Vegan and Omnivore Gut Microbiomes Differ Sharply, But Plant Diversity Matters Most

A team led by Nicola Segata at the University of Trento has produced the most detailed gut microbiome map of dietary patterns to date, analyzing stool samples from 21,561 individuals across five independent cohorts in the US, UK, and Italy. Published in January 2025 in Nature Microbiology, the study found that microbial profiles alone could distinguish vegan, vegetarian, and omnivore diets with a mean AUC of 0.85, a classification accuracy that underscores just how deeply long-term food choices restructure the microbial ecosystem of the gut.

Omnivore Gut Microbes Linked to Colon Cancer and Heart Disease Risk

Red meat emerged as the single strongest driver of the omnivore microbial signature. Omnivore guts were enriched in bacteria specialized for digesting animal proteins, most notably Alistipes putredinis, along with Ruminococcus torques and Bilophila wadsworthia. These species carry a clinical concern: they have been independently associated with increased risk of colorectal cancer, inflammatory bowel disease, and cardiovascular disease. Omnivore-signature microbes also negatively correlated with host cardiometabolic markers across the cohorts, a pattern consistent with broader evidence linking gut dysbiosis to inflammatory, metabolic, and cardiovascular disorders. The finding does not prove causation, but the correlation across five independent datasets strengthens the signal considerably.

Alpha diversity, a measure of the variety of species within a single gut sample, was actually higher in omnivores than in vegans or vegetarians. That result sounds counterintuitive, but the researchers note that higher diversity in omnivores partly reflects the presence of meat-digesting specialists rather than a broader community of health-promoting microbes. Diversity alone, in other words, is not the whole story.

Vegan Microbiomes Favor Fiber Fermentation and Cardiometabolic Health

Vegan gut microbiomes told a different story. They were enriched in bacteria from the Bacteroides and Firmicutes phyla that specialize in fermenting dietary fiber, producing short-chain fatty acids including butyrate as a byproduct. Butyrate feeds colonocytes, reduces intestinal inflammation, and supports metabolic and immune balance. Vegan-signature microbes correlated positively with favorable cardiometabolic markers, and notably, omnivores who consumed more diverse plant-based foods showed enrichment of those same vegan-associated microbes. The finding points directly to diet quality as a mechanism: it is not the absence of meat per se, but the presence of varied fiber-rich plants, that appears to tip the microbial community toward a health-promoting profile.

This aligns with earlier work. A study of Italian omnivores, vegetarians, and vegans found that vegetable-based diets increased fiber-degrading bacteria and elevated fecal short-chain fatty acid levels, echoing the metabolic pathway the new, much larger study now confirms at scale.

The vegetarian microbiome sat squarely between the two extremes. It shared some vegan features, particularly fiber-fermenting species, but also carried bacteria associated with dairy consumption. Streptococcus thermophilus, a bacterium used in yogurt and cheese fermentation, appeared in both vegetarian and omnivore guts but was largely absent in vegans. Vegans, conversely, harbored microbes typically found in soil, arriving via fruits and vegetables, while showing the fewest food-associated bacterial species overall, except those traceable to plant surfaces.

How 21,561 Participants and Shotgun Metagenomics Made This Study Credible

Most prior diet-microbiome studies have relied on 16S rRNA sequencing, which identifies bacteria at the genus level at best. This team used shotgun metagenomics of stool samples, a method that resolves microorganisms down to the strain level, giving far finer resolution. Participants self-reported their long-term dietary patterns, and statistical differences between groups were assessed via PERMANOVA, correcting for sex, age, and BMI. Results were replicated through a meta-analysis pooling species-level genome bins across all five cohorts.

The scale also matters. A large cohort gut microbiome study like the Rotterdam Study previously examined around 1,130 participants. Conducting that kind of analysis across 21,561 individuals, drawn from three countries and five independent datasets, substantially reduces the risk that any single cohort's idiosyncrasies are driving the results. The multi-cohort design is the methodological backbone that makes the headline findings credible rather than preliminary.

Open Questions About Self-Reported Diet and Long-Term Causality

The study carries real limitations. Dietary patterns were self-reported, which introduces classification noise: a self-described omnivore who rarely eats red meat looks different from one who eats it daily, and those distinctions can blur at the population level. The cross-sectional design, which captures a snapshot rather than following people over time, means the study cannot establish whether the microbial differences cause differences in cardiometabolic health or merely reflect them.

Standardizing methods and moving toward longer-term intervention studies remain key challenges across the field. The researchers acknowledge that translating population-level microbial signatures into actionable clinical guidance will require prospective designs and, eventually, controlled dietary interventions that can test whether shifting someone's diet actually shifts their disease risk through the microbial pathway.

Precision Nutrition and the Fiber Diversity Message

The research feeds into a larger precision nutrition agenda. The goal, as Segata's group frames it, is to tailor dietary advice to individual microbiome profiles and cardiometabolic markers rather than issuing blanket population-level guidelines. Health policy is already beginning to engage with microbiome evidence, with recent Dietary Guidelines for Americans reports acknowledging that diet-microbiota research should inform future recommendations.

Segata himself drew the practical takeaway plainly: "The quantity and diversity of plant-based foods have a very positive impact on the microbiome. Avoiding meat or dairy products does not necessarily have a positive effect if it does not come with a variety of quality plant-based products. From the point of view of the microbiome, what we can generally recommend is that it is important to eat many plant-based foods, especially those rich in fiber. And that food diversity is important."

The microbial signatures identified here are also positioned as tools for future epidemiological studies and nutritional interventions, providing a validated shorthand for diet quality that researchers can apply without relying on imprecise food-frequency questionnaires alone. For anyone watching the plant-based diet debate, the headline from 21,561 guts is less about veganism as a label and more about what ends up on the plate: variety, fiber, and plants in quantity appear to be what the microbiome is actually voting for.