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Antioxidants are substances that inactivate free radicals, highly unstable molecules that can damage cell membranes and scramble the genetic information (DNA) in cells. Free radicals are produced in the body during normal cell metabolism, and at a higher rate following tissue injury or exposure to tobacco smoke, sunlight, x-rays, and other environmental influences. To fight these dangerous chemicals, the body deploys a powerful antioxidant defense system, but it is hypothesized that in some cases the quantity of free radicals may overwhelm the body's natural defenses. This in turn could theoretically accelerate or cause various illnesses.

Vitamins A , C , and E , beta-carotene , and the mineral selenium function as antioxidants, agents with the ability to deactivate naturally occurring toxic compounds called free radicals . Other substances in foods such as lutein , astaxanthin and lycopene also have antioxidant activity; in addition, as noted above, the body produces its own antioxidants.

The “antioxidant hypothesis” states that herbs and supplements with antioxidant properties may prevent illnesses and health problems associated with excess free-radical activity. For example, the conditions heart disease , cancer , macular degeneration , and aging skin are just a few of the hundreds of illnesses in which free radicals are thought to play a role. Based on this hypothesis, some of the largest double-blind, placebo-controlled studies in history were set up and performed to evaluate the potential disease-preventive benefits of various antioxidant supplements. The results, unfortunately, have been quite disappointing. 1-6 At present, there is no firm evidence that antioxidants provide any meaningful health benefits. Indeed, in a 2012 review of 78 randomized trials involving 296,707 participants, researchers found no evidence that antioxidant supplements (eg, beta-carotene, vitamin A, vitamin C, vitamin E, selenium) could reduce mortality.8

On the contrary, there was some evidence that vitamin A , beta-carotene , and vitamin E supplementation may actually increase mortality. 7

What is the reason for this discrepancy? It is possible that studies have not involved the right combinations of antioxidants. It is also possible that the antioxidant hypothesis is wrong.

For more information, see the individual articles on the antioxidant nutrients noted above.

  • 1

    Bjelakovic G, Nikolova D, Gluud LL, et al. Mortality in randomized trials of antioxidant supplements for primary and secondary prevention: systematic review and meta-analysis. JAMA . 2007;297:842-57.

  • 2

    Bjelakovic G, Nagorni A, Nikolova D, et al. Meta-analysis: antioxidant supplements for primary and secondary prevention of colorectal adenoma. Aliment Pharmacol Ther. 2006;24:281-91.

  • 3

    Coulter ID, Hardy ML, Morton SC, et al. Antioxidants vitamin C and vitamin E for the prevention and treatment of cancer. J Gen Intern Med. 2006;21:735-744.

  • 4

    Pham DQ, Plakogiannis R. Vitamin E supplementation in cardiovascular disease and cancer prevention: Part 1. Ann Pharmacother. 2005;39:1870-1878.

  • 5

    Bleys J, Miller ER 3rd, Pastor-Barriuso R, et al. Vitamin-mineral supplementation and then progression of atherosclerosis: a meta-analysis of randomized controlled trials. Am J Clin Nutr. 2006;84:880-887,954-955.

  • 6

    Bardia A, Tleyjeh IM, Cerhan JR et al. Efficacy of antioxidant supplementation in reducing primary cancer incidence and mortality: systematic review and meta-analysis. Mayo Clin Proc. 2008;83:23-34.

  • 7

    Bjelakovic G, Nikolova D, Gluud LL, Simonetti RG, Gluud C. Antioxidant supplements for prevention of mortality in healthy participants and patients with various diseases. Cochrane Database of Systematic Reviews. 2008;(2):CD007176.

  • 8

    Bjelakovic G, Nikolova D, Gluud LL, Simonetti RG, Gluud C. Antioxidant supplements for prevention of mortality in healthy participants and patients with various diseases. Cochrane Database Syst Rev. 2012 Mar 14;3:CD007176.