Quercetin Part 1-Chemical Structure, Content in Foods, Daily Intake and Bioavailability

The flavonoid quercetin is thought to help in preventing diseases linked to oxidative stress. Quercetin occurs in many foods of plant origin, in the form of various beta-glycosides. The daily intake is 10-20 mg. Quercetin glucosides can be hydrolysed in the small intestine by lactase. The released aglycone is taken up into the mucus membrane cells by diffusion. Alternatively, glucosides can be transported into mucus membrane cells through the glucose transporter SGLT1 and then hydrolysed intracellularly by beta-glucosidases. Glycosides containing rhamnose (rutin) are hydrolyed in the large intestine by intestinal bacteria. The aglycone is absorbed in the large intestine or broken down into phenolic acids. Quercetin in the body is subject to biotransformation. In plasma, quercetin is almost exclusively present as conjugates and methylated metabolites. There is little information on the absolute level of quercetin absorption. The plasma concentration and the area under the concentration-time curve are measures of the bioavailability. The bioavailability depends on the type of glycoside, individual differences and other factors. The bioavailability of quercetin from glucosides is greater than that from rutin or that of the isolated aglycone.