The powerful influence of diet: How gut microbes shape health and fight disease.

In a review article in the magazine BiomedicineScientists have provided a detailed description of the individual differences in gut microbiota and its relationship to diet and health.

Review: Relationships between diet, gut anaerobes, microbial metabolites and health. Image credit: POLIGOONE / Shutterstock

Human gut microbiota

The human gut microbiota refers to a diverse collection of microorganisms, including bacteria, fungi, archaea, and viruses. Anaerobic organisms that do not require oxygen for growth and survival comprise the largest microbial biomass in the large intestine.

Although certain organisms predominate and are prevalent in the healthy human gut, considerable variation in gut microbiota composition and diversity is commonly observed between individuals. The gut microbiota produces a number of primary and secondary metabolites, such as short-chain fatty acids (SCFAs), that play a critical role in maintaining body homeostasis.

Diet is considered a major driver in modulating the composition and activities of the gut microbiota, which is associated with both positive and negative health outcomes. An imbalance in the gut microbial community, also known as gut dysbiosis, is known to be associated with a variety of diseases, including metabolic diseases and colorectal cancer.

The relationship between diet, gut microbiota and health

Dietary macro and micronutrients play an important role in shaping the composition and functions of the human gut microbiota. The influence of nutrition on the gut microbiota begins at birth. In breastfed infants, the gut microbiota consists primarily of bifidobacterial populations, which are important for the utilization of indigestible human milk oligosaccharides. In contrast, formula-fed infants exhibit a more complex adult-like gut microbiota composition.

The introduction of solid food in infants leads to the proliferation of obligate anaerobic bacterial populations capable of metabolizing more complex polysaccharides. Diet alters metabolite production by the gut microbial community. People living in rural areas generally show higher levels of SCFAs, probably due to higher dietary fiber consumption.

The intestinal microbial community obtains energy from dietary compounds such as resistant starch, non-starch polysaccharides, oligosaccharides and proteins that escape digestion by host enzymes.

Resistant starch is the primary dietary polysaccharide degraded by gut microbiota. Bacterial populations containing complex extracellular starch-degrading structures (amylosome) can degrade resistant starch that cannot be digested by host enzymes.

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

Regarding dietary fats, only 7% of the ingested amount reaches the large intestine for use by the gut microbiota. Dietary fats can affect the composition of the gut microbiota in several ways, such as reducing the number of microbes and increasing bile acid secretion. Diets high in saturated fat are known to suppress the immune system, increase inflammation, disrupt gut barrier integrity, and trigger systemic diseases.

Food proteins are broken down by both host- and bacteria-derived proteases and peptidases to produce peptides and amino acids. Depending on the intake, 3-18 grams of dietary protein reach the large intestine daily for use by the microbiota. Peptides and amino acids produced from dietary proteins are directly incorporated into microbial proteins or fermented to produce energy for the microbiota.

Fermentation of amino acids results in the production of ammonia, major SCFAs, and branched-chain fatty acids (BCFAs), which are commonly used as stool markers for protein fermentation. Moreover, the bacterial decomposition of aromatic amino acids leads to the formation of various phenolic compounds.

A diet high in protein is known to cause many health problems, including inflammatory diseases and some types of cancer. Excessive consumption of a high-fat, high-protein diet can increase bacterial toxic metabolites associated with many health conditions, including migraines, hypersensitivity syndrome, portal-systemic encephalopathy, and colorectal cancer.

The relationship between diet, gut microbiota and the immune system

The immune system plays an important role in mediating the interplay between diet, gut microbiota, and health. The anaerobic microbial community in the gut ferments dietary carbohydrates and proteins to produce SCFAs and many other breakdown products, which are then regulated by G-protein-coupled receptors on intestinal epithelial cells. T cellsIt leads to inhibition of the effector T cell response.

Gut microbiota ferments dietary fiber to produce the SCFA butyrate, which plays a critical role in maintaining T cell function. These immune cells increase the production of anti-inflammatory cytokines by T cells, which are needed to activate the immune system against common antigens from food products and primary bacteria.

SCFAs obtained by the gut microbiota increase the immune response to pathogens. In particular, SCFAs inhibit pathogen colonization by increasing the capacity of intestinal macrophages to continuously eliminate pathogens.

In addition to SCFA, other metabolites derived from the gut microbiota influence the host immune system in several ways by promoting regulatory T cells in the small intestine and preventing intestinal inflammation.

Written by

Dr. Sanchari Sinha Dutta

Dr. Sanchari Sinha Dutta believes in science communication in spreading the power of science to all corners of the world. She has a Bachelor of Science (BSc) and a Master of Science (MSc) in Biology and Human Physiology. Sanchari followed her Masters degree and went on to pursue her Ph.D. In human physiology. She has authored more than 10 original research articles, all of which have been published in world-renowned international journals.

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