Everyday it seems that new supplements are appearing on the market. One group of nutritional supplements that has received a lot of attention in the past few years is antioxidants. Vitamins such as C, E and beta-carotene along with fruits like blueberries and strawberries are considered powerful antioxidants. Some claim that these have many health benefits including enhanced exercise performance. Is this true? Can antioxidants benefit athletes? Dr. Scott Powers of the University of Florida is one of the leading researchers on exercise and antioxidants. He has published a review article focusing on antioxidant supplementation and performance. In his review, he concludes that there is actually little scientific evidence in favor of supplementation and that supplementation with high doses may do more harm than good.
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What exactly are antioxidants? Antioxidants are a class of compounds that fight off reactive oxygen. Chemically speaking, reactive oxygen (or oxygen radicals) is formed when oxygen molecules lose an electron. This makes the molecule highly unstable and reactive. These molecules can then damage proteins, membranes and DNA. Such “oxidative damage” can lead to cell death, and is linked to many disorders including some cancers. Antioxidants scavenge reactive oxygen and protect cells from damage.
Reactive oxygen is formed during the normal course of metabolism. Some conditions lead to greater production and a condition known as oxidative stress occurs. Cells are equipped with its own antioxidant defenses so that most conditions of oxidative stress are easily handled by the cell. In some pathological conditions, oxidative stress can be very large greater or the cells own antioxidant defenses are suspect. Under either these conditions, reactive oxygen accumulates and the damaging to various cells can be severe.
During exercise, metabolism increases 10-20 times normal. As a result, the muscle cells undergo a period of oxidative stress and there is a rise the in amount of reactive oxygen produced. However, this rise is relatively small and very temporary compared to clinical conditions. Nevertheless, some researchers have linked this exercise-induced oxidative stress to exercise performance. They feel that it plays a key role in fatigue during prolonged activity lasting more than 30 minutes. By providing greater defense against reactive oxygen through supplementation, fatigue can be delayed.
On the other hand, many researchers point out that normal, healthy individuals have more than enough natural antioxidants to protect against oxygen radicals. This is especially true during exercise. Enzymes and antioxidant compounds found within muscle cells are more than capable of fighting off reactive oxygen. In fact, one of the benefits of exercise training is enhanced protection against oxidative stress – training builds greater defense. This adaptation leads this group to argue that reactive oxygen plays little if any role in the development of fatigue.
Given this debate, will do antioxidant supplements improve exercise performance?
The compound N-acetylcystine (NAC) may have some positive effects on performance. A few laboratory studies show that NAC delays fatigue under some conditions. Unfortunately, the methods used in these studies raise questions about the practicality of using NAC. For instance, one study shows positive effects after 35 minutes of intravenous infusion of NAC. Needles to say, most athletes cannot nor should not take such a pre-match approach. A few other studies show little or no effect of ingesting NAC on performance. So while there may be some potential for NAC to delay fatigue, there is not enough evidence to suggest that athletes use it as a performance enhancer.
As for nutritional antioxidants such as vitamins C, E or beta carotene, the research is much clearer. The vast majority of scientific studies show that supplementation with these vitamins has very little effect on performance. The vast majority of research studies all agree on this point. Further, a review of more than 68 clinical trials emphasize that dietary supplementation with these vitamins does not appreciably improve health. As it turns out, a well balanced diet that is high in fresh fruits and vegetables provides plenty of antioxidants and little benefits are gained by supplementation.
In his review, Dr. Powers emphasizes an important point. New evidence indicates combating exercise-induced oxidative stress may actually do more harm than good. Reactive oxygen seems to trigger the muscle to adapt to training. The brief rise during exercise stimulates the muscle to produce the machinery needed to improve function (and to combat oxidative stress). This includes muscle proteins as well as mitochondria and enzymes needed for energy production. Two recent studies emphasize this point. Both show that supplementing the diet with large amounts of vitamin E and C (~16 times the RDA for an adult) blunt the training adaptation to exercise.
The bottom line is that antioxidant supplements are not needed for otherwise healthy athletes who have solid nutritional habits. The only exception is athletes who do not eat a proper diet. Those who skip meals, don’t eat proper servings of fruits and vegetables or try to lose excess weight by cutting calories may benefit from a vitamin supplement that contains antioxidants. Otherwise, a healthy diet will provide more than adequate defense against oxidative stress and allow training adaptations. Supplementing the diet with antioxidants will not provide an added training benefit.
Finally, it should be pointed out that nearly all of the studies addressing the ability of antioxidants to improve performance were carried out with adult subjects. There is little if any research on adolescents or pre-adolescents. This makes it even more difficult to recommend antioxidant supplements to young athletes. Neither the positive nor potentially negative effects of short- or long-term use are understood. Thus, at this time, young players should focus on proper nutrition rather than supplement use.
Stear SJ, Burke LM, Castell LM, Powers SK, Kavazis AN, Nelson WB, Ernst E (2009) BJSM Reviews: A-Z of nutritional supplements, sports nutrition foods and ergogenic aids for health and performance Part 3. British Journal of Sports Medicine, 43:890-892.
Posted by Jay Williams, Ph.D. Labels: Current Research, Nutrition, Supplements