Oral bioavailability of the flavonol quercetin - A review

Flavonoids are secondary plant metabolites with a polyphenol structure. In the 1930ies, flavonoids were thought to have vitamin properties, whereas they were considered as potential mutagens and carcinogens in the 1970ies. Attention focused on their anti-mutagenic and anti-carcinogenic activities in the 1980ies. In recent years, the antioxidant properties of flavonoids and their potential role in both, inhibition of low-density lipoprotein oxidation and platelet aggregation, were reported (Hertog, 1996). Protective properties of flavonoids in conjunction with so-called free radical diseases, such as cardiovascular diseases (CVD3), cancer, or cataract, are actually discussed and, in case of CVD, are supported by epidemiologic studies. These findings have resulted in increased interest in the health-promoting aspects of flavonoids. In order to evaluate their bioactivity in vivo, it is necessary to understand the factors influencing the fate of flavonoids within the gastrointestinal tract and the nature of the conjugates and metabolites present in the circulation. This review provides an overview on the present knowledge on flavonol bioavailability, thereby focusing on the flavonol quercetin. Quercetin is an abundant flavonoid in vegetal food, and due to its potent antioxidative properties it is also one of the most investigated polyphenols. Emphasize is put on mechanisms of intestinal absorption, and on factors influencing intestinal absorption of quercetin. Furthermore, metabolism, distribution and elimination of quercetin are reviewed.