Are tennis players prone to more free-radical production and damage, and do they need antioxidant supplements? The human body has sufficient antioxidant defences to counterbalance the normal rate of free-radical production. However, a decrease in antioxidant activity or an increase in the production of free radicals may result in an imbalance, known as oxidative stress. During exercise, O2 production increases 10- to 15-fold, resulting in more free radical production than at rest. Some components of the of the defence system, such as antioxidant enzymes, increase in trained subjects following regular exercise. Important antioxidant enzymes are vitamins E and C, B-carotene and glutathione, capable of scavenging reactive oxygen species. The three most important antioxidant enzymes are superoxide dismutase, catalase, and glutathione peroxidase. Studies using vitamin E-deficient animals have demonstrated an increased predisposition to lipid peroxidation. Endurance exercise, associated with a rise in lipid peroxidation, may produce an increased need for vitamin E. However, a beneficial effect of vitamin E supplements on human performance has not been shown, except at high altitudes. Vitamin C is the most effective aqueous-phase antioxidant in human plasma. There is limited evidence that individuals who exercise regularly require additional vitamin C. Glutathione concentration in tissues seems to be related to their oxidative capacity, with a synergistic action between vitamins C and E and glutathione. Glutathione supplements do not influence endurance performance. Selenium, an essential trace element, is needed for glutathione peroxidase activity. Lack of selenium causes a drastic fall in such activity and incrases vitamin E oxidation. However, selenium deficiency is infrequent, and studies of the effects of selenium supplements on exercise performance have produced contradictory results. The researchers concluded that although a single bout of tennis increases antioxidant production, the natural antioxidant defences of well-nourished individuals are suffiecient and generally, no supplementation is needed.
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