I read about an interesting study today in the NYTimes, published in this week’s  Proceedings of the National Academy of Sciences. Antioxidant vitamins like C and E  may block some of the beneficial effects of exercise and dieting, including the body’s sensitivity to insulin.

As summarized in the article:

…exercise makes the muscle cells metabolize glucose, by combining its carbon atoms with oxygen and extracting the energy that is released. In the process, some highly reactive oxygen molecules escape and make chemical attacks on anything in sight…These reactive oxygen compounds are known to damage the body’s tissues. The amount of oxidative damage increases with age, and according to one theory of aging it is a major cause of the body’s decline…The body has its own defense system for combating oxidative damage, but it does not always do enough. So antioxidants, which mop up the reactive oxygen compounds, may seem like a logical solution.

The researchers  at the University of Jena in Germany, tested this proposition by having young men exercise, giving half of them moderate doses of vitamins C and E and measuring sensitivity to insulin as well as indicators of the body’s natural defenses to oxidative damage.

They found that in the group taking the vitamins there was no improvement in insulin sensitivity and almost no activation of the body’s natural defense mechanism against oxidative damage.

The reason, they suggest, is that the reactive oxygen compounds, inevitable byproducts of exercise, are a natural trigger for both of these responses. The vitamins, by efficiently destroying the reactive oxygen, short-circuit the body’s natural response to exercise.

The researcher’s conclusions:

If you exercise to promote health, you shouldn’t take large amounts of antioxidants, and antioxidants in general cause certain effects that inhibit otherwise positive effects of exercise, dieting and other interventions.

It might be that reactive oxygen is beneficial in small doses, because it touches off the body’s natural defense system, but harmful in higher doses.