Are antioxidant supplements necessary for those who exercise regularly? Should antioxidant supplements be part of the "nutritional game plan" of athletes? These are common questions directed to fitness leaders, athletic trainers, and other health professionals who are consulted about the role of antioxidants in a healthy, active lifestyle.
The reason for this interest in antioxidants is the finding that certain highly reactive chemical species called free radicals increase during exercise. Free radicals contain one or more unpaired electrons in their outer orbit that allows them to attack cellular components. During oxidative metabolism, most of the consumed oxygen ends up bound to hydrogen, forming water. Because this process is not 100 percent effective, 4-5 percent of the oxygen is not completely reduced and forms free radicals which, in turn, lead to other harmful oxidation products. When free radicals attack cellular membranes, a chain of reactions called lipid peroxidation produces additional damage. Thus, as oxygen consumption is increased during exercise, there will be a concomitant increase in free radicals and lipid peroxidation in skeletal muscle cells.
Exercise can also generate free radicals by other means including (1) increased intake of oxygen which itself is a diradical, (2) increased amounts of epinephrine and other catecholamines that
produce oxygen radicals when they are metabolically inactivated, (3) production of lactic acid that can convert a weakly damaging free radical (superoxide) into a
(4) response to of membranes and an increase in macrophages and white blood cells in damaged muscle.
The body contains an elaborate antioxidant defense system that depends on dietary intake of antioxidant vitamins and minerals and the endogenous production of antioxidant compounds such as glutathione. Vitamins C, E, and beta carotene are the primary vitamin antioxidants. In addition to glutathione, there are numerous enzymes involved in the quenching or removal of free radicals.
Whether the body's natural antioxidant defense system is sufficient to counteract the increase in free radicals with exercise or whether additional supplements are needed is not fully known. Those who engage in chronic physical activity, placing a constant oxidative stress on the muscles and other cells, may require additional antioxidants to help them recover from exercise. However, physical training may enhance the antioxidant system to counteract the barrage of free radicals produced during exercise. Antioxidant supplementation may benefit the "weekend athlete" whose defenses may not be prepared to handle a sudden increase in oxidative stress. Although it has been suggested that antioxidant supplements will enhance physical performance, the data are equivocal.
Exercise and Oxidative Stress: How is it Detected?
Because there is no way to directly detect free radical production in humans, indirect methods have been developed. Basically these methods rely on the breakdown products of lipid peroxidation, such as conjugated diene, malondialdehyde (MDA), and hydrocarbons. Measurement of malondialdehyde and conjugated dienes in the blood or urine and the assessment of hydrocarbon production by measurement of expired pentane provide evidence of lipid peroxidation. Malondialdehyde is most commonly measured by its reaction with thiobarbituric acid, which generates thiobarbituric acid reactive substances (TBARS). These methods have been criticized for not representing an accurate measure of lipid peroxidation. Because lipid peroxidation can occur in all tissues, blood levels of peroxidation products or expired pentane can provide no information on where lipid peroxidation is occurring. Furthermore, expired pentane can reflect flushing of hydrocarbons from adipose tissue, and many natural compounds other than lipids can produce TBARS.