Recent research has studied the role of the gut microbiome in modulating risk of several metabolic and immune-mediated diseases, including inflammatory bowel disease, obesity, type 2 diabetes, cardiovascular disease, autoimmune psoriasis and arthritis, and cancer. Although it is a well-known fact that diet is a major player in modulating both composition and function of the gut microbiome, little is known about the possible therapeutic utility of modulating the gut microbiome through dietary components.
A recent review, led by Dr. Wilson Liao from the Department of Dermatology at the University of California (San Francisco), has explored how specific dietary components and dietary patterns interact with the gut microbiome.
Regarding macronutrients, dietary proteins differentially affect gut microbiota depending on the source. Plant protein increases Bifidobacterium and Lactobacillus genera, leading to an increase of short-chain fatty acids (SCFAs), a subsequent gut barrier strengthening, and a decrease in inflammatory response. However, animal protein leads to an increase of Bacteroides, Alistipes, Bilophila and Ruminococcus genera and a decrease in Bifidobacterium genera. This is associated with an overall increase of trimethylamine-N-oxide (TMAO) -a pro-atherogenic substrate that may increase the risk of cardiovascular disease- and a decrease in SCFA production.
Fat also may differentially affect the gut microbiome. Saturated fatty acids icrease bacteria that promote metabolic inflammation through toll-like receptor signalling, such as Bacteroides and Bilophila. On the other hand, unsaturated fatty acids increase Streptococcus, Lactobacillus, Bifidobacteria and Akkermansia muciniphila, bacteria that promote an anti-inflammatory phenotype and are related to decreased plasma total- and LDL-cholesterol.
The effects of both types of carbohydrates (digestible-starch/sugars and non-digestible-fibre) are also reported. Artificial sweeteners and natural sugars (glucose, fructose, and sucrose) differentially affect the gut microbiota. Regarding non-digestible carbohydrates, the researchers mention that intake of prebiotics (from several sources) may result in an increase in gut microbiota gene richness, and some of them may lead to an increase of intestinal Bifidobacterium and lactic acid bacteria. Beyond gut health, prebiotics may also have both metabolic and immune effects, as previously reported.
The review also explores the effects of probiotics on the gut microbiota. Specifically, it assesses the stimulating effect of probiotics on counts of commensal bacteria and inhibitory effects on enteropathogens like Escherichia coli and Helicobacter pylori. Besides this, health benefits from consuming probiotics are reported for different conditions. An update on probiotic use in clinical practice can be reviewed in the latest World Gastroenterology Organisation “Probiotics and prebiotics” Guideline.
Finally, the review explores the role of dietary polyphenols on gut microbiota richness and diversity. The final outcome depends on the food matrix source and can be summarized overall with an increase of Bifidobacteria and Lactobacilli genera and a decrease in Bacteroides, Clostridia, Salmonella typhimurium, and Staphylococcus aureus.
When it comes to focusing on overall dietary patterns in human studies, special diets such Western, Mediterranean and gluten-free diets have differential effects on gut microbiota. The Western diet (high in animal fat and protein and low in fibre) leads to a decrease of total bacteria and beneficial Bifidobacterium and Eubacterium, together with increased production of cancer-promoting nitrosamines and inflammation. Secondly, short-term gluten-free diets increase numbers of E. coli and Enterobacteriaceae, which include further opportunistic pathogens. Vegan and vegetarian diets (rich in fermentable plant-based foods) have differential and sometimes contradictory effects on the gut microbiome and the authors emphasize that more long-term follow up studies are needed in order to elucidate its effects. Finally, a high adherence to a Mediterranean diet (high in fibre and antioxidants and low in red meat) is related to increased levels of faecal SCFAs, Bifidobacteria, Lactobacilli, Eubacteria, Bacteroides and Prevotella. The authors explain that the beneficial effects of the Mediterranean diet on chronic conditions such as obesity, cardiovascular disease, and inflammation could be explained, at least in part, by changes in gut microbiota.
On the whole, dietary components (from macronutrients to polyphenols, prebiotics and probiotics) have specific effects on gut microbial communities. Long-term follow-up studies in homogeneous samples are needed in order to have a better understanding of the effect of diet on the gut microbiome.
Singh RK, Chang HW, Yan D, et al. Influence of diet on the gut microbiome and implications for human health. J Transl Med. 2017; 15(1):73. doi: 10.1186/s12967-017-1175-y.