In April 2019 researchers from Slovakia and the USA published a review on the effect of vegetarian and vegan diets on gut microbiota. Whilst humans are about 99.9% identical to each other in terms of their genome (complete set of DNA, including all of its genes), but their gut microbiome (a vast community of trillions of bacteria and fungi that inhabit every part of the gastrointestinal tract) can be up to 80–90% different. The gut microbiome can have a major influence on metabolism, body weight, risk of illness, immune system, appetite and mood. The difference in gut microbiota composition between individuals following vegan or vegetarian diets and those following omnivorous diets is well documented. Several studies have suggested that there are three basic bacterial types, namely (1) genus Prevotella (considered to be mostly anti-inflammatory and therefore protective), (2) genus Bacteroides (more pro-inflammatory and possibly linked to a heightened risk of metabolic syndrome and other conditions), and (3) genus Ruminococcus (of which the significance is less evident). In addition to these three, Bifidobacterium, Eubacterium, Clostridium, Streptococcus, and Enterobacteriaceae are most commonly found. However, it should be noted that stool samples are only estimations of the gut microbiota rather than a complete representation as anaerobic species often attach to the gut mucosa, making it difficult to identify all bacterial species present in the large intestine.
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 other medical 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 unfavourable changes in gut microbiota composition. Therefore current research is focusing on the development of interventions such as personalized nutrition.
A plant-based diet appears to be beneficial for human health by promoting the development of a more diverse microbial system. Additionally, vegans and vegetarians have significantly higher counts of certain Bacteroidetes-related organisms compared to omnivores. Fibre (ie non-digestible carbohydrates, found exclusively in plants) most consistently increases 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 fibre intake also encourages the growth of species that form metabolites as short-chain fatty acids, including acetate, propionate, and butyrate. Short-chain fatty acids have many positive health effects, including improved immunity against pathogens, blood–brain barrier integrity, provision of energy substrates, and regulation of critical functions of the intestine.
Tomova A et al. The Effects of Vegetarian and Vegan Diets on Gut Microbiota. Front Nutr. 2019 Apr 17;6:47.