Antioxidants: What are They and What Role Do They Play in Physical Activity and Health?

Background

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.

Measurements of the body’s antioxidant defense system, such as gluthiaone levels and activities of glutathione peroxidase, superoxide dismutases, catalase, and glutathione reductase, have been used to assess changes in antioxidant status. Blood levels of vitamins E (tocopherol), C, and A have also been assessed.

Because no one of these measures provides an accurate assessment of lipid peroxidation or antioxidant status, studies have incorporated several assessments. However, until more valid techniques are developed, our knowledge of free radical generation, oxidative stress, and antioxidant status is limited.

Effects of Exercise, Training, and Antioxidant Supplementation

In general, there is indication that acute bouts of strenuous exercise can increase lipid peroxidation and that regular participation in physical training can enhance antioxidant status. Studies have shown that exercise increases expired pentane and plasma MDA and conjugated dienes levels. Glutathione concentration and glutathione reductase activity are also altered by exercise. Training appears to augment the antioxidant defense system. For example, highly trained runners have elevated levels of erythrocyte vitamin E, glutathione, and catalase activity.

There is limited information on the effects of antioxidant supplementation on exercise oxidative stress. Vitamin E supplementation has been shown to result in a significant reduction in the increase of expired pentane and MDA during exercise. Selenium supplementation has also been found to reduce the MDA response to exercise. A combination of vitamin C and glutathione supplementation reduced resting levels of expired pentane and MDA. Increased ingestion of vitamin C has been associated with a low incidence of respiratory tract infection in ultra marathon runners.

Because oxidation causes damage to muscle fibers, it has been suggested that oxidation could also result in exercise-induced muscle soreness. Several studies have examined whether antioxidant supplements would reduce soreness. Although vitamin E supplements have not been shown to be effective, vitamin C has been found to reduce soreness. These results are interesting, but further studies with a larger sample size are needed to fully document whether antioxidants will reduce muscle soreness
or the damage associated with soreness. Furthermore, there is no clear documentation that oxidative stress contributes to soreness.

Vitamin Supplementation and Exercise Performance

Antioxidant supplements have been touted by manufacturers as allowing individuals to recover more quickly and fully from vigorous exercise and/or allowing them to train more strenuously. However, the theoretical basis for why antioxidants should enhance performance is not clear.

Several studies have investigated the effects of vitamin E on exercise performance and have found no beneficial effects on measures of endurance or aerobic capacity. The only study to find that vitamin E supplementation enhanced performance was done at high altitude. In one study where diets poor in vitamin E were administered to subjects, there muscle weakness.

The results regarding vitamin C supplementation are equivocal, but most well-controlled studies report no beneficial effect on either endurance or strength performance. Likewise, studies of vitamin C restriction showed that a marginal vitamin C deficiency did not affect performance.

Antioxidant Status of Athletes

Studies have assessed vitamin status and dietary intakes of athletes to determine whether chronic participation in exercise results in compromised antioxidant status. Most athletes ingest adequate amounts of antioxidants as compared with the RDA, with the exception of those maintaining low body weights, such as dancers, gymnasts, and wrestlers. The studies that have assessed the vitamin A, C, or E status of athletes by analyzing blood samples have found no evidence of a deficiency. Most athletes had adequate or above adequate blood levels of these vitamins. However, there are no data available for athletes who may be at risk of inadequate antioxidant intake and compromised status because of a restricted caloric intake to maintain low body weights. Those on very low fat diets may have compromised status because fat ingestion is important to vitamin E intake.

On the other hand, it has also been suggested that the “weekend athletes,” those who only exercise strenuously on occasion, should be sure that their antioxidant levels are adequate. The reason for this is that physical training enhances the body’s antioxidant defense system so that athletes who overexert themselves are prepared to deal with tissue oxidative damage. Weekend athletes would not have this augmented defense system and may be more susceptible to tissue damage. Further studies are needed to determine the levels of antioxidants that are necessary for an optimally functioning defense system.

Recommendations for Antioxidant Use by Athletes

Although there are data documenting that antioxidants will reduce lipid peroxidation, it is still uncertain exactly what amounts are needed for a beneficial effect. Popular belief is that high doses of vitamins C, E, and beta carotene are not harmful. However, in recent years concern has arisen over the long-term use of megadoses of selected nutrients. When one considers that the body operates on a
finely tuned homeostasis, it would appear that megadoses of any nutrient could upset the delicate balance. Moreover, it may take a long time before the resulting negative effects are evident.

Many sports nutritionists recommend that adequate amounts of antioxidants be obtained from the diet, and that athletes should make intelligent food choices. As an “insurance policy,” athletes could be encouraged to take a multivitamin/mineral supplement containing no more than the recommended dietary allowance (RDA). In contrast, other experts believe that there is suff’cient information to suggest that athletes supplement their diets with antioxidants in excess of the RDA. Some have suggested that it borders on malpractice not to recommend antioxidant supplements to athletes.

Although the issue of whether to supplement with antioxidants and how much to supplement remains unresolved, what is clear is the importance of ingesting foods rich in antioxidants for those who exercise regularly as well as those who exercise on occasion.

1. Alessio HM. Exercise-induced oxidative stress. Med Sci Sports Exerc 1993;25:218-24.

2. Bendich A. Exercise and free radicals: effects of antioxidant vitamins. Med Sport Sci 1991;32:59-78.

3. Clarkson PM. Antioxidants and physical performance. Clin Rev Food Sci Nutr 1995;35:131 41.

4. Clarkson PM. Micronutrients and exercise: anti-oxidants and minerals. J Sports Sci 1995;13:SII-S24.

5. Dekkers JC, van Doornen LJP, Kemper HCG. The role of antioxidant vitamins and enzymes in the prevention of exercise-induced muscle damage. Sports Med 1996;21:213-38.

6. Goldfarb AH. Antioxidants: role of supplementation to prevent exercise-induced oxidative stress. Med Sci Sports Exerc 1993;25:232-6.

7. Sjodin B. Hellsten Westing Y. Apple FS. Biochemical mechanisms for oxygen free radical formation during exercise. Sports Med 1990;10:236-54.