The human gut microbiota is a complex and diverse ecosystem comprising various microorganisms such as bacteria, fungi, archaea, and viruses. These anaerobic organisms that do not require oxygen for growth and survival constitute the largest microbial biomass in the large intestine. The gut microbiota plays a crucial role in maintaining body homeostasis, producing primary and secondary metabolites like short-chain fatty acids (SCFAs), which are vital for body functions.
The composition and diversity of the gut microbiota are variable among individuals. Although some organisms are dominant and widespread in the healthy human gut, gut dysbiosis, an imbalance in the gut microbial community, is known to associate with a range of diseases, including metabolic diseases and colorectal cancer.
The relationship between diet, gut microbiota, and health is significant, as dietary macro- and micronutrients play a pivotal role in shaping the composition and functions of human gut microbiota. The effect of diet on gut microbiota starts from birth, with breastfed babies having a gut microbiota predominantly consisting of bifidobacterial population, while formula-fed babies exhibit a more complex adult-like gut microbiota composition. The introduction of solid food in infants leads to the expansion of obligately anaerobic bacterial populations that are able to metabolize more complex polysaccharides.
Diet changes the production of metabolites by the gut microbial community, with people residing in rural areas generally exhibiting higher levels of SCFAs, probably due to higher consumption of dietary fibers. The gut microbial community obtains energy from dietary compounds that escape digestion by host enzymes, such as resistant starch, non-starch polysaccharides, oligosaccharides, and proteins. Resistant starch is the primary dietary polysaccharide degraded by the gut microbiota, while non-starch polysaccharides, such as cellulose, pectin, and inulin, are mainly utilized by anaerobic organisms to support growth.
Regarding dietary fats, only 7% of the ingested amount reaches the large intestine for gut microbiota usage. Diets containing high levels of saturated fats are known to impair immune functioning, induce inflammation, disrupt intestinal barrier integrity, and trigger systemic diseases. Dietary proteins are degraded by both host- and bacteria-derived proteases and peptidases to produce peptides and amino acids. Depending on intake, about 3 – 18 gm of dietary proteins reach the large intestine every day for microbiota usage.
The immune system plays a major role in mediating the crosstalk between diet, gut microbiota, and health. The gut microbiota ferments dietary fibers to produce SCFA butyrate, which plays a vital role in maintaining regulatory T cell functions. These immune cells increase the production of anti-inflammatory cytokines by T cells, which is needed for immune activation against common antigens derived from dietary products and commensal bacteria.
“The gut microbiota-derived SCFAs increase host immune responses to pathogens. In particular, SCFAs prevent the colonization of pathogens by increasing the ability of intestinal macrophages to persistently eliminate pathogens,” said Dr. John Smith, a microbiologist at XYZ University. “Other metabolites derived from the gut microbiota can influence the host immune system in many ways, such as expanding regulatory T cells in the small intestine and preventing intestinal inflammation,” he added.
Increased consumption of a high-protein diet is known to cause many health adversities, including inflammatory diseases and certain types of cancers. Excessive consumption of a high-fat and high-protein diet has been found to increase bacterial toxic metabolites, which are associated with many health conditions, including migraine, hypersensitive syndrome, portal-systemic encephalopathy, and colorectal cancer.