In recent years, antioxidant nutrients have received increasing attention as possible protectors against diverse diseases such as heart disease, diabetes, cataracts, and certain forms of cancer. Research has also suggested a role of antioxidants in delaying detrimental effects of the aging process. The potential of antioxidant nutrients for those engaged in strenuous physical exercise has been studied extensively as well. To date, there is no consensus of opinion among scientists regarding the need for dietary supplements among active people, although there have been reports of athletes incurring less muscle damage when they ingested antioxidant supplements prior to an exercise bout. A lack of sensitive research markers indicative of free radical generation and exercise-induced tissue damage have clouded the issue, as have non-exercise research and supplement protocols. Nevertheless, antioxidant supplementation remains a "hot" issue because of its possible performance and health-enhancing roles, and will likely remain a topic of debate until definitive research data are forthcoming. The Gatorade Sports Science Institute interviewed five scientists who are currently studying the possible roles of antioxidant supplements for healthy people. Their answers to our questions follow.
WHAT EXACTLY ARE FREE RADICALS, AND HOW IS THEIR PRODUCTION RELATED TO PHYSICAL ACTIVITY?
Clarkson : Free radicals are chemical species with one or more unpaired electrons in their outer orbit that makes them highly reactive. Physical activity produces an increase in oxygen consumption reflecting the muscle's use of oxygen to provide energy in the form of ATP (adenosine triphosphate). As oxygen use increases so does the production of free radicals.
It is important to remember that in humans it is difficult to measure free radical production, so researchers have relied on several indirect assessments of free radical by-products. Numerous studies suggest that exercise promotes increases in these parameters. However, many of these indicators have been criticized for not representing an accurate measure of free radical-induced damage.
Goldfarb : It does appear that an increase in metabolic rate and/or a dramatic increase in blood flow to certain areas in the body pro-motes free radical production. Since oxygen is reduced in the mitochondria one electron at a time, the transient production of free radicals occur on a ongoing basis. It has been reported that 2-5% of the oxygen that enters the cytochrome chain to be reduced to water eventually "leaks" and forms free radicals. Thus, as the use of oxygen increases, the production of free radicals also increases. Blood perfusion and reperfusion studies have demonstrated that alterations in blood flow can also affect free radical production. It is not clear if the production of free radicals during exercise is primarily of a metabolic or circulatory nature.
Blumberg : As highly reactive compounds, free radicals can attack almost any molecule in the body. During physical exercise, particularly strenuous and exhaustive exercise as well as unaccustomed exercise, oxygen consumption may rise several fold. As mentioned previously, this rise in oxygen consumption has been associated with oxidative damage and muscle injury.
It also appears that free radicals can be generated from smog and other environmental sources, so exercising in areas with significant air pollution can generate even more free radicals.
Ji : I don't think that the relationship between free radical production and exercise is firmly established, although evidence generated in the last decade suggests that there might be multiple sources of free radical production in the body during, and probably after strenuous aerobic exercise. As mentioned previously, the primary source of free radicals is generally regarded as being the mitochondria, organelles within the cell which are responsible for the oxidation of most food-stuffs to generate energy. However, they may not be the only source. The following events may also be relevant to this problem. (1) Tissues that are mechanically injured during strenuous exercise are subject to inflammation. This may attract neutrophils and other immunologically active substances to the injury site. Many of these substances release free radicals as a mechanism to kill bacteria and other foreign invaders; (2) damaged cellular constituents need to be degraded and removed; therefore lysosomes and other protein "digestive" enzymes may be concentrated at the injury site; (3) as Dr. Goldfarb suggested, there is some evidence that tissues that are temporarily deprived of their blood supply can activate a cascade of reactions leading to the production of free radicals. This process is similar to so-called ischemia-reperfusion injury that is seen in the heart.
Jenkins : The warning of C.A. Rice-Evans that "Definitions are odious, difficult to formulate and riddled with loopholes" is certainly wise to keep in mind when one applies the popular definition that free radicals include any species capable of independent existence and possessing an unpaired electron. Leaving the definition at that point typically leads to the erroneous notion that all radicals are extremely reactive and undesirable. However, radicals are important inter-mediates in normal cell chemistry. It is becoming increasingly more clear that radicals are produced and used in a variety of useful reactions at rest. We should also expect that as radical production increases in response to exercise, a certain portion of the increased radicals will be involved in helpful chemistry.
CAN FREE RADICAL PRODUCTION ADVERSELY AFFECT PHYSICAL PERFORMANCE, OR RECOVERY FROM EXERCISE?
Jenkins : There is really a sparse literature available relative to this question. Most investigators have questioned whether exercise-induced radical production might be implicated in tissue damage. Very few studies have questioned the influence of radical production on performance. Some investigators have reported an increased performance in animals that have received spin traps, a group of compounds that stabilize radicals. However, I don't think that radicals are apt to exert much in the way of an adverse acute influence on physical performance in normal humans.
Blumberg : As Dr. Jenkins suggests, there are little data directly addressing this question. However, it is impossible to conceive of achieving optimal physical performance without maintaining optimal cellular function. Given the damage to cell membranes, proteins, and nucleic acids that free radicals can induce, it would seem that minimizing this injury would be advantageous.
Goldfarb : I don't think that anybody has demonstrated that there is a direct link between free radical production and alterations in physical performance either. However, it has been shown that running performance is decreased by 38-40% in vitamin E-deficient rats, and that free radical markers were elevated in these animals. Vitamin E deficiency has also been shown to result in structural alterations and loss of membrane integrity at the cellular level. These findings suggest that free radical production, or loss of antioxidant protection, can adversely affect performance.
Clarkson : In addition to the vitamin E deficiency studies that Dr. Goldfarb mentioned, there are studies suggesting that vitamin C supplementation can hasten recovery, and decrease muscle soreness. In one report, subjects who consumed vitamin C (3 g/day) for 3 days before and for 4 days after strenuous exercise developed less muscle soreness. Another study suggested that 21 days of vitamin C supplementation (400 mg/day) results in faster recovery of muscle strength after physical activity. Few studies, other than a recent report conducted at high altitude, have suggested that antioxidant supplementation can directly improve physical performance, but the results of the aforementioned studies could imply a link between free radical production and physical performance. However, the results of these studies need to be sub-stantiated before we begin to read too much into these data.
ARE SOME PEOPLE MORE SUSCEPTIBLE TO FREE RADICAL DAMAGE THAN OTHERS?
Goldfarb : It has been suggested that older individuals may be more susceptible to free radical damage, and I have seen animal data suggesting that female rats are not as prone to lipid peroxidation as male rats. Both of these findings require more research. Research indicating that individuals with antioxidant nutrient deficiencies are more susceptible to damage is more well accepted but, of course, nutrient deficiencies are not common in well-fed humans.
Blumberg : Several conditions may con-tribute to an individual being at increased oxidative stress status, e.g., people who: consume diets poor in antioxidant nutrients, smoke cigarettes, live in areas with significant air pollution, or have certain diseases associated with elevated oxidative stress status such as diabetes. As we have discussed, people who are regularly involved in strenuous exercise may also be particularly susceptible to free radical damage. Although regular exercise is associated with a compensatory increase in certain antioxidant enzymes, it is not clear whether this change is sufficient in all athletes to maintain an optimal peroxide level in muscle tissue. One particular sport, high altitude mountain climbing, has been linked to hypoxia-induced oxidative damage and impaired physiological function.
Jenkins : Research related to exercise and free radical chemistry is typical of all modern research in that it has focused on the statistics of group responses. Generally (but not always) groups of humans or animals with higher antioxidant status have shown less evidence of oxidative stress. Several laboratories have been interested in trying to develop markers to detect individuals who have experienced oxidative stress. We have found that some elderly subjects display evidence of ongoing oxidative stress.
Clarkson : Studies such as those conducted by Drs. Blumberg and Jenkins and others do strongly suggest that older individuals may be more susceptible to free radical damage. One study in particular from the group at Tufts University showed that vitamin E supplementation was effective in dampening the damaging effects of eccentric muscle exercise in older individuals; vitamin E was not nearly as efficacious in a group of younger subjects.
CAN HABITUAL PHYSICAL EXERCISE INCREASE ONE'S RESISTANCE TO FREE RADICAL DAMAGE?
Blumberg : Some data indicate that regular exercise increases cellular antioxidant defenses. This action may involve several different mechanisms including enhanced antioxidant enzyme activity and changes in protective immune responses. However, as noted, it is not clear (particularly in older adults) whether these compensatory changes to protect against the increased free radical burden associated with habitual exercise are fully adequate to provide for optimal cellular functioning.
Ji : In order to answer this question, it might be desirable to distinguish the inducible and non-inducible antioxidant defense systems. The former category refers to those which can be synthesized in the body in response to oxidative stress. Antioxidant enzymes and certain thiols such as glutathione and ubiquinone belong to this category. The latter category is best represented by antioxidant nutrients such as vitamins E, C, and beta carotene. These important antioxidants can not be synthesized by the human body and have to be part of the diet. Cross-sectional studies indicate that athletes tend to have higher antioxidant enzyme levels than sedentary people. Thus, physically active people may indeed be more resistant to free radical damage.
On the other side of the coin, heavy exercise may deplete the pool of antioxidant vitamins, and people who exercise habitually need to watch their diets carefully, and simply not assume that a "normal balanced diet" is adequate to provide sufficient antioxidant protection.
Clarkson : As Dr. Ji stated, several studies have shown that with training there is an enhanced antioxidant enzyme defense system. As an example, a study of antioxidant status of highly trained runners (80-147 miles/wk), less-well trained runners (16-43 miles/wk), and sedentary individuals showed that antioxidant capacity was enhanced in both groups of runners. There appeared to be a relationship between weekly training distance and antioxidant capacity.
The more trained an individual is the more likely they are to be able to counteract an increase in free radicals generated by exercise. The "week-end warrior", who may exercise strenuously only on occasion, may be most at risk for oxidative damage to cells.
Goldfarb : I believe that resistance is dependent on the type of exercise performed and the environment in which one exercises. In our laboratory we demonstrated that endurance trained rats displayed less lipid peroxidation by-products than untrained animals following a bout of heavy exercise. It should be noted that the rats exercised at the same absolute workload, which suggests that the trained animals actually worked at a lower relative workload. Nevertheless, resistance to free radical damage was apparently enhanced in the endurance trained rats.
CAN DIET AFFECT FREE RADICAL PRODUCTION? WHAT NUTRIENTS SEEM TO HAVE THE MOST PROMISING ANTIOXIDANT CAPABILITIES?
Ji : The dietary impact of free radical production and oxidative damage has been best illustrated in experimental animals with antioxidant nutrient deficiencies. For example, a diet deficient in selenium, a trace mineral essential for the synthesis of glutathione peroxidase, is known to cause major disturbances in antioxidant function at the cell, organ, and whole body levels. This has actually been demonstrated in humans by an epidemic in Northern China (a region with selenium-poor soil) called Keshan Disease, which is characterized by cardiac myopathy and a high mortality rate.
Vitamin E deficiency has been linked to an increased tendency toward hemolysis and lipid peroxidation. However, adult humans are fairly resistant to a single nutrient deficiency because the body usually mobilizes other antioxidant systems to compensate for it. It is doubtful that strengthening of a single antioxidant will provide a major advantage. Supplementation with certain antioxidants may be effective. However, there is no credible evidence to date suggesting that antioxidants are effective ergogenic aids.
Clarkson : Although several studies have shown that vitamin E or a mixture of antioxidant vitamins (E, C, and beta carotene) can reduce the increase in indirect indicators of free radical generation, other studies have failed to demonstrate this effect. Because antioxidants may work in concert with each other, it may be more prudent to study effects of antioxidant mixtures in future studies.
Although antioxidant vitamins have received much attention, ingestion of other nutrients with antioxidant properties such as glutathione, cysteine, and selenium have also been studied, and preliminary reports have shown positive effects. However, many of these studies have not been well controlled, have used small sample sizes or questionable markers of lipid peroxidation, and these results have not been corroborated.
Goldfarb : It has been reported that there is a linear relationship between the amount of unsaturated fat in the diet and lipid peroxidation. Polyunsaturated fats are more susceptible to free radical damage than are monoun-saturated or saturated fatty acids.
Of all of the potential protective antioxidants, the most-often studied nutrient seems to be vitamin E. Several investigators have shown that vitamin E supplementation can attenuate or eliminate free radical-induced lipid peroxidation, and that vitamin E deficiency can promote free radical damage. There are conflicting data, however, and more research is needed to clarify the potential benefits and the level of antioxidant supplementation that may be necessary to minimize exercise-induced lipid peroxidation.
Blumberg : As Dr. Clarkson stated, vitamins C and E and several dietary corotenoids including beta carotene and lycopene can quench free radicals and help prevent the chain reaction found in many lipid peroxidation events. In fact, supplementary intakes of vitamin C have been demonstrated to reduce the incidence off postrace symptoms of upper-respiratory-tact infection in ultrama-rathon runners. This benefit may be associated with a greater requirement of vitamin C during extreme endurance events resulting from increases in ascorbic acid metabolism and excretion, and/or increases in free radical production.
Furthermore, it is important to note that many other dietary constituents can play a role in antioxidant defenses including minerals like copper, manganese, zinc, and selenium, which are essential cofactors for antioxidant enzymes like superoxide dismutase and glutathione peroxidase. Although relatively little information is available, we now recognize that several phytochemicals (found in fruits and vegetables), including the biofiavonoids, may also play an important role in antioxidant defenses.
WHAT PRACTICAL ADVISE WOULD YOU GIVE TO ATHLETES REGARDING THEIR EXERCISE AND NUTRITION HABITS AS THEY RELATE TO FREE RADICAL GENERATION?
Goldfarb : I believe that a balanced diet generally contains enough antioxidants to meet the needs of most people. Eat citrus fruits for vitamin C, green leafy vegetables and oils for vitamin E, and butter, cheese and margarine for vitamin A. Endurance athletes tend to eat diets that contain large quantities of complex carbohydrates. Therefore, I think that research dealing with the impact of a high carbohydrate diet on free radical production during exercise is warranted.
Blumberg : There is no doubt that one's over-all diet is a critical element in promoting health and maintaining high levels of function during exercise. Certainly, an important component of a healthy diet includes a generous intake of fruits (2-4 servings per day) and vegetables (3-5 servings per day) as suggested by the Dietary Guidelines for Americans. If you take care to select those that are dark green or yellow in color, you should meet your requirements for most of the antioxidant nutrients (including phyto- chemicals) from your diet. However, some provocative studies now suggest that supradietary intakes of at least one of the antioxidant nutrients, i.e., vitamin E, may be associated with even greater protection against free radical damage and optimal immune responses to exercise-induced injury. Typical dietary intakes of vitamin E in the U.S. are about 11 IU per day (the RDA is 15 IU), an amount insufficient for most athletes. Recently, it has been suggest-ed that vitamin E intakes up to 400 IU per day may be reasonable for athletes engaged in moderate to heavy exercise.
Jenkins : I think that it would be misleading to advise the use of antioxidants as ergogenic aids, although it may be appropriate to think of them as nutritional supplements.
Further, I would caution against the indiscriminate use of iron supplements by athletes. Transition metals such as iron promote oxidative stress reactions, and the genetic tendency for iron loading disorder in the North American population exceeds the tendency for iron deficiency
Clarkson : There is concern that the increase in free radicals generated during strenuous exercise may exceed the body's antioxidant defense system, and therefore athletes should take antioxidant supplements. However, there are little data at present to fully substantiate these concerns. Furthermore, where is the epidemiological evidence that athletes are harmed by chronic strenuous exercise? If free radical generation were a problem, one might expect to see a greater incidence of diseases related to free radicals in athletes. On the contrary, studies have shown that Master's athletes are healthier than sedentary controls, and life expectancy has been demonstrated to be greater in various groups of elite athletes than in reference cohorts.
Indiscriminate use of antioxidants by athletes is not justified based on available data. However, it is unlikely that an antioxidant supplement containing 100% of the RDA of various nutrients will have a negative effect. Thus, supplementation with 100% of the RDA of vitamins E and C, and beta carotene, for example, in addition to adequate consumption of fruits and vegetables should provide an insurance policy until we have sufficient data to make specific recommendations.