Vitamins and Exercise Add up to a Surprise

Wellness Letter

Featured Article
August 2009

There’s no doubt that exercise—especially the aerobic kind, which works your cardiovascular system—is 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. We’ve 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. Should you run away from these supplements?

Some background

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 oxygen—10 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 hope—or 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 body’s 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.

The bigger picture

Even if antioxidant supplements turn out not to be harmful in exercisers, there’s no convincing evidence that they are beneficial. Here are some other reasons not to take them:

• It’s not clear how much exercise-induced oxidative damage is actually produced and what effect it has on health. There’s some evidence that only high-intensity or exhaustive exercise—such as running a marathon—significantly increases oxidative damage.

• The body seems to adapt to regular exercise over time by boosting antioxidant activity, thus decreasing oxidative damage and making supplements unnecessary.

• Free radicals are actually good in some ways. Some, for example, are involved in cell signaling, immunity, and other physiological functions. And small amounts also stimulate the body’s antioxidant defenses.

• No research has shown that athletes suffer long-term negative health effects from free radical damage. Rather, the research is clear that exercise makes people healthier, reducing the risk of heart disease, diabetes, obesity, and some cancers.

Words to the wise: We don’t recommend vitamin C or E or any other antioxidant supplements for anyone—athlete or couch potato. It’s not known if free radicals generated during exercise are even harmful and need to be blocked. The proven benefits of exercise far outweigh any theoretical risks. Get your antioxidants from fruits, vegetables, and whole grains. And adopt a consistent, moderate workout routine to better enhance your body’s antioxidant defenses over time.

UC Berkeley Wellness Letter, August 2009