Bioavailability and metabolism of flavonoids

Abstract

Flavonoids are natural polyhydroxylated compounds with proved positive impact on human health. However, majority of the evidence relates to in vitro properties. In literature, attention has been focused on the in vitro mechanism of the flavonoid action, while their metabolic transformations in humans have been almost omitted. Currently, only little information is available on flavonoid bioavailability, formation of conjugates and their bioactivity in humans. It has been established that after flavonoids enter the gastrointestinal tract, the process of absorption in the small intestine takes place. The degree of absorption depends on several factors and differs among the individual flavonoid subclasses. The highest bioavailability has been determined for isoflavones, followed by flavanols, flavanones and flavonol glycosides. Flavonoid glycosides are first deglycosylated prior to the intestinal uptake, whereas aglycones can freely penetrate through cell membranes. Absorbed flavonoids are transported to the liver where they undergo extensive metabolism generating different conjugation forms such as glucuronides, sulphates and methylated derivatives. It has been proposed that these conjugates are responsible for the health-promoting effects of flavonoids. This review provides the latest information on flavonoid research as far as biochemistry, absorption, metabolism and biological activity of flavonoid conjugates in living systems is concerned. © 2008 VÚP Food Research Institute, Bratislava.