There's no doubt that exerciseespecially the aerobic kind, which works your cardiovascular systemis good for you. But strenuous physical activity does increase the production of potentially harmful free radicals in the body. Thus many exercisers, following the advice of some researchers and supplement marketers, take high doses of antioxidants such as vitamin C and E to help mop up these cellular troublemakers. Weve questioned this advice in the past, and recently a German study got lots of media attention when it suggested that vitamin C and E can, in fact, block some of the benefits of exercise.
Free radicals are unstable molecules created during the processing of oxygen and are thus part of life. Simply breathing creates them, as does exposure to ultraviolet light, cigarette smoke, and other air pollutants. When we exercise, we produce more free radicals because we consume more oxygen10 to 20 times more during aerobic activity. The body has a natural antioxidant defense system, partly fueled by the antioxidants we get from food. But high levels of free radicals can overwhelm this system, resulting in oxidative damage, similar to the rusting of metal or the browning that occurs when an apple slice is exposed to oxygen. Oxidative damage is implicated in heart disease, cancer, and other age-related conditions. It's theorized that antioxidant supplements can help prevent all this, but so far they have not lived up to the hopeor the hype.
What the new study found
In the German study, published in the Proceedings of the National Academy of Sciences, researchers had 39 young men (trained and untrained) run, cycle, or cross-train for 85 minutes five days a week. Half took 400 IU of vitamin E and 1,000 milligrams of vitamin C a day; those are common doses, equal to 12 to 18 times the recommended dietary allowances (RDAs). After four weeks, only the men not taking the supplements had an improvement in insulin sensitivity and a boost in the bodys natural antioxidant system. (Improved insulin sensitivity, a known benefit of exercise, means the body is better able to control blood sugar, which could help reduce the risk of Type 2 diabetes.) These benefits, however, were suppressed in the men taking the vitamins.
This was only one small, short study with a narrow focus, and it raised more questions than it answered. No studies have found that exercisers (including those with diabetes) who take antioxidants have worse blood sugar control than other exercisers. Still, the findings raised some red flags.