Many antioxidant products are marketed with claims that, by blocking the action free radicals, they can help prevent heart disease, cancer, and various other conditions associated with aging.
Free radicals are atoms or groups of atoms that have at least one unpaired electron, which makes them highly reactive. Free radicals promote beneficial oxidation that produces energy and kills bacterial invaders. In excess, however, they produce harmful oxidation that can damage cell membranes and cell contents.
It is known that people who eat adequate amounts of fruits and vegetables high in antioxidants have a lower incidence of cardiovascular disease, certain cancers, and cataracts. Fruits and vegetables are rich in antioxidants, but it is not known which dietary factors are responsible for the beneficial effects.
Each plant contains hundreds of phytochemicals (plant chemicals) whose presence is dictated by hereditary factors. Only well-designed long-term research can determine whether any of these chemicals, taken in a pill, would be useful for preventing any disease.
The most publicized phytochemicals with antioxidant properties have been vitamin C, vitamin E, and beta-carotene (which the body converts into vitamin A).
Evidence exists that vitamin E can help prevent atherosclerosis by interfering with the oxidation of low-density lipoproteins (LDL), a factor associated with increased risk of heart disease. However, vitamin E also has an anticoagulant effect that can promote excessive bleeding. In 1993, the New England Journal of
Medicine published two epidemiologic studies which found that people who took vitamin E supplements had fewer deaths from heart disease. These studies did not prove that taking vitamin E was useful because they did not rule out the effects of other lifestyle factors or consider death rates from other diseases.
Moreover, other studies have had conflicting results. The only way to settle the question scientifically is to conduct long-term double-blind clinical studies comparing vitamin users to nonusers and checking death rates from all causes.