By NICHOLAS WADE
Published: May 11, 2009
If you exercise to improve your metabolism and prevent diabetes, you may want to avoid antioxidants like vitamins C and E.
That is the message of a surprising new look at the bodys reaction to exercise, reported on Monday by researchers in Germany and Boston.
Exercise is known to have many beneficial effects on health, including on the bodys sensitivity to insulin. Get more exercise is often among the first recommendations given by doctors to people at risk of diabetes.
But 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 bodys tissues. The amount of oxidative damage increases with age, and according to one theory of aging it is a major cause of the bodys 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, led by Dr. Michael Ristow, a nutritionist 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 bodys natural defenses to oxidative damage.
The Jena team found that in the group taking the vitamins there was no improvement in insulin sensitivity and almost no activation of the bodys 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 bodys natural response to exercise.
If you exercise to promote health, you shouldnt take large amounts of antioxidants, Dr. Ristow said. A second message of the study, he said, is that antioxidants in general cause certain effects that inhibit otherwise positive effects of exercise, dieting and other interventions. The findings appear in this weeks issue of The Proceedings of the National Academy of Sciences.
The effect of vitamins on exercise and glucose metabolism is really quite significant, said Dr. C. Ronald Kahn of the Joslin Diabetes Center in Boston, a co-author of the report. If people are trying to exercise, this is blocking the effects of insulin on the metabolic response.
The advice does not apply to fruits and vegetables, Dr. Ristow said; even though they are high in antioxidants, the many other substances they contain presumably outweigh any negative effect.
Dr. Kahn said it might be that reactive oxygen is beneficial in small doses, because it touches off the bodys natural defense system, but harmful in higher doses.
Andrew Shao of the Council for Responsible Nutrition, a trade association of dietary supplement makers, said the new study was well designed but was just one bit of evidence in a complex issue. Most available evidence points to the opposite conclusion, that antioxidants benefit health by reducing oxidative stress, he said.
I wouldnt change recommendations for anyone based on one study, he said. This is one small piece of the puzzle.