Absorption and metabolism of flavonoids.

The benefits of flavonoids as chemopreventive dietary or dietary supplemental agents are still only "potential." Much has been learned about possible mechanisms of action of these agents, but whether they can reach their multiple intended sites of action, particularly in humans, is largely unknown. The biological fate of the flavonoids, including their dietary glycoside forms, is highly complex, dependent on a large number of processes. This review is intended to bring some order into this complex area and deals with the fate of the naturally occurring glycosides, their enzymatic hydrolysis, as well as the resulting aglycones. The impact of membrane transporters as well as metabolic enzymes on the cellular availability of these phytochemicals is examined. A reevaluation of the concept of oral bioavailability applied to the dietary flavonoids is presented.