Excess free-radical formation has been hypothesized to contribute to cancer, atherosclerosis, aging, and exercise-associated muscle damage. Antioxidant supplements such as vitamin C, vitamin E, and beta-carotene have been touted as beneficial for enhancing exercise performance and for preventing certain diseases. Before physicians routinely recommend supplements to prevent exercise-associated damage, more study will be required. Recommendations for the prevention of cardiovascular disease and cancer are more complex. Because study results have been contradictory, individual supplement recommendations must be offered with caution. Physicians must be cognizant of which supplements patients are taking and be prepared to discuss risks and benefits. The most beneficial prescription is probably a daily diet containing five to seven servings of fruits and vegetables..Muscular exercise promotes the production of radicals and other reactive oxygen species in the working muscle. Growing evidence indicates that reactive oxygen species are responsible for exercise-induced protein oxidation and contribute to muscle fatigue. To protect against exercise-induced oxidative injury, muscle cells contain complex endogenous cellular defence mechanisms (enzymatic and non-enzymatic antioxidants) to eliminate reactive oxygen species. Furthermore, exogenous dietary antioxidants interact with endogenous antioxidants to form a cooperative network of cellular antioxidants. Knowledge that exercise-induced oxidant formation can contribute to muscle fatigue has resulted in numerous investigations examining the effects of antioxidant supplementation on human exercise performance. To date, there is limited evidence that dietary supplementation with antioxidants will improve human performance. Furthermore, it is currently unclear whether regular vigorous exercise increases the need for dietary intake of antioxidants. Clearly, additional research that analyses the antioxidant requirements of individual athletes is needed