When it comes to preventing cancer, vitamin C may do as much harm as good, researchers now say. According to the results of a study described today in the journal Science, the well-known antioxidant can also produce DNA-damaging compounds. And mutations caused by such agents are known from tumors. Though the findings do not indicate that vitamin C causes cancer or that people should eliminate it from their diets, they may help explain why supplements of the vitamin have proved ineffective at combating cancer.

Scientists have known for some time that vitamin C acts to render free radicals harmless. But some have wondered whether it might also prompt compounds known as lipid hydroperoxides to degrade into DNA-damaging genotoxins in the same way that certain metal ions do. To assess vitamin C's effects, Ian A. Blair of the University of Pennsylvania and his colleagues added it to test tubes containing lipid hydroperoxides. The vitamin, they found, induced genotoxin formation more than twice as effectively as the metal ions did. Taking that into consideration, "it's possible that vitamin C isn't working in cancer prevention studies because it's causing as much damage as it's preventing," Blair remarks.

Looking forward, the researchers plan to investigate whether vitamin C spurs genotoxin production in intact cells. In the meantime, however, the argument that it can stave off cancer is unsupported. The logic some researchers have followed in recommending vitamin C supplements to cancer patients "is that fruits, vegetables, et cetera, contain vitamin C; these foods prevent cancer; thus vitamin C prevents cancer," Blair notes. "But our message is that it's the total diet that's important, not just one antioxidant."