TABLE 1.

Chief metabolites produced by the gut microbiota.

Chief metabolites produced by gut microbiota	Important functions	References
SCFAs Ex: Acetate, propionate, butyrate	Maintain the integrity of intestinal mucosa, regulate lipid and glucose metabolism, regulation of the immune system, control inflammatory responses, regulates appetite, maturation of microglia, play a role in the synthesis and release of neurotransmitters like 5-HT, acetylcholine from ECC, nor-adrenaline from sympathetic neurons	Koh et al., 2016; Morrison and Preston, 2016; Kho and Lal, 2018; Caspani et al., 2019; Agus et al., 2021
Branched chain amino acids Ex: Valine, isoleucine, leucine	Regulate the synthesis of proteins, metabolism of glucose and lipids, immunity, proliferation of Hepatocyte, insulin resistance	Koh et al., 2016
Secondary bile acids Ex: Deoxycholic acid, lithocholic acid	Metabolic functions like promoting glycogen synthesis, secretion of insulin, thermogenesis, mediation of satiety, anti-inflammatory properties, anti-microbial effects	Koh et al., 2016; Kho and Lal, 2018; Rowland et al., 2018
Tryptophan and indole derivate metabolites Serotonin, aryl hydrocarbon receptor agonists like indole-3-propionic acid (IPA)	Regulation of tryptophan metabolism in the cells, induce the production of serotonin (5-HT3), IPA has neuroprotective action. It inhibits the amyloid beta fibril formation	Koh et al., 2016; Kho and Lal, 2018
Vitamins Vitamin K and B group	Vitamin K2 reduces the risk of cardiovascular disease Vitamin B5, B12 are necessary for neurologic functioning Red blood cell production Cofactors for various biochemical reactions	Kho and Lal, 2018
Polyphenol metabolites Ex: Urolithin A&B, equol, n-phenyl naringenin	Reduces oxidative stress, estrogen modulation, platelet inhibition, anti-inflammatory, and anti-microbial effects	Kho and Lal, 2018; Rowland et al., 2018
Neurotransmitters Ex: GABA, 5-HT, acetylcholine, dopamine	Modulate the neurotransmission by acting on the CNS receptors or receptors present on the peripheral neurons or immune cells, afferent nerves or VN	Caspani et al., 2019