Research has found that antioxidant vitamins can block the health benefits of exercise.
A study in the journal Proceedings of the National Academy of Sciences suggests that taking vitamins C and E after exercise prevents any improvements in the body's energy regulation.
Exercise is known to increase sensitivity to the hormone insulin, but this latest study claims that some vitamins can block this beneficial effect of exercise.
Vitamins such as C and E neutralise destructive "free radical" molecules called reactive oxygen species that can damage cells and DNA, and speed up ageing.
The molecules are a by-product of metabolism and generated in larger numbers during vigorous exercise.
But the findings suggest that as well as causing damage, oxygen free radicals make the body more sensitive to insulin.
By absorbing the molecules, antioxidant vitamins cancel out this beneficial effect.
Scientists also 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, Germany, said: "We find that antioxidant supplements prevent the induction of molecular regulators of insulin sensitivity and endogenous antioxidant defence by physical exercise."