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How do antioxidants work?

Antioxidants act as cell protectors. 

Oxygen, an essential element for life, can create damaging by-products during normal cellular metabolism. Antioxidants counteract these cellular by-products, called free radicals, and bind with them before they can cause damage. If left unchecked, free radicals may cause heart damage, cancer, cataracts, and a weak immune system.

Antioxidants work by binding to the free radicals; transforming them into non-damaging compounds; or repairing cellular damage. Antioxidants come in a variety of forms and include Vitamin C, Vitamin E, the Carotenoids, and Selenium.

"Antioxidant defense systems limit [free-radical] formation and scavenge them, but antioxidant defenses are not 100 percent efficient," said Barry Halliwell, D.Sc., of the University of London and the University of California at Davis, speaking at a symposium sponsored by the International Life Sciences Institute (ILSI). Over time, "Oxidative stress can damage proteins, lipids, DNA and carbohydrates."

In an effort to beef up bodily defenses to combat free-radical activity, scientists are studying the effects of increasing individuals' antioxidant levels through the diet and dietary supplements.

Antioxidants and cancer prevention

Considerable laboratory evidence from chemical, cell culture, and animal studies indicates that antioxidants may slow or possibly prevent the development of cancer. However, information from recent clinical trials is less clear. In recent years, large-scale, randomized clinical trials reached inconsistent conclusions.

How might antioxidants prevent cancer?

Antioxidants neutralize free radicals as the natural by-product of normal cell processes. Free radicals are molecules with incomplete electron shells which make them more chemically reactive than those with complete electron shells. Exposure to various environmental factors, including tobacco smoke and radiation, can also lead to free radical formation. In humans, the most common form of free radicals is oxygen. When an oxygen molecule (O2) becomes electrically charged or "radicalized" it tries to steal electrons from other molecules, causing damage to the DNA and other molecules. Over time, such damage may become irreversible and lead to disease including cancer. Antioxidants are often described as "mopping up" free radicals, meaning they neutralize the electrical charge and prevent the free radical from taking electrons from other molecules.

Antioxidants and exercise

Although it is well known that vitamin deficiencies can create difficulties in training and recovery, the role of antioxidant supplementation in a well nourished athlete is controversial. The experimental studies are often conflicting and conclusions are difficult to reach. Nevertheless, most of the data suggest that increased intake of vitamin E is protective against exercise induced oxidative damage. It is hypothesized that vitamin E is also involved in the recovery process following exercise. Currently, the amount of vitamin E needed to produce these effects is unknown. The diet may supply enough vitamin E in most athletes, but some may require supplementation. There is no firm data to support the use of increased amounts of the other antioxidants.

Antioxidants and performance

In general, antioxidant supplements have not been shown to be useful as performance enhancers. The one exception to this is vitamin E which has been shown to be useful in athletes exercising at high altitudes. A placebo controlled study done on mountaineers demonstrated less free radical damage and decline in anaerobic threshold in those athletes supplemented with vitamin E. Although difficult to generalize, this finding suggests that supplementation with vitamin E might be beneficial in those triathletes who are adapting to higher elevations.

Antioxidants in clinical trials

Among the most widely-publicized research trials on antioxidants was a five-year study published in the Journal of the National Cancer Institute involving approximately 30,000 residents of north-central China. Participants were given either a placebo or a dietary supplement containing one of seven vitamin-mineral combinations. Persons who received a daily dose of beta carotene, vitamin E and selenium had a reduced cancer rate of 13 percent.

Although many questions remain as to the significance of these findings for other populations, the study represents the first large-scale randomized, prospective, placebo-controlled study showing the benefits of dietary supplementation with antioxidant vitamins and minerals. Much of the previous evidence was based on epidemiological studies of populations, which suggested an association between antioxidants and disease prevention but were not designed to reveal cause and effect relationships.

In another recent study reported at the American Heart Association (AHA) Scientific Session in November, women who consumed high amounts of antioxidant containing foods had a 33 percent lower risk of heart attack and a 71 percent lower risk of stroke, than women who ate few antioxidant-containing foods.

The study involved 1,795 female nurses, each of whom had a history of heart attack, chest pain due to coronary disease, or treatment for a blockage in a coronary artery. Food intake was analyzed according to subjects' estimated consumption of vitamins C and E, carotene and riboflavin.

Even after controlling for other variables that can contribute to cardiovascular risk such as age and high blood pressure, the nurses who consumed the most dietary antioxidants had the greatest disease reduction. Individual foods most closely associated with the health benefits included carrots, spinach and other greens.

"These data suggest that people at high risk because of a history of cardiovascular disease or events may benefit the most from increased consumption of fruits and vegetables," said principal investigator JoAnn Manson, M.D., of Brigham and Women's Hospital in Boston.

"These women were at especially high risk, and any intervention that impacts on that risk could potentially save a large number of lives," Manson added. Health care costs related to cardiovascular disease are expected to exceed $117.4 billion in 1993 alone, according to AHA.

Food sources of antioxidants

Antioxidants are abundant in fruits and vegetables, as well as in other foods including nuts, grains and some meats, poultry and fish. The list below describes food sources of common antioxidants.

  • Beta-carotene is found in many foods that are orange in color, including sweet potatoes, carrots, cantaloupe, squash, apricots, pumpkin, and mangos. Some green leafy vegetables including collard greens, spinach, and kale are also rich in beta-carotene.
  • Lutein, best known for its association with healthy eyes, is abundant in green, leafy vegetables such as collard greens, spinach, and kale.
  • Lycopene is a potent antioxidant found in tomatoes, watermelon, guava, papaya, apricots, pink grapefruit, blood oranges, and other foods. Estimates suggest 85 percent of American dietary intake of lycopene comes from tomatoes and tomato products.
  • Selenium is a mineral, not an antioxidant nutrient. However, it is a component of antioxidant enzymes. Plant foods like rice and wheat are the major dietary sources of selenium in most countries. The amount of selenium in soil, which varies by region, determines the amount of selenium in the foods grown in that soil. Animals that eat grains or plants grown in selenium-rich soil have higher levels of selenium in their muscle. In the United States, meats and bread are common sources of dietary selenium. Brazil nuts also contain large quantities of selenium.
  • Vitamin A is found in three main forms: retinol (Vitamin A1), 3,4-didehydroretinol (Vitamin A2), and 3-hydroxy-retinol (Vitamin A3). Foods rich in vitamin A include liver, sweet potatoes, carrots, milk, egg yolks and mozzarella cheese.
  • Vitamin C is also called ascorbic acid, and can be found in high abundance in many fruits and vegetables and is also found in cereals, beef, poultry and fish.
  • Vitamin E, also known as alpha-tocopherol, is found in almonds, in many oils including wheat germ, safflower, corn and soybean oils, and also found in mangos, nuts, broccoli and other foods.


  • Antioxidants protect cells from damage caused by unstable molecules known as free radicals.
  • Laboratory and animal research has shown antioxidants help prevent the free radical damage that is associated with cancer. However, results from recent studies in people (clinical trials) are not consistent.
  • Antioxidants are provided by a healthy diet that includes a variety of fruits and vegetables.


  • Follow a balanced training program that emphasizes regular exercise and eat 5 servings of fruit or vegetables per day. This will ensure that you are developing your inherent antioxidant systems and that your diet is providing the necessary components.
  • Weekend warriors should strongly consider a more balanced approach to exercise. Failing that, consider supplementation.
  • For extremely demanding races (such as an ultradistance event), or when adapting to high altitude, consider taking a vitamin E supplement (100 to 200 IU, approximately 10 times the RDA) per day for several weeks up to and following the race.
  • Look for upcoming FDA recommendations, but be wary of advertising and media hype.
  • Do not oversupplement.


  • Frei B (ed), Grei B. Natural antioxidants in human health and disease. New York, NY: Academic Press, 1994.
  • Blot WJ, Li JY, Taylor PR, et al. Nutrition intervention trials in Linxian, China: supplementation with specific vitamin/mineral combinations, cancer incidence, and disease-specific mortality in the general population. J Natl Cancer Inst 1993;85:1483-91.
  • The Alpha-Tocopherol, Beta Carotene Cancer Prevention Study Group. The effects of vitamin E and beta carotene on the incidence of lung cancer and other cancers in male smokers. N Engl J Med 1994;330:1029-35.
  • Omenn GS, Goodman G, Thomquist M, et al. The beta-carotene and retinol efficacy trial (CARET) for chemoprevention of lung cancer in high risk populations: smokers and asbestos-exposed workers. Cancer Res 1994;54(7 Suppl):2038s-43s.
  • Hennekens CH, Buring JE, Manson JE, Stampfer M, Rosner B, Cook NR, et al. Lack of effect of long-term supplementation with beta carotene on the incidence of malignant neoplasms and cardiovascular disease. N Engl J Med 1996;334:1145-9.
  • Lee IM, Cook NR, Manson JE. Beta-carotene supplementation and incidence of cancer and cardiovascular disease: Women's Health Study. J Natl Cancer Inst 1999;91:2102-6.

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