Fig. 3

Schematic representation of typical quercetin metabolism. Quercetin is one of the major flavonoids, and present in plant foods, principally as glycosides such as quercetin-3-O-glucoside (isoquercitrin), −3-O-rutinoside (rutin) and -4′-O-glucoside. When consumed, quercetin glycosides are deglycosylated into their aglycone form (quercetin) by mucosal and bacterial enzymes in the alimentary canal. Subsequently, quercetin aglycone is metabolized to glucuronidated and/or sulfated derivatives (ex. Quercetin-3-O-glucronide and -3′-O-sulphate) by sulfotransferase (SULT) and uridine 5′-diphospho-glucuronosyltransferase (UGT), respectively. Additionally, quercetin aglycone is also methylated to methoxide quercetin such as tamarixetin and isorhamnetin by catechol-O-methyl transferase (COMT), and further glucronidated. Hece, the glucuronidated and/or sulfated derivatives may be more appropriate metabolites for quercetin aglycone for evaluation of the beneficial effects of quercetin under physiological conditions than their glycosides and aglycone form