Understanding the New Research on Antioxidant Supplements

Clinical studies on antioxidant supplements have been mixed, with some finding benefits and others finding no benefits.

Antioxidant supplements have been popular in the U.S. for decades, especially among athletes. Early laboratory studies on their chemical properties suggested that they might help neutralize harmful chemicals called free radicals that are produced in the body as a part of normal cellular metabolism, and in response to environmental pollution. Oxidative damage caused by free radicals is thought to accelerate the aging process and artery disease, and to be potentially carcinogenic.

Clinical studies in humans have generally been mixed, however, with some finding benefits and others finding no benefits. And recent analyses suggest that megadoses of beta-carotene and vitamin E may even be harmful to the people researchers once thought would benefit most from supplementation: smokers and people with artery disease.

Beta-carotene and cancer

Beta-carotene is one of more than 50 chemicals called carotenoids in the human diet. Carotenoids are plant pigments responsible for the red, orange and deep yellow colors found in many fruits and vegetables. Many dark green vegetables contain carotenoids as well, but the dark green of chlorophyll masks the lighter shades of the carotenoids.

Several members of the carotenoid family have been associated with potential health benefits. For example, higher intakes of lycopene, found in cooked tomato products, have been associated with lower risk for prostate cancers. In general, studies finding a positive association measured intake from food, not supplements.

Because beta-carotene looks like a powerful antioxidant in the laboratory, several well-designed studies have tested the effect of beta-carotene supplements on rates of cancer development in people. While some studies found no effect, a few found that beta-carotene supplements were associated with higher cancer rates, especially in current and former smokers. A recent meta-analysis of research looking at the effect of beta-carotene supplements on rates of digestive system cancers in humans concluded that mortality rates were slightly higher for subjects in the beta-carotene groups than subjects receiving a placebo.1

Vitamin E and heart disease

Similar conclusions have been reached concerning vitamin E. In the laboratory, vitamin E seems to help protect arteries from damage by preventing oxidation of cholesterol carriers called low-density lipoproteins (LDLs). However, a recent meta-analysis concluded that human subjects receiving megadoses of vitamin E, versus a placebo, had somewhat higher mortality rates.2,3

How harmful are antioxidant supplements?

The current research does not suggest that all antioxidant supplements are harmful, but that people should re-evaluate their use of vitamin E and beta-carotene. Risk of mortality only increased in the meta-analyses by a very small amount. Critics have pointed out that subjects in many of the studies were unhealthy to begin with, so the effect of these supplements on healthy people may be different. Risk of harm from vitamin E seems to increase at doses of 400 IU and higher; lower doses may be safe, although more research is needed.

Because your body benefits from consuming at least 50 different carotenoids, most researchers suggest that it is too early to take megadoses of single carotenoid supplements such as beta-carotene. Fitness professionals would be wise to recommend a colorful, healthful diet, rather than beta-carotene supplements.


1. Bjelakovic,G., D. Nikolova, R.G. Simonetti and C. Gluud. Antioxidant supplements for prevention of gastrointestinal cancers: A systematic review and meta-analysis. Lancet 364 (9441): 1219-28, 2004.

2. Greenberg, E.R. Vitamin E supplements: Good in theory, but is the theory good? Annals of Internal Medicine 142 (1), 2005. Accessed 12/1/04 at www.annals.org.

3. Miller, E.R., R. Pastor-Barriuso, D. Dalal, et al. Meta-analysis: High-dosage vitamin E supplementation may increase all-cause mortality. Annals of Internal Medicine 142 (1), 2005. Accessed 12/1/04 at www.annals.org.

4. Powers, S.K., K.C. DeRuisseau, J. Quindry and K.L. Hamilton. Dietary antioxidants and exercise. Journal of Sports Sciences 22 (1): 81-95, 2004.

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