Do Antioxidants Block Exercise Benefits?
Opinion by Alan R. Gaby, MD
We have always been taught that free radicals, caused by oxidation, can damage our bodies and promote disease development. A new study now suggests that the oxidative stress induced by exercise may actually be good for us, since it improves insulin sensitivity and increases the body’s natural antioxidant defenses. Moreover, according to the researchers, taking antioxidants such as vitamins C and E may block some of the benefits of exercise by decreasing the amount of exercise-induced oxidative stress. However, concluding from these findings that exercise and antioxidants don’t mix is premature, as that conclusion is contradicted by a wealth of other research.
To C or not to C (or E)?
In the new study, healthy young men underwent a four-week exercise training program, during which they were randomly assigned to take vitamin C (500 mg twice a day) and vitamin E (400 IU per day) or a placebo.
Insulin sensitivity was measured before and after four weeks of exercise. Insulin sensitivity is a measure of how efficiently the body responds to insulin. Impaired insulin sensitivity (insulin resistance) is a risk factor for diabetes, heart disease, and other conditions.
The levels of two of the body’s natural antioxidant defenses (superoxide dismutase and glutathione peroxidase) were also measured before and at the end of the exercise program. In the men who received the placebo, physical training resulted in an increase in both insulin sensitivity and the levels of superoxide dismutase and glutathione peroxidase. In contrast, none of these measures increased in response to exercise in people taking vitamins C and E.
Look before you leap…to conclusions
It might seem logical to conclude from this study that taking antioxidants could block the beneficial effect of exercise in preventing diabetes and heart disease. However, there are several reasons such a conclusion may not be correct.
First, the men in the study were all young, healthy, and not overweight, and therefore probably had normal insulin sensitivity. It is not clear whether increasing already-normal insulin sensitivity aids in preventing diabetes and heart disease. The study did not examine how antioxidants interact with exercise in people who are insulin-resistant (and therefore at increased risk for diabetes and heart disease). Previous studies have shown that supplementing with vitamin E helps reverse insulin resistance (although those studies did not investigate the influence of exercise).
Second, the fact that insulin sensitivity did not increase in response to exercise does not mean the exercise was not beneficial. The increase in insulin sensitivity that results from exercise training is a compensatory response that allows the body more efficiently to take up carbohydrates (glucose) into cells and burn them to produce energy. However, most of the energy produced during prolonged exercise comes not from burning carbohydrates, but from burning fat. Previous studies have shown that vitamin E levels in the blood rise after intensive exercise, possibly to mitigate the oxidative stress that results from increased burning of fat. If vitamin E somehow enhances the fat-burning response to exercise, such an improvement might eliminate the need to increase carbohydrate-burning, and thus no increase in insulin sensitivity would be necessary.
The bigger picture
Earlier research has found that supplementing with vitamins C and E improves immune function in joggers, but not in sedentary individuals, indicating a positive interaction between these vitamins and exercise. In addition, the combination of vitamin E supplementation and exercise improved heart and lung fitness in men living at high altitude (7,200 feet above sea level), whereas exercise alone or vitamin E alone had no effect. Moreover, supplementing with vitamin C has been reported to decrease muscle soreness after intense exercise.
The fact that antioxidant supplementation did not increase superoxide dismutase and glutathione peroxidase levels in the new study is also not a cause for concern. Since extra vitamin C and vitamin E were available to counter exercise-induced oxidative stress, the body presumably had no need to manufacture more of its own antioxidants. There is no reason to believe that exercisers who have high levels of superoxide dismutase and glutathione peroxidase are any better off than exercisers who have high levels of vitamins C and E.
Clearly, the interactions between antioxidants, exercise, and health are complex, and the results of the new study do not provide convincing evidence that taking antioxidants blocks the beneficial effects of exercise.
May 21, 2009
(Proc Natl Acad Sci 2009 May 11; e-pub ahead of print)
An expert in nutritional therapies, Chief Medical Editor Alan R. Gaby, MD, is a former professor at Bastyr University of Natural Health Sciences, where he served as the Endowed Professor of Nutrition. He is past-president of the American Holistic Medical Association and gave expert testimony to the White House Commission on Complementary and Alternative Medicine on the cost-effectiveness of nutritional supplements. Dr. Gaby has conducted nutritional seminars for physicians and has collected over 30,000 scientific papers related to the field of nutritional and natural medicine. In addition to editing and contributing to The Natural Pharmacy (Three Rivers Press, 1999), and the A–Z Guide to Drug-Herb-Vitamin Interactions (Three Rivers Press, 1999), Dr. Gaby has authored Preventing and Reversing Osteoporosis (Prima Lifestyles, 1995) and B6: The Natural Healer (Keats, 1987) and coauthored The Patient's Book of Natural Healing (Prima, 1999).
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