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 body’s 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 body’s 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.