The Council for Responsible Nutrition, a trade association representing dietary supplement suppliers and manufacturers, questioned the researchers' decision to do a meta-analysis of studies that investigated a variety of antioxidants, rather than analyzing them as individual nutrients.
"They're violating one of the basic tenets [of meta-analysis] by combining things that shouldn't be combined," said John Hathcock, vice president of scientific and international affairs at the council. Some antioxidants are known to have opposing effects, so in a study population taking two supplements, the benefits of one may be masked by the negative effects of the second, he said.
A new study that suggests antioxidants may not protect against several cancers looked only at pills, not at antioxidant vitamins found in foods.
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_____Special Report_____
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Deep Purple (The Washington Post, Oct 6, 2004)
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Overfed, Undernourished (The Washington Post, Sep 21, 2004)
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Accept No Substitutes (The Washington Post, Aug 25, 2004)
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Interactions (The Washington Post, Aug 3, 2004)
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Belly Up To the Bars (The Washington Post, Jul 27, 2004)
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Dietary Supplements
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In addition, participants in the studies took high doses of antioxidants, which the authors said might contribute to the negative findings.
Researchers said that the antioxidant levels that provide protective effects are not known and probably differ between individuals. Some nutrients may have benefits at lower levels but negative effects at higher ones.
Participants in the studied trials took doses considerably higher than those found in multivitamins and some antioxidant supplements. For instance, every one to two days they took between 1,500 and 15,000 micrograms of vitamin A, compared with the recommended intake (known as a Dietary Reference Intake, published by the Food and Nutrition Board of the Institute of Medicine) of 700 micrograms for most women and 900 micrograms for most men. Study participants took 30 to 600 milligrams of vitamin E, compared with a recommended intake of 15 milligrams daily for most adults. (The labeling on many bottles of over-the-counter supplements measures some pill contents in international units; some use the metric scale.)
Not all antioxidant supplements were found to be without benefit or potentially risky. Selenium appears to be an exception, Bjelakovic said, potentially leading to a reduction of gastrointestinal cancers.In four trials, selenium supplements reduced the incidence of gastrointestinal cancers, according to the study. The reductions occurred within two to four years after beginning supplementation.
Selenium has been found to offer other potential benefits, leading the National Cancer Institute (NCI) to sponsor further study into its effects. NCI is funding a clinical trial to see if selenium and vitamin E reduce the risk of prostate cancer.
Two trials of beta carotene included in the analysis drew criticism from some experts. The trials, which found that beta carotene had harmful effects, included participants who were smokers or had been exposed to asbestos and were therefore at higher risk for cardiovascular disease and lung cancer.
"No one can tell what was responsible for their increased risk," said Jeffrey Blumberg, a nutrition professor at Tufts University. "Basically this [beta carotene research] is driving the entire conclusion of this report." Blumberg sits on advisory boards for several nutritional pharmaceutical companies and receives honoraria for attending some meetings.
But other previous reports conclude that antioxidant supplements are of limited value. A meta-analysis in the July issue of the Archives of Internal Medicine looked at seven trials involving vitamin E supplementation. It found that the supplements did not help prevent cardiovascular disease and that patients' assumptions about unproven benefits may keep them from developing a healthy lifestyle.
The U. S. Preventive Services Task Force last year said there is insufficient evidence to recommend for or against the use of supplements of vitamins A, C or E; multivitamins with folic acid; or antioxidant combinations for the prevention of cancer or cardiovascular disease.
The task force also recommended against the use of beta carotene supplements, alone or in combination, for prevention of those diseases.
Howard Parnes, chief of NCI's prostate group in the division of cancer prevention, said that, regardless of the infighting about this new analysis, it's clear that consumers should focus more on eating healthy diets and less on taking supplements. With studies showing harm to some populations, and others showing no benefit, there are good reasons to be a cautious consumer of vitamin pills beyond doses found in a multivitamin.
"I really don't think people should be going out and taking large amounts of antioxidants," said Parnes, who said he doesn't take supplements but does eat a "colorful" diet rich in vegetables and fruit. "Eating a good diet is the number one way [to stay healthy]. If people for whatever reason can't or don't eat a good diet, a multivitamin is a good alternative."•
