Occurrence in food, metabolic rate and health benefits of quercetin

Quercetin, the most common member of the flavonoids family, is a group of chemical compounds that can be found in the plant kingdom. Fruits and vegetables - particularly citrus fruits, apples, onions, parsley, tea, and red wine - are the primary dietary sources of quercetin. The flavonol is present in plants in many different glycosidic forms, and the highest concentrations are found in onion where quercetin is bound to 1 or 2 glucose molecules (quercetin-3'-glucoside and quercetin-4'-glucoside). Other important sources of quercetin glycosides are apples (quercetin galactosides) and berries (quercetin arabinoses). In addition to the chemical form of the flavonol, the applied vehicule or the composition of the diet seems to have a substantial effect on quercetin bioavailability. In general, flavonoids have been described as health-promoting, disease-preventing dietary supplements, and have activity as cancer preventive agents. Specifically, quercetin, one of the most prominent dietary antioxidants, is combating the destructive "free radical" molecules that play a part in many diseases. Moreover, quercetin has healthful anti-inflammatory and anti-allergy effects, it can also improve the health of capillaries and it has shown to have anti-cancer effects.