A new study suggests that some vitamin supplements may reduce some of the beneficial effects of exercise.
Michael Ristow, of the University of Jena in Germany, has shown that antioxidant supplements like vitamin C or E can interfere with the benefits of exercising.
Previous studies have also revealed that taking antioxidants may hasten death through an unknown mechanism.
One possibility, according to the researcher, is that they interfere with the beneficial effects of exercise, as there are hints that free radicals might be used by the body to prevent cellular damage after exercise.
During the study, the researchers recruited 40 volunteers. Half of them were asked to take 1000 milligrams of vitamin C and 400 international units of vitamin E per day - equivalent to amounts in some vitamin supplements.
The volunteers were also asked to exercise for 85 minutes a day, five days a week, for four weeks.
The researchers found that muscle biopsies showed a two-fold increase in a marker of free radicals called TBARS (thiobarbituric acid-reactive substances) in those volunteers who didn't take antioxidants
However, they found no increase in those who did take the supplements - suggesting that they were indeed mopping them up.
Ristow pointed out that exercise is well known to have a beneficial effect on insulin resistance - a precursor condition to type 2 diabetes.
However, when the team measured the effects of exercise on insulin sensitivity, they found no increase in those volunteers taking antioxidants, but a significant increase in those who didn't take the supplements.
"These data are fully in accord with recent work on the actions of reactive oxygen species in cells, although clearly at odds with the popular concept that dietary antioxidants are inevitably beneficial," New Scientist magazine quoted Malcolm Jackson at the University of Liverpool, UK, who was not involved in the research as saying.
In fact, in this case, "antioxidants are preventing the health effects of exercise," said Ristow,
He, however, cautions that not all vitamin supplements contain such high doses of vitamin C and E.
The study appears in Proceedings of the National Academy of Sciences.