Because many flavonoids—and especially those belonging to two flavonoid subgroups called flavonols and flavan-3-ols—can be effective in reducing free radical damage to cells and other components in body tissue, they provide antioxidant benefits. It is not clear, however, if we should be thinking about flavonoids as falling into the same category as more widely known antioxidant nutrients like vitamin C or vitamin E.
One reason for this is because their concentration in the bloodstream is so much lower. Another reason lies in the fact that many of the antioxidant functions of the flavonoids are not performed by the flavonoids themselves, but by forms of the flavonoids that have been altered by our metabolism. Even though we do not know all the details about the way flavonoids function as antioxidants, however, studies have documented better protection of certain cell types—for example, red blood cells—following consumption of flavonoid-rich foods. Blueberries, for example, have been repeatedly studied in this context for their flavonoid-related antioxidant benefits.
In this antioxidant context, it is also worth pointing out the potentially unique relationship between flavonoids and vitamin C. Recent studies have shown the ability of flavonoids to alter transport of vitamin C, as well as to alter function of an enzyme called ascorbate oxidase, which converts vitamin C into a non-vitamin form (monodehydroascorbate). While we do not yet know the full meaning of these relationships, it is clear that the transport and cycling of vitamin C is flavonoid related. This association makes sense to us, since so many foods high in vitamin C (such as our top five WHFoods for vitamin C are papaya, bell peppers, broccoli, Brussels sprouts, and strawberries) are also high in flavonoids.