Was Pythagoras right? The father of vegetarianism (and veganism, free of any animal product) did not yet know that a healthy gut microbiota is the result of the balance between two large groups of bacteria: Bacteroidetes and Firmicutes. A balanced flora is typically associated to a diet rich in fruits, vegetables and cereals, and poor in sugars, fat and animal proteins. Western diet, which includes opposite dietary intakes, could be detrimental to the microbiota and promote obesity.
Beware of popular beliefs!
Reality is not that simple: extensive work on Bacteroidetes and Firmicutes showed that if some species from this bacterial group are truly more abundant in case of vegetarian and vegan diets (Prevotella and Ruminococcus), others (such as Bacteroides) are more abundant in people who frequently eat animal proteins and fats. This is why several studies have shown that an imbalance between Bacteroidetes and Firmicutes impacts the body mass index and corpulence... but with contradictory results!
Beneficial fibers
Fibers are favorable to “good” bacteria in the gut microbiota. They also promote the growth of species that degrade fibers into short-chain fatty acids, i.e., molecules with anti-inflammatory properties. Polyphenols, antioxidants very abundant in plants such as apples and grapes, also promote the growth of some bacterial species with protective effects (Bifidobacterium and Lactobacillus). Conclusion: the gut microbiota of vegetarians and vegans, more abundant and more diversified, could prevent inflammatory and cardiovascular diseases and obesity.
These interactions should be further explored
Long-term vegetarian and vegan diets seem to be the most effective way to promote diversity and abundance in the gut microbiota, which are a prerequisite for good health. However, the authors concluded that because of inter-individual differences (ethnicity...) and because it is a complex subject, additional studies should be carried out in order to characterize interactions between diet and gut microbiota and better measure the impact on health.
