Most research has demonstrated that strenuous exercise increases production of harmful substances called free radicals, which can damage muscle tissue and result in inflammation and muscle soreness. Exercising in cities or smoggy areas also increases exposure to free radicals. Antioxidents, including vitamin C and vitamin E, neutralize free radicals before they can damage the body, so antioxidants may aid in exercise recovery. Regular exercise increases the efficiency of the antioxidant defence system, potentially reducing the amount of supplemental antioxidants that might otherwise be needed for protection. However, at least theoretically, supplements of antioxidant vitamins may be beneficial for older or untrained people or athletes who are undertaking an especially vigourous training protocol or athletic event.^{1, 2}

Placebo-controlled research, some of it double-blind, has shown that taking 400 to 3,000 mg of vitamin C per day for a few days before and after intense exercise may reduce pain and speed up muscle strength recovery.^{3, 4, 5} However, taking vitamin C only after such exercise was not effective in another double-blind study.⁶ While some research has reported that vitamin E supplementation in the amount of 800 to 1,200 IU per day reduces biochemical measures of free-radical activity and muscle damage caused by strenuous exercise,^{7, 8, 9} a few studies have not found such benefits,^{10, 11, 12, 13} and no research has investigated the effect of vitamin E on performance-related measures of strenuous exercise recovery. A combination of 90 mg per day of co-enzyme Q10 and a very small amount of vitamin E did not produce any protective effects for marathon runners in one double-blind trial,¹⁴ while in another double-blind trial a combination of 50 mg per day of zinc and 3 mg per day of copper significantly reduced evidence of post-exercise free radical activity.¹⁵

In most well-controlled studies, exercise performance has not been shown to improve following supplementation with vitamin C, unless a deficiency exists, as might occur in athletes with unhealthy or irrational eating patterns.^{16, 17} Similarly, vitamin E has not benefited exercise performance,^{18, 19} except possibly at high altitudes.^{20, 21}