Abstract
Phenols are universal components of plant material. Flavonoids are polyphenolic constituents of the diet with antioxidant properties by virtue of the structural arrangements and hydrogen-donating potential of their phenolic groups. The ability of specific phenolics to chelate transition metal ions is also of significance. In vitro , the antioxidant activity of the aglycone forms of these polyphenols is, in the main, greater than that of vitamin E and vitamin C. In the diet they are present principally as glycosylated, esterified or polymerized derivatives. Many factors, such as the extent of their gastrointestinal absorption, metabolism (possibly into derivatives with enhanced activity) and interaction with target cells, affect the bioavailability and activity of dietary polyphenols. The balance between beneficial and detrimental effects after their consumption is thus somewhat unpredictable. The possible sites at which these compounds might act as significant dietary micronutrient antioxidants are: (1) in food, during storage and cooking; (2) in the gut, sparing ascorbic acid and f -tocopherol from oxidative decomposition during digestion; and (3) after absorption into the plasma and tissues, where they may be active either as the glycosylated or aglycone flavonoid or as their constituent hydroxybenzoic or hydroxycinnamic acids, depending on their bioavailability and metabolism. These mechanisms may explain the epidemiological association between high dietary flavonoid content and low incidence of cardiovascular disease and certain cancers.