans from omnivores, followed by separating vegetarians from vegans, and finally vegetarians from omnivores. Because it was not logged when diet patterns may have been switched, it is hypothesized that the non-perfect classification might be due to individuals who switched diet patterns recently, and some associations may actually be stronger than what was observed. Altogether, these results warranted further investigation into the specific microbiome components responsible for these differences.
Gut Microbe Signatures of Vegans, Vegetarians, and Omnivores
To explore which microbes are associated with the different gut microbial compositions between vegans, vegetarians, and omnivores, a meta-analysis was performed across the five cohorts on the differential relative abundance of each SGB within each individual and their respective diet pattern. In total, 488 SGBs were significantly differentially abundant in omnivores compared with 112 SGBs in vegetarian microbiomes; 626 SGBs were significantly differentially abundant in omnivores compared with 98 SGBs in vegans; and 30 SGBs were significantly differentially abundant in vegetarians compared to 11 SGBs in vegans. When focusing on the top 30 microbial markers, the majority of these strongest associations were linked to the least restrictive diet pattern.
Knowledge of the predicted functions of the SGBs linked to the various diet patterns revealed potential dietary-specific niches. Several SGBs increased in omnivore microbiomes are linked to meat consumption by aiding in its digestion through, for example, protein fermentation (Alistipes putredinis), utilizing amino acids and via bile-acid resistance (Bilophila wadsworthia), or are mucolytic indicators of inflammation that have been linked to inflammatory bowel diseases (Ruminococcus torques). In contrast, several SGBs overrepresented in vegan microbiomes are known butyrate producers (Lachnospiraceae, Butyricicoccus sp., and Roseburia hominis) and are highly specialized in fibre degradation (Lachnospiraceae). In addition, Streptococcus thermophilus, a common dairy starter and component, had the highest effect size in vegetarian versus vegan gut microbiomes and second highest effect size in omnivore versus vegan gut microbiomes. Thus, when a major differentiating characteristic between diet patterns lies in dairy consumption, the SGB with the greatest ability to differentiate between those diets is abundantly found in cheese and yogurt products. This was supported by other dairy-linked SGBs associated more with omnivore and vegetarian than vegan diets such as Lactobacillus acidophilus, Lactobacillus delbrueckii.
Vegan Diet and Gut Microbiome: Recent Research
New research shows that following a vegan diet for about 4 months can boost your gut microbiome. In turn, that can lead to improvements in body weight and blood sugar management.
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Researchers studied 147 participants, randomized into two groups. One followed a low-fat vegan diet. The other made no changes to their diet. After the 16-week study was completed, researchers reported the vegan group saw their body weight, fat mass, and visceral fat levels go down.
The microorganisms that live in the digestive tract, when properly balanced, promote a healthy digestive tract, along with the immune system, bowel movements, metabolism, and hormones that help with appetite regulation. But when the microbiome is unbalanced, things can get out of whack.
The Importance of Plant-Based Foods
Multiple studies show benefits to a plant-based diet. One of the greatest predictors of good gut health is a variety of antioxidant-, phytonutrient- and fiber-rich foods. Plants provide the bulk of these.
A vegan diet that promotes high-fiber foods that come from plants will improve the gut microbiome. But when we start to take out all animal protein, we tend to limit ourselves with where our protein is coming from. If you’re [eating] from a vegan diet, it’s mostly coming from beans and some vegetables. So it’s really important to make sure you don’t fall short on any nutrients.
A vegan diet can be less advantageous if all your foods are frozen dinners and white grains. Doing your research and meeting with your doctor or dietitian to help you get started is recommended.
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How to Incorporate More Plant-Based Foods into Your Diet
The first step is familiarizing yourself with the different vegetables that are out there, specifically the vegetables that have the prebiotic fibers. These are the initial phase of what probiotics feed on: indigestible fibers that help encourage the growth and proliferation of the probiotics. High-prebiotic foods include asparagus, onions, Jerusalem artichokes, cabbage, garlic, cashews, lentils, and chickpeas.
When planning a meal, it’s helpful to think in terms of thirds. A third of the plate should be vegetables, a third should be lean protein sources, and a third should be complex carbs, such as sweet potatoes, beets, quinoa, bran, and oats. There’s also room for healthy fat, such as olive or avocado oil, because they help improve heart health.
It's recommended entirely cutting out red and processed meat, or at least limiting these products to twice a month. Eat as close to nature as possible. Think about what you’re putting in your body. How many steps did it have to go through to get to you? Choose foods that are close to nature, the ones that include one ingredient.
Systematic Review of Plant-Based Diets and Gut Microbiome
Plant-based diets have grown increasingly popular across the globe, mainly for their health and environmental benefits. Several studies have identified a link between plant-based diets and the decreased risk of developing cardiovascular diseases, obesity, and other health issues. A systematic review of human interventions was conducted to identify the relationship between various plant-based food items and the gut microbiome, alongside the biochemical and anthropometric measurements as secondary findings.
The most prevalent plant-based diet practiced is vegetarianism, with an estimated 1.5 billion followers worldwide. With Asia being the leading continent for plant-based diet adoption, it is estimated that almost one-fifth of the Asian population embraces vegetarianism. More predominantly, India has the highest vegetarianism following at nearly 40% of the population. Moreover, vegetarianism has been linked to religious followings that promote nonviolence and respect for all living beings, including Hinduism, Sikhism, Jainism, and Buddhism. Subsequently, the increase in the number of plant-based diet consumers is accredited to the increasing social awareness of animal rights and animal cruelty when producing food sourced from animals.
Plant-based protein sources are also increasing in developed countries via foods such as tempeh, lentils, and quinoa. Many believe that plant-based foods are healthier and beneficial to their gut microbiome, as they generally undergo less chemical processing and are more natural. In general, plant-based diets are lower in saturated fats and high in fiber and phytochemicals, contributing to lower concentrations of blood low-density lipoprotein cholesterol. Thus, integrating these diets has decreased the risk of followers developing cardiovascular diseases, obesity, diabetes, hypertension, and certain cancers.
Plant-Based Diets: A Summary of Findings
The database search found 203 records. In total, 101 titles and abstracts were evaluated after 102 duplicate entries of articles were removed. Subsequently, 23 full texts were reviewed using the review eligibility criteria. A manual search yielded six additional articles; one was excluded after not fulfilling the criteria. Most reports were excluded because they did not report on a plant-based intervention, did not report findings on gut microbiota, or were conference abstracts.
The summary of the main characteristics of the included studies is displayed in Table 1. The comprehensive details of the studies are included in Supplementary Table S3. All 12 included studies were assigned to interventional study design. Two studies were published in the 1990s, three studies were published between 2010 and 2018, and all other studies were retrieved from the year 2020 to 2021.
The link between diet and microbiota composition in vegan or vegetarian intervention is displayed at the class, family, genus, or species level, according to the information retrieved from the included studies.
At the family level, contradictory results were observed for Enterobacteriaceae. One study found higher abundances of Enterobacteriaceae in the adult with CVD risk with a plant-based diet and low abundances in vegans. Four studies found statistically significantly increased levels of Ruminococcaceae in vegan and plant-based diets. Bacteroidaceae was reduced significantly in vegan and plant-based diets. Ahrens et al. reported a significant increase in Lachnospiraceae, Ruminococcaceae, Monoglobaceae, Eggerthellaceae, Christensenellaceae, Butyricicoccaceae, and Erysipelatoclostridiaceae in individuals who followed a six-day plant-based diet. The same applies to the genus Faecalibacterium, which tended to be less abundant in vegans and had a high abundance in plant-based diets. Genus Alitispes was increased in animal-based and vegan diets. In contrast, Ahren et al. reported a reduced abundance of Alistipes in individuals with a plant-based diet. Bacteroides were found to be statistically increased in animal-based and vegan diets, compared to a significant reduction in plant-based diets. The genus Faecalibacterium and Coprococcus were higher in vegans and significantly reduced in plant-based diets. The plant-based diet, vegan diet, and ovo-lacto vegetarian diet had increased abundances of the genus Ruminococcus. Finally, the genus Roseburia was found at statistically increased levels in the plant-based diet and ovo-lacto vegetarian diet; however, the genus abundance was decreased in the animal-based diet and vegans. The Faecalibacterium prausnitzii sp, was significantly increased in plant-based and vegan diets. The Bacteroides fragilis was markedly increased in vegans, but, Kahleova et al. also reported a significant reduction among individuals with a low-fat vegan diet.
Various microbial metabolites have favorable health benefits. These include anti-inflammatory, immunomodulatory, systemic anti-obesogenic, antihypertensive, hypocholesterolemic, antiproliferative, and antioxidant effects. These postbiotic effects depend on the composition and substrates of the microbiota and are mainly influenced by diet. They arise from the control of gene expression, metabolism, and intestinal function.
The Key to a Healthy Gut: Diversity of Plant-Based Foods
Our guts are home to trillions of microbes that have a profound influence on our overall health. New studies find that - whether you're vegan, vegetarian, or omnivore - the key to a healthy gut microbiome is the same: Eat lots of different plant-based foods.
The more plant-based foods people ate, the more gut bacteria they had that are linked with better health - such as less inflammation and a stronger immune system. As for omnivores, their gut microbiomes tended to have more bacteria associated with meat digestion, which makes sense. But they also had more bacteria linked to inflammatory bowel disease and a higher risk of colon cancer. But the good news for meat eaters is that, when it comes to having a healthy composition of good bacteria, their gut microbiomes looked a lot like those of vegans and vegetarians - so long as they ate an array of plant-based foods in large amounts.
Eating an array of plant-based foods is more important than strictly adhering to a vegan or vegetarian diet pattern. It's the overall quality of the diet that counts. For better health, eat the rainbow - meaning lots of different types of colorful plant foods. Think tomatoes, butternut squash, pineapple, avocado, broccoli and kale, blueberries and eggplant. And really, don't forget about those whole grains, nuts, seeds, and legumes.
People who ate more than 30 different types of plant foods each week also had more gut microbes that produce chemical messengers known to boost health. But you don't have to aim for 30 to improve your health. A simple goal is to eat five different plant-based foods every day. For example, consider making bean or lentil soups with lots of vegetables, or mixing up some pears or berries in your salad.
As you try to diversify the plants in your diet, look for fiber-rich options, because most Americans don't consume enough of this dietary-hero nutrient. One easy idea: Try adding seeds like chia seeds, pumpkin seeds, or flax seeds, as an easy way to diversify your plant intake and boost the fiber content of any meal.
Frozen foods - whether it's berries or vegetable medleys - are also a great option. It's usually cheaper than buying fresh, and it's still just as healthy because you're still getting vitamins and minerals, antioxidants. The vegetables are picked at their peak, so they are still providing tons of nutrients.
Unilever's Perspective on Plant-Based Diets and Gut Health
Research has revealed insights into how a plant-based diet can improve the gut microbiome. Over half of the microbes in our gut can be altered by what we eat, so the trick is to ‘feed’ the beneficial ones that are critical to maintaining good health. Research has shown that plant-based diets are particularly good at providing the nutrients needed to support a wide diversity of helpful microbes in our gut. The result is that these ‘good’ microbes become influential, improving the microbiome’s composition in a matter of weeks or even days.
With 3.4 billion people using our products every day, Unilever has a real opportunity to help make plant-based foods an affordable, accessible choice for consumers. That’s why we’ve set ourselves a target to sell double the number of products that include meaningful amounts of healthy ingredients such as vegetables and fruits.
Unilever recently published two nutrient diet reviews which jointly analysed over 170 papers comparing the nutritional impact of vegetarian and vegan diets on both adult and child and adolescent health compared to meat-based diets. The reports showed that in general plant-based diets are higher in fibre, polyunsaturated fatty acids (PUFAs), folate, vitamin C, vitamin E and magnesium than meat-based diets. This is great news as fibre and PUFAs are prebiotics that can feed health-boosting gut microbes.
Based on this research, we should be aiming to eat 30 or more different plant foods a week to keep our gut microbes happy.