Vegetable flavonoids and cardiovascular disease.
Studies have suggested that dietary flavonoids are helpful in the prevention of atherosclerosis and cardiovascular disease. Antioxidant activity should be noted as underlying mechanism of their health impact in the vascular system, as atherosclerosis is closely related to oxidative events such as oxidized LDL accumulation in the macrophages. Vegetables contain a variety of flavonoids, such as flavonols, flavones and anthocyanidins. We focused on quercetin (3,3',4',5,7- pentahydroxyflavone), a major flavonoid in onion, and its anti-atherosclerotic effect was examined from the aspect of the bioavailability and translocation to the target site. Although quercetin exists as its glucoside form in onion, it is metabolized into several glucuronides and/or sulfate conjugates with or without methylation during its intestinal absorption. We found that these metabolites circulating in the human blood stream were mostly localized in plasma albumin fraction, but not LDL fraction. Onion consumption failed to enhance the antioxidant activity of plasma fraction against LDL oxidation, indicating that the level of quercetin metabolites bound to albumin is insufficient to exert the antioxidative effect in vivo. In contrast, we discovered that quercetin metabolites accumulate in the aorta tissue and exerted their antioxidant activity, when rabbits were fed with quercetin glucoside and high cholesterol diet. Furthermore, quercetin metabolites were detected in human atherosclerotic aorta exclusively. These imply that quercetin metabolites are incorporated into the atherosclerotic region and act as complementary antioxidants, when oxidative stress is loaded in the vascular system. It is likely that plasma albumin is a carrier for translocation of quercetin metabolites to vascular target.