How Gut Microbes Shape Health and Fight Disease

A review article published in a magazine biomedicalscientists provide an in-depth overview of inter-individual variability in the gut microbiota and the link between diet and health.

review: Links between diet, intestinal anaerobes, microbial metabolites and healthImage Credit: POLIGOONE / Shutterstock

human intestinal flora

The human gut microbiome refers to a diverse collection of microorganisms, including bacteria, fungi, archaea, and viruses. Anaerobes, which do not require oxygen for growth and survival, constitute the largest microbial biomass in the large intestine.

Although several organisms are dominant and widespread in the healthy human gut, considerable inter-individual variability in the composition and diversity of the gut microbiota is commonly observed. The microbiota produces many primary and secondary metabolites, such as short-chain fatty acids (SCFAs), that play important roles in maintaining body homeostasis.

Diet is considered an important factor regulating the composition and activity of the gut microbiota, which is associated with both positive and negative health effects. An imbalance in the gut microbiota, also known as gut microbiota dysbiosis, is known to be associated with various diseases, including metabolic disorders and colorectal cancer.

Relationship between diet, intestinal microbiota, and health

Dietary macro- and micronutrients play an important role in shaping the composition and function of the human gut microbiota. The effect of diet on gut microbiota begins at birth. In breastfed babies, the gut microbiota consists primarily of bifidobacterial populations, which are required for the utilization of non-digestible breast milk oligosaccharides. In contrast, formula-fed babies exhibit a more complex adult-like gut microbiota composition.

Introduction of solid food to young children leads to expansion of obligate anaerobic bacterial populations capable of metabolizing more complex polysaccharides. Diet alters the production of metabolites by the gut microbial community. People living in rural areas generally show higher levels of his SCFA, probably due to their higher dietary fiber consumption.

The gut microbial community obtains energy from food compounds that escape digestion by host enzymes, including resistant starch, non-starch polysaccharides, oligosaccharides and proteins.

Resistant starch is the major dietary polysaccharide degraded by the intestinal microbiota. Bacterial populations containing complex extracellular starch-degrading structures (amylosomes) are able to degrade resistant starches that cannot be digested by host enzymes.

The second major dietary polysaccharides degraded by the gut microbiota are non-starch polysaccharides such as cellulose, pectin and insulin. Insulin and oligosaccharides are primarily utilized to support anaerobe growth. These compounds are also used as prebiotics to promote bifidobacterial communities.

As for dietary fat, only 7% of the intake reaches the large intestine due to utilization of the gut microbiota. Dietary fat can affect the composition of the gut microbiota in many ways, including reducing microbial diversity and increasing bile acid secretion. It is known to impair blood flow, induce inflammation, disrupt the integrity of the intestinal barrier, and cause systemic disease.

Food proteins are degraded by both host- and bacterial-derived proteases and peptidases to produce peptides and amino acids. Depending on intake, approximately 3-18 g of dietary protein reaches the large intestine each day for use by the microbiota. Peptides and amino acids produced from dietary proteins are directly incorporated into microbial proteins or fermented to generate energy for the microbiota.

Fermentation of amino acids leads to the production of ammonia, major SCFAs, and branched-chain fatty acids (BCFAs) commonly used as fecal markers of protein fermentation. In addition, bacterial deamination of aromatic amino acids produces various phenolic compounds.

Increased intake of high-protein diets is known to cause many health problems, including inflammatory diseases and certain types of cancer. It has been found to increase toxic metabolites of bacteria associated with many health conditions, including headache, hypersensitivity syndrome, portal systemic encephalopathy, and colorectal cancer.

Relationship between diet, intestinal microbiota, and the immune system

The immune system plays a major role in mediating the crosstalk between diet, gut microbiota, and health. The anaerobic microbial community present in the gut ferments dietary carbohydrates and proteins to form SCFAs and many other degradation products, which are then released to G protein-coupled receptors present on intestinal epithelial and regulatory cells. join T cellsleading to inhibition of effector T-cell responses.

The gut microbiota ferments dietary fiber to produce the butyrate SCFA, which plays an important role in maintaining regulatory T-cell function. These immune cells increase the production of anti-inflammatory cytokines by T cells. This is required for immune activation against common antigens derived from food and commensal bacteria.

Gut microbiota-derived SCFAs increase host immune responses to pathogens. In particular, SCFAs prevent pathogen colonization by enhancing the ability of intestinal macrophages to persistently clear pathogens.

In addition to SCFAs, other metabolites derived from the gut microbiota may affect the host immune system in various ways, including proliferation of regulatory T cells in the small intestine and prevention of intestinal inflammation. I have.