Exercise increases the production of free radicals, and free radicals have been associated with a number of diseases and with aging. To combat those free radicals people have taken to popping Vitamins C and D, antioxidants, to decrease the number of free radicals. New research indicates that that might not be a good idea.
In one study with two groups of exercising rats, one group was injected with antioxidants, and the other group was not. The group of rats injected with antioxidants showed almost no free-radical activity, while the non-injected group had a high level of free radicals.
There were other differences as well. From this NYT article, Phys Ed: Free the Free Radicals more findings from the study: