Flavonoids as antioxidants

Flavonoids are phenolic substances isolated from a wide range of vascular p lants, with over 8000 individual compounds known. They act in plants as ant ioxidants, antimicrobials, photoreceptors, visual attractors, feeding repel lants, and for light screening. Many studies have suggested that flavonoids exhibit biological activities, including antiallergenic, antiviral, antiin flammatory, and vasodilating actions. However, most interest has been devot ed to the antioxidant activity of flavonoids, which is due to their ability to reduce free radical formation and to scavenge free radicals. The capaci ty of flavonoids to act as antioxidants in vitro has been the subject of se veral studies in the past years, and important structure-activity relations hips of the antioxidant activity have been established. The antioxidant eff icacy of flavonoids in vivo is less documented, presumably because of the l imited knowledge on their uptake in humans. Most ingested flavonoids are ex tensively degraded to various phenolic acids, some of which still possess a radical-scavenging ability. Both the absorbed flavonoids and their metabol ites may display an in vivo antioxidant activity, which is evidenced experi mentally by the increase of the plasma antioxidant status, the sparing effe ct on vitamin E of erythrocyte membranes and low-density lipoproteins, and the preservation of erythrocyte membrane polyunsaturated fatty acids. This review presents the current knowledge on structural aspects and in vitro an tioxidant capacity of most common flavonoids as well as in vivo antioxidant activity and effects on endogenous antioxidants.