Hamburg  - Athletes who take vitamins after a workout may be reversing the benefits of their sweat-inducing exercise, according to startling new research by a team of German scientists.

Antioxidant vitamins C and E, taken immediately after a workout, appear to prevent physical exercise from improving the body's energy regulation, the new study found.

Exercise increases sensitivity to the hormone insulin, which governs the way cells use sugar as an energy source. Type 2 diabetes occurs when the body becomes resistant and less sensitive to insulin.

Some vitamins block this beneficial effect of exercise, the new findings suggest, according to a news release by the scientists.

Ironically, it is the supplements' health-boosting properties that appear to be to blame.

Vitamins such as C and E neutralise destructive "free radical" molecules called reactive oxygen species (ROS) that can damage cells and DNA, and speed up ageing. These molecules are a by-product of metabolism and are generated in larger numbers during vigorous exercise.

But the new research suggests that as well as causing damage, oxygen free radicals also spur the body into becoming more sensitive to insulin. By mopping up the molecules, antioxidant vitamins cancel out this beneficial effect.

In addition, scientists found that the vitamins weakened the body's own exercise-induced free radical defence system, which relies on the production of natural antioxidants.

Dr Michael Ristow, from the University of Jena in Germany, and colleagues wrote in the journal Proceedings of the National Academy of Sciences: "We find that antioxidant supplements prevent the induction of molecular regulators of insulin sensitivity and endogenous antioxidant defence by physical exercise.

"We propose that transiently increased levels of oxidative stress reflect a potentially health-promoting process, at least in regards to prevention of insulin resistance and Type 2 diabetes mellitus."

The findings add to the growing evidence that vitamins have complex effects on the body which can do harm as well as good.

The German researchers compared two groups of 20 individuals who undertook 85-minute exercise sessions five days a week for a month. One group took daily vitamin C and E supplements during their training and the other did not.

Only men taking the vitamins showed no changes in their ROS levels caused by exercise. But at the same time only those not given vitamins saw their insulin sensitivity increase.

The daily supplement doses used in the study were 1000 milligrams for vitamin C and 400 International Units (or 268 milligrams) of vitamin E. (dpa)