Table 3.

Metabolites produced by gut microbiota and their functions

Metabolites	Functions	Ref.
Bile acid metabolites; including deoxycholic acid and lithocholic acid	Regulate bile acid, cholesterol, lipid, glucose, and activate host nuclear receptors and cell signaling pathways	Ramírez-Macías et al[39]
Short-chain fatty acids metabolites	Regulate food intake and insulin secretion, also aid in maintaining body weight	Psichas et al[40]
Branched-chain fatty acids including isobutyrate	Histone deacetylase inhibition, increased histone acetylation	Mischke et al[41]
Indole derivatives including indoxyl sulfate and IPA	IPA exhibits neuroprotective effects, acts as a powerful antioxidant and regulates intestinal barrier function	Hendrikx et al[42]
Lipopolysaccharide, peptidoglycan, lipoteicholic acid	Epigenetic regulation of genes in colorectal cancer, modulation of chromatine structure and transcriptional activity	Lightfoot et al[43]
Phenolic derivatives include 4-OH phenylacetic acid, urolithins, enterodiol and 9-prenylaringenin	Exhibit antimicrobial effect, maintain intestinal health and protect against oxidative stress	Larrosa et al[44]
Choline metabolites include choline, trimethylamine N-oxide, and betaine	Regulating lipid metabolism, and glucose synthesis contribute to the development of cardiovascular disease	Smallwood et al[45]
Polyamines include putrescine, spermidine and spermine	Sustaining the high proliferation rate of intestinal epithelial cells enhances intestinal barrier integrity and enhances the systematic adaptive immune system	Rooks et al[46]
Vitamins including thiamine (B1), riboflavin (B2), niacin (B3), pyridoxine (B6) panthotenic acid (B5), biotin (B7), folate (B11, B9), cobalamin (B12), and menaquinone (K2)	Help in red blood cell formation, DNA replication, and repair, work as an enzymatic co-factor, and enhance immune functioning	Nicholson et al[47]
Ethanol	Protein fermentation metabolism may be involved in NAFLD progression	Yao et al[48]
Hydrogen sulfide	Reduction/neutralization of reactive oxygen species	Afanas'ev et al[49]

IPA: Indole-3-propionic acid; NAFLD: Non-alcoholic fatty liver disease.