Background And Relevant Pharmacokinetics

Quercetin is a flavonol belonging to a group of polyphenols substances known as flavonoids or bioflavonoids. The first flavonoids were identified in 1936 by Albert Szent-Gyorgyi, who was awarded the Nobel Prize for his discovery of vitamin C (Challem 1998).

Studies on the absorption, bioavailability, and metabolism of quercetin after oral intake in humans have produced contradictory results (Graefe et al 1999). The nature of quercetin metabolites in plasma is currently unclear and requires further elucidation (Day & Williamson 2001), which may in part explain these inconsistencies.

There appears to be marked individual variation in absorption rates ranging from 0% to over 50% (Erlund 2004, Graefe et al 1999). Factors that may improve bioavailability include: gender (especially females taking oral contraceptives), gastrointestinal flora (Erlund 2004), and concurrent intake of bromelain and papain (Shoskes et al 1999). Absorption from onions is three times that of apples (Hollman et al 1997) and twice that of black tea (deVries et al 1998).

The main determinant for the absorption of quercetin conjugates is the nature of the sugar moiety. Glucose-bound glycosides (quercertin glucosides) are effectively absorbed from the small intestine because the cells possess glucoside-hydrolysing activity and their glucose transport system is capable of participating in glucoside absorption, whereas quercetin glycosides are subject to deglycosidation by enterobacteria before absorption in the large intestine (Murota & Terao 2003).

After absorption, quercetin is transported to the liver via the portal circulation, where it undergoes significant first pass metabolism. Peak plasma levels of quercetin occur from 0.7 to 9 hours following ingestion, and the elimination half-life of quercetin is approximately 23-28 hours (Hollman et al 1997, PDRHealth 2005). Due to its long half-life, repeated consumption of quercetin-containing foods should cause accumulation of quercetin in the body. Excretion is likely to be via the biliary system (Erlund 2004).

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