LEICESTER, ENGLAND. Antioxidants such as vitamin-E and beta-carotene can under certain conditions behave as prooxidants, that is, they promote oxidative reactions rather than quench them. Fortunately, the conditions which encourage the prooxidant activities are rare so both vitamin E and beta-carotene are generally considered to be safe antioxidants. Now researchers at the University of Leicester report in a letter to Nature that they have found evidence that vitamin C can also act as a prooxidant. Their study involved 30 healthy volunteers who were given 500 mg/day of ascorbic acid for a six-week period in addition to their regular diet. The researchers noted a marked increase in oxidative damage to the DNA of the volunteers' lymphocytes during the vitamin-C supplementation as compared to another six weeks when they were supplemented with 500 mg/day of calcium carbonate (placebo). The DNA damage was deduced from a significant decrease in 8-oxoguanine levels and an equally significant increase in 8-oxoadenine levels during the vitamin C supplementation period. Both 8-oxoguanine and 8-oxoadenine are markers of DNA damage. The researchers conclude that their findings raise some concern about vitamin C supplementation, but concede that at doses less than 500 mg/day the antioxidant effects of vitamin C may well predominate. Editor's Note: Although these new findings cannot be dismissed out of hand they should not be blown out of proportion either. The communication to Nature was in the form of a letter, not a peer-reviewed research article. There are numerous peer-reviewed research reports attesting to the safety and benefits of vitamin C which cannot be disregarded when evaluating this new information. Also, while 200-500 mg/day of vitamin C could well be optimum for healthy people the new information does not address the question as to whether people suffering from atherosclerosis, cancer or rheumatoid arthritis need much larger amounts in order to reduce the excessive oxidative stress involved in their diseases.
Podmore, Ian D., et al. Vitamin C exhibits pro-oxidant properties. Nature, Vol. 392, April 9, 1998, p. 559 (scientific correspondence)