This fact sheet provides a general overview of antioxidants—with a focus on dietary supplements—and suggests sources for additional information.
Several decades of dietary research findings suggested that consuming greater amounts of antioxidant-rich foods might help to protect against diseases. Because of these results, there has been a lot of research on antioxidant supplements. Rigorous trials of antioxidant supplements in large numbers of people have not found that high doses of antioxidant supplements prevent disease. This section describes the preliminary research findings, the results of the clinical trials, and possible explanations for the differences in study results.
Observational studies on the typical eating habits, lifestyles, and health histories of large groups of people have shown that those who ate more vegetables and fruits had lower risks of several diseases, including cardiovascular disease, stroke, cancer, and cataracts. Observational studies can provide ideas about possible relationships between dietary or lifestyle factors and disease risk, but they cannot show that one factor causes another because they cannot account for other factors that may be involved. For example, people who eat more antioxidant-rich foods might also be more likely to exercise and less likely to smoke. It may be that these factors, rather than antioxidants, account for their lower disease risk.
Researchers have also studied antioxidants in laboratory experiments. These experiments showed that antioxidants interacted with free radicals and stabilized them, thus preventing the free radicals from causing cell damage.
Because the results of such research seemed very promising, large, long-term studies—many of which were funded by the National Institutes of Health (NIH)—were conducted to test whether antioxidant supplements, when taken for periods of at least a few years, could help prevent diseases such as cardiovascular diseases and cancer in people. In these studies, volunteers were randomly assigned to take either an antioxidant or a placebo (an identical-looking product that did not contain the antioxidant). The research was conducted in a double-blind manner (neither the study participants nor the investigators knew which product was being taken). Studies of this type—called clinical trials—are designed to provide clear answers to specific questions about how a substance affects people’s health.
Among the earliest of these studies were three large NIH-sponsored trials of high-dose supplements of beta-carotene, alone or in combination with other nutrients. These trials, completed in the mid-1990s, all showed that beta-carotene did not protect against cancer or cardiovascular disease. In one trial, beta-carotene supplements increased the risk of lung cancer in smokers, and in another trial, supplements containing both beta-carotene and vitamin A had the same effect.
More recent studies have also found that in most instances antioxidant supplements did not help to prevent disease. For example:
Unlike the studies described above, two large studies on age-related macular degeneration (AMD), a leading cause of vision loss in older adults, found a beneficial effect of dietary supplements that include antioxidants. Both studies were sponsored by the National Institutes of Health.
The first study, called AREDS (for Age-Related Eye Disease Study), showed that a combination of antioxidants (vitamin C, vitamin E, and beta-carotene) plus zinc and copper reduced the risk of progression of intermediate AMD to advanced AMD by 25 percent over 5 years. However, during the period when this study was performed, other research showed that taking beta-carotene supplements increases the risk of lung cancer in people who smoke cigarettes.
The second study, called AREDS2 (for Age-Related Eye Disease Study 2), showed that the carotenoids lutein and zeaxanthin are an appropriate replacement for beta-carotene in the supplement formula. Unlike beta-carotene, lutein and zeaxanthin did not increase lung cancer risk, and 10-year followup data on the study participants showed that lutein and zeaxanthin were more effective than beta-carotene in reducing the risk of progression of AMD.
Most clinical studies of antioxidant supplements have not found them to provide substantial health benefits. Researchers have suggested several reasons for this, including the following: