The aim of this work was to determine the antioxidant activities of a range of phytoestrogenic isoflavones. The antioxidant activity in the aqueous phase was determined by means of the ABTS.+ total antioxidant activity assay. The results show that the order of reactivity in scavenging the radical in the aqueous phase is genistein > daidzein = genistin approximately equal to biochanin A = daidzin > formononetin approximately equal to ononin, the latter displaying no antioxidant activity. The importance of the single 4'-hydroxyl group in the reactivity of the isoflavones, as scavengers of aqueous phase radicals, as well as the 5'7-dihydroxy structure is demonstrated. Examination of their abilities to enhance the resistance of low density lipoproteins to oxidation supports the observation that genistein is the most potent antioxidant among this family of compound studied, both in the aqueous and in the lipophilic phases.

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.