Exercisers take the antioxidant based on the assumption that it protects their muscles from the oxidative stress that results from physical exertion, Vina's group reports in the American Journal of Clinical Nutrition. However, the researchers point out, oxidative stress during exercise may actually help the body to build endurance by forcing muscles to adapt.

To investigate, Vina and his team had 14 sedentary men undergo 8 weeks of training, during which 5 of them took 1 gram of vitamin C daily. They also put 24 rats through 3- and 6-week exercise training protocols, and gave half of the animals vitamin C daily.

The men who didn't take vitamin C showed a 22-percent increase in their body's ability to take up and use oxygen during exercise, compared with just a 10.8-percent increase for the men given vitamin C. Similar results were seen for the rats.

When the animals were forced to run to the point of exhaustion after 6 weeks of endurance training, those that weren't dosed with vitamin C were able to run nearly twice as far as they had before training, while those given the vitamin only increased their distance by 25 percent. Tests on the muscle tissue of the animals found that rats that weren't given vitamin C had added more mitochondria, the "engines" within cells that convert nutrients into energy.

The findings suggest that the release of free radicals during exercise helps muscles to adapt by changing gene expression, Vina and his team note, while Antioxidants such as vitamin C may interfere with this process. In fact, they add, exercise itself could act as an antioxidant by boosting the body's expression of antioxidant enzymes.

"Taking into account that a high fitness level is associated with a lower risk of premature death from any cause, the effect of vitamin C administration on endurance capacity has important implications for nutritionists, physicians, and exercise trainers and practitioners," they conclude.

SOURCE: American Journal of Clinical Nutrition, January 2008