When exercise lasts longer than seconds or minutes, energy is produced from carbohydrate, fat, or protein under aerobic (“with oxygen”) conditions. The majority of the oxygen used in these reactions will eventually become water (H2O), but approximately 4 percent will not. A small number of the oxygen molecules become free radicals, which are known as oxidants. Oxidants can damage cell membranes and tissues if they are not neutralized by antioxidants. An antioxidant is any compound that can slow or prevent the action of an oxidant. Some of the most potent antioxidants are vitamins, especially vitamin E. Oxidative stress occurs when oxidants outnumber antioxidants. The goal is to have antioxidants equal or slightly exceed oxidants (Urso & Clarkson, 2003).
Endurance athletes are at greater risk than others for oxidative stress because more free radicals are produced as the intensity and duration of exercise increases. Endurance training enhances the body’s antioxidant defenses, but athletes must have enough antioxidants to counter the increased production of free radicals. This concern has naturally led to the study of antioxidant vitamin supplements, such as those containing vitamins A, C, and E, in endurance athletes.
There are few data that suggest that vitamin C supplements are effective for reducing oxidative stress in endurance athletes, and no data for the effectiveness of vitamin A supplements. Vitamin E is the most potent of the antioxidant vitamins, has the most potential to prevent damage from oxidants, and is the most studied. Unfortunately, the results of studies of vitamin E have been inconclusive, and experts do not agree about whether endurance athletes should consume vitamin E supplements. To date, about half of the studies report that oxidative stress is decreased with vitamin E supplementation, and half report no effect. A handful of studies report that vitamin E supplements increase oxidative stress because at high concentrations antioxidants act as pro-oxidants (Williams, Stobel, Lexis, & Coombes, 2006).
Until more definitive data suggest otherwise, experts recommend that athletes consume foods that are rich in antioxidant vitamins. This means eating a variety of fruits, vegetables, whole grains, nuts, and vegetable oils daily. Endurance athletes should assess their consumption of vitamin E–containing foods because surveys suggest that intake is often low. A common reason is the tendency of endurance athletes to consume a low-fat diet, which restricts their intake of vitamin E–rich foods. Excellent sources of vitamin E are vegetable oils, wheat germ, almonds, and sunflower seeds.