ANTIOXIDANT AND OTHER EFFECTS OF PHYTOCHEMICALS ON DNA INTEGRITY

Epidemiological evidence that links the consumption of fruits and vegetables with a decreased risk of cancer is strong, and yet the underlying mechanisms responsible for this protection have still not been determined. One popular hypothesis is that antioxidants of various kinds that are present in virtually all plant-derived foods protect us against the DNA-damaging effects of reactive oxygen species, since it is known that oxidation of bases in DNA can result in mutation and thus promote carcinogenesis. But large-scale clinical trials with β-carotene and other antioxidant supplements have given disappointing results, in some cases inducing an increase in cancer incidence in those taking the supplement. More basic information about the effects of phytochemicals is required. Antioxidant activity of plant extracts is easy to demonstrate in vitro, but this activity bears little relationship to the effectiveness of a phytochemical in the body, since bioavailability and metabolism must be taken into account. It is possible to assess the activity of antioxidants, or antioxidant-rich foods, in humans, by isolating lymphocytes and testing the resistance of the DNA to breakage induced by H²O². Another approach is to measure the level of endogenous base oxidation in lymphocyte DNA. Kiwifruit, onions, soya milk, tomato puree, carrot juice and antioxidant supplements have all been shown to enhance antioxidant status in vivo. However, the emphasis on antioxidant activity tends to obscure the many other ways in which phytochemicals might affect metabolic processes relevant to carcinogenesis. One example is DNA repair; consumption of kiwifruit was able to increase markedly the ability of lymphocytes to repair DNA base oxidation damage.