In these diseases, free radicals that cause oxidative damage may play a critical role in causing the disease or may work with other factors in producing the condition.

Learn about the latest research on the role of free radicals and oxidative damage in:
Cancer
Heart disease
Macular degeneration
Diabetes
Alzheimer's disease
Parkinson's disease

Free radicals are believed to play a role in many cancers because of the damage they do to DNA. In response, researchers have proposed a role for antioxidants in protecting against lung cancer, invasive cervical cancer, esophageal and stomach cancer, and colonic polyps⁴, but the results of clinical trials have been inconsistent.

In two major studies, the Alpha-Tocopherol, Beta Carotene trial and the Beta-Carotene and Retinol Efficacy Trial, male smokers were given supplements of beta-carotene (a form of vitamin A) or placebo⁵. Unexpectedly, the men who received beta-carotene had a higher incidence of lung cancer than did those taking placebo. Interestingly, in the Women's Health Study, beta carotene supplements had no effect, either in promoting or preventing, cancer in women smokers⁶. However, while the evidence for benefits for antioxidant supplements in cancer is inconclusive, diets high in natural antioxidants do appear to be associated with lower risks of disease, including cancer.⁷

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Perhaps the most promising and best-studied area in which antioxidants might play a valuable role is in heart disease. Research has demonstrated that the circulation of oxidized LDL (often called the "bad" cholesterol) promotes atherosclerosis or hardening of the arteries. In vitro or laboratory studies have shown that the antioxidant vitamin E can counteract some of the effects of oxidized LDL. The U.S. Nurses Health Study and the Health Professionals Follow-up Study both showed a greater than 30% reduction in the risk of heart disease among those who took supplemental vitamin E. However, the Women's Health Study of over 40,000 women showed no difference in the rate of death from heart disease when comparing those who took vitamin E with those who did not.⁸

One review of several different studies of the benefits of vitamin E supplements did not find enough benefits to recommend routine use of supplements, but the author did note that, despite that lack of conclusive evidence, about half of America's cardiologists take vitamin E themselves!⁹

A recently recognized risk factor for coronary artery disease is high circulating blood levels of the substance homocysteine. One small trial looked at the use of folate plus antioxidant vitamins in patients with known heart disease, and found that their homocysteine levels dropped after two weeks on supplements.¹⁰ This study was too brief and involved too few patients to offer any definitive conclusions, but it does suggest that more work should be done in this area.

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Age-related macular degeneration, a form of eye damage, is the leading cause of blindness in Americans over the age of 65. Some researchers have suggested that when the retina of the eye absorbs light, excessive levels of free radicals are produced, leading to oxidative damage and eventually blindness. Thus, some have recommended that supplements of antioxidants might reduce the incidence of macular degeneration.

In one Finnish study, male smokers received supplements of vitamin E and/or vitamin A or placebo. The antioxidants did not reduce the frequency of age-related macular degeneration.¹¹ In one small study of 20 people, gingko biloba, another antioxidant, was shown to have some benefit in improving macular degeneration.¹²

Other studies have shown that diets high in the antioxidants called carotenoids, especially lutein and zeaxanthin, did have a protective benefit in reducing the risk of macular degeneration.¹³ These are found primarily in green leafy vegetables, and the intake of spinach or collard greens had the greatest benefit. Neither vitamins A, C, nor E was especially helpful. Thus, the preliminary studies suggest that dietary antioxidants, rather than supplements might be beneficial in age-related macular degeneration.

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Among the complications of longstanding diabetes are heart disease, hardening of the arteries, kidney disease, and decreased circulation to the legs. Research has linked those complications with high levels of free radicals.¹⁴

Elderly patients with diabetes have been found to have low blood levels of vitamins A and E and carotenoids (another class of antioxidants), especially beta-carotene and lutein.¹⁵ Studies conducted in diabetic rats have shown that supplementing their diets with vitamin E resulted in improved control of their diabetes, which should lead to fewer complications.¹⁶ Much work needs to be done before these preliminary results can be applied to humans.

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In Alzheimer disease, a protein called beta amyloid is associated with the cell damage that occurs with the disease. The effects of beta amyloid are worsened by free radicals.¹⁷ In theory, antioxidants that scavenge or scoop up free radicals in brain cells should slow the progression of Alzheimer disease.

Preliminary studies of the antioxidants vitamin E, gingko biloba and selegeline have suggested some benefit for Alzheimer patients.¹⁸ Other laboratory studies conducted on brain cells in culture, have shown that melatonin can retard some of the oxidative damage of beta amyloid,¹⁹ but further studies need to be conducted in patients before melatonin can be recommended as a potential treatment for Alzheimer disease.

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Parkinson's disease occurs when damage to certain parts of the brain produces tremors and other complications. Oxidative damage appears to play a role in the onset of Parkinson's disease. Early studies suggested that deficiencies of the antioxidants glutathione and glutathione peroxidase in specific areas of the brain can lead to the disease.²⁰ Recent laboratory studies looking at the protective benefits of vitamin E and selegeline have shown that both offer promise of protecting the vulnerable neurons affected by Parkinson's.²¹ Trials that will test these antioxidants in humans are just beginning.

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