Since antioxidants counteract the harmful effects of free radicals, you would think that we should consume as much as of them as possible. The truth is, although there is little doubt that antioxidants are a necessary component for good health, it is not clear if supplements should be taken and, if so, how much. Once thought to be harmless, we now know that consuming mega-doses of antioxidants can be harmful due to their potential toxicity and interactions with medications. Remember -- antioxidants themselves may act as pro-oxidants at high levels.
So, is there any basis for the hoopla? The experimental (intervention) studies completed so far have had mixed results:
The Alpha-Tocopherol Beta-Carotene Cancer Prevention Study (ATBC) involved Finnish men who were heavy smokers and alcohol drinkers. The volunteers were either given 20 milligrams (mg) of synthetic beta-carotene, 50 mg of vitamin E, a combination of the two, or a placebo. After eight years, the men who took vitamin E had 32 percent fewer diagnoses of prostate cancer and 41 percent fewer prostate cancer deaths compared to men who did not. However, after only four years, there were 16 percent more cases of lung cancer and 14 percent more lung cancer deaths in the beta-carotene-only group.
In the Carotenoid and Retinol Efficacy Trial (CARET), volunteers were either smokers or asbestos workers. They were given a combination of 30 mg of synthetic beta-carotene and 25,000 International Units (IU) of retinol (pre-formed vitamin A) or placebo. This study was stopped early due to the fact that preliminary findings suggested that there was a 28-percent increase in lung cancer rates in the beta-carotene group compared to the placebo group.
The Physicians' Health Study (PHS) of 22,000 physicians, 11 percent of whom were smokers and 40 percent were past smokers, showed neither a protective effect nor a toxic effect after 12 years of follow-up. The participants were randomized to receive either 50 mg of beta-carotene every other day or placebo. (A second PHS underway is testing beta-carotene, vitamin E, vitamin C, and a multivitamin with folate in healthy men age 65 and older for slowing cognitive decline.)
A 1997 study published in the Journal of the American Medical Association found that 60 mg of vitamin E a day strengthened the immune system in a group of healthy patients at least 65 years old, and 200 mg generated a four-to-six fold improvement after four months. However, 800 mg of vitamin E resulted in worse immunity than receiving no vitamin E at all.
In 2001, the Age-Related Eye Disease Study (AREDS), a randomized, placebo-controlled clinical trial, showed that high-dose supplementation of 500 mg of vitamin C, 400 IU of vitamin E, 15 mg of beta-carotene, 80 mg of zinc, and 2 mg of copper significantly reduced development of advanced age-related macular degeneration (AMD) compared to placebo. In addition, the antioxidant-plus-zinc group had significant reduction in rates of at least moderate visual acuity loss.
There are several possible explanations to account for the results.
- The amount of antioxidants in supplements may be so high compared with that in the diet that it leads to a toxic effect.
- Other nutrients may be present in fruits and vegetables that work in sync with antioxidants and are necessary to provide a protective effect.
- The study participants may have been too old to start taking antioxidants, or they may have led lifestyles that were too unhealthy for the antioxidants to make a positive difference.
Furthermore, literally hundreds, if not thousands, of observational studies (where investigators look for associations without giving participants supplements to take) have linked diets rich in antioxidant-rich fruits and vegetables to a lower risk for diseases like cancer, heart disease, stroke, cataracts, Parkinson's, Alzheimer's, and arthritis. So, despite the disappointing findings of trials, scientists remain certain of the many potential benefits of dietary antioxidants -- they simply haven't figured out exactly how the different antioxidant systems work together in our bodies to protect us from free-radical damage.