Several dietary flavonoids exhibit anti-oxidative, anti-inflammatory, and anti-osteoporotic activities relevant to prevention of chronic diseases, including lifestyle-related diseases. Dietary flavonoids (glycoside forms) are enzymatically hydrolyzed and absorbed in the intestine, and are conjugated to their glucuronide/sulfate forms by phase II enzymes in epithelial cells and the liver. The intestinal microbiota plays an important role in the metabolism of flavonoids found in foods. Some specific products of bacterial transformation, such as ring-fission products and reduced metabolites, exhibit enhanced properties. Studies on the metabolism of flavonoids by the intestinal microbiota are crucial for understanding the role of these compounds and their impact on our health. This review focused on the metabolic pathways, bioavailability, and physiological role of flavonoids, especially metabolites of quercetin and isoflavone produced by the intestinal microbiota.
Keywords: ALDH: aldehyde dehydrogenase; AhR: aryl hydrocarbon receptor; CBG: cytosolic β-glucosidase; COMT: catechol-O-methyltransferases; DHD: dihydrodaidzein; DOPAC: 3,4-dihydroxyphenylacetic acid; FOS: fructooligosaccharides; Flavonoid; Keap1: Kelch-like erythroid cell-derived protein 1; LPH: lactase-phlorizin hydrolase; MCT: monocarboxylic acid transporter; Nrf2: NFE2-related factor 2; O-DMA: O-desmethylangolensin; OPAC: 3-hydroxyphenylacetic acid; OVX: ovariectomized; PCA: 3,4-dihydroxybenzoic (protocatechuic) acid; Q3G: quercetin 3-O-glucoside; Q4ʹG: quercetin 4ʹ-O-glucoside; SGLT1: sodium-glucose co-transporter type 1; SULT: sulfotransferases, UGT: uridine-5ʹ-diphosphate-glucuronosyltransferases; THD: tetrahydrodaidzein; equol; l-DDRC: DHD racemase; l-DHDR: DHD reductase; l-DZNR, NADP(H)-dependent daidzein reductase; l-THDR: THD reductase; microbial metabolism; phase II metabolism; ring-fission products.