Fruits and vegetables in the prevention of cellular oxidative damage.

Numerous studies have demonstrated in vitro effects of flavonoid components from fruits and vegetables on various measures of oxidative cellular damage. However, the questions that have not been answered satisfactorily deal with the absorption/metabolism of antioxidant components in fruits and vegetables and whether they are absorbed in sufficient quantities and in a form in which effects on in vivo measures of oxidative cellular damage could be observed. The focus of this review is on clinical studies that provide information about possible in vivo changes in antioxidant status with fruit and vegetable consumption. Two classes of flavonoids as antioxidants are considered in detail (anthocyanins and flavonols). Absorption of anthocyanins appears to be much less than that of the flavonol quercetin, perhaps as little as one tenth. Relatively high dietary levels of anthocyanins appear to be necessary to observe antioxidant effects in vivo. Metabolism of cyanidin 3-glucoside and quercetin by methylation or conjugation with glucuronide or sulfate will decrease antioxidant activity. However, quercetin metabolites seem to maintain at least part of their antioxidant activity in vivo. A potential role for food flavonoids and polyphenolics as antioxidants is discussed.