Of course thousands of people supplement vitamin C for its antioxidant properties. In fact, many consume several grams per day in an effort to reduce the damage that free radical compounds can cause. But is consuming such high doses beneficial In addition to the inefficiency of absorption as the vitamin C dose is increased, evidence also exists that large acute doses can result in opposing effects to what is intended. How can this be Because of the nature of redox reactions, a substance such as vitamin C could reduce certain cellular components (an antioxidant effect) while oxidizing others. The ability of vitamin C to do this has been reported repeatedly and may be related to dose. Podmore and colleagues (1998) showed that administration of 500 mg/ day to healthy humans for 6 weeks induced pro­oxidant effects on particular segments of nuclear material in lymphocytes. This suggests that higher doses actually act in a manner that is opposite to their intend­ed purpose for many people. And in an effort to elucidate a mechanism for vitamin C's pro-oxidant effects, Paolini et al. examined very high dose supplementation (250 and 500 mg/kg for 4 days) in rats. The researchers showed a dose-response effect on superoxide anion production and an increase in microsomal oxidative enzymes, with the 500 mg/kg dose being substantially worse. As shown in the new RDA for vitamin C was set by the Institute of Medicine's Food and Nutrition Board to reflect tissue saturation. This 75-90 mg/ day recommendation may be exceeded with relative safety up to 2500 mg/day (the "Upper Limit") but this does not ensure a total lack of pro-oxidant effects. To prevent selective oxidation of circulating blood components, it may be prudent to limit the daily dose of vitamin C to below 500 mg/day.

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