The phytochemicals quercetin is a flavonol, which is present in many foods, such as apples, broccoli and onions. Studies show that consumption of foods rich in quercetin is associated with a lower risk of heart disease and certain types of cancer. In vitro studies have demonstrated the antioxidant and anticancer activity of quercetin. However, it is not very clear if quercetin also works in vivo because of limited knowledge about bioavailability. In nature, quercetin is present as conjugates with glucuronyl, sulphuryl or methyl moieties. Previous studies have shown that quercetin glycoside is first transformed in aglycone quercetin by different enzymes. Other enzymes transform the quercetin into glucuronated, sulphated and methylated quercetin conjugates. Different quercetin conjugates have specific properties.
The aim of this study was to investigate the metabolism of ingested quercetin in rats, by measuring quercetin metabolites in the gut content, intestinal tissues and other internal tissues. The study was carried out on rats, which received a diet rich in quercetin (0.45%) during a period of six weeks. After this period, in total 16 different quercetin metabolites were identified. The gut content mainly contained unmetabolized quercetin whereas the intestinal tissues contained different levels of metabolized quercetin. In the other tissues quercetin was mainly found as sulphated methyl-quercetin glucuronide. Each intestinal tissue showed its own pattern of quercetin conjugates whereas tissues of the plasma, liver and kidney showed the same pattern. All quercetin found in plasma was metabolized. The study showed that quercetin undergoes extensive biotransformation during metabolism.
Source: Graf BA, Ameho C, Dolnikowski GG, Milbury PE, Chen CY and Blumberg JB. Rat Gastrointestinal Tissues Metabolize Quercetin. Journal of Nutrition. 136:39-44, January 2006