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Report Warns Against Taking Large Vitamin Doses for Prevention

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WebMD Health News

April 11, 2000 (Washington) -- If you're taking large doses of vitamins C and E or other antioxidant supplements like selenium or carotenoids to ward off chronic diseases, you may be doing yourself more harm than good. That's the major conclusion of a new report from the Institute of Medicine (IOM), a division of the National Academy of Sciences.

In its analysis, titled "Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids," the IOM's expert panel looked at the controversial question of whether taking these antioxidants could help prevent conditions like cardiovascular disease, Alzheimer's disease, or certain types of cancer. It is believed that antioxidants interfere with certain forms of oxygen and nitrogen in the blood called free radicals that can damage normal cells.

However, after a review of the available studies, the panel concluded that, at least for now, there's not enough evidence about the benefits of antioxidants to say they prevent disease, even though many suggest that eating foods rich in these substances lowers the risk of illness.

"What is known, however, is that taking very large quantities of some antioxidants can actually cause health problems," said panel chair Norman Krinsky, PhD, at a news conference Tuesday.

Krinsky, a biochemist at the Tufts University School of Medicine, says the panel's recommendation is to consume either the recommended daily allowance (RDA) or no more than what is considered the maximum safe amount, or "tolerable upper intake level." Anything in that range should be safe, according to the IOM.

The panel says too much vitamin C can cause diarrhea, excess amounts of vitamin E have been linked to bleeding, and selenium in megadoses is known to trigger hair loss as well as cause brittle fingernails and toenails. These side effects are increasingly a cause for concern as more Americans choose to manage their health care by taking supplements.

You do have some reason to be interested in antioxidants, however. Some studies show that vitamin E may slow the rate of mental decline in the early phases of Alzheimer's disease. Vitamin C may not prevent the common cold, but research indicates it shortens the duration of symptoms.

However, food should be a sufficient source of antioxidants, says the report. Those who take supplements may wonder whether certain types or forms are preferable. "It doesn't have any impact on the upper limit, but if you are going to take a supplement, it really should be taken with a meal [to be better absorbed]," says panelist Maret Traber, PhD, a nutritionist at Oregon State University.

For vitamin C, the daily recommendation is 75 mg for women and 90 mg for men. These numbers actually represent a 50% increase for men and a 25% jump for women. Smokers should get another 35 mg of C daily, as the antioxidant may counter the damage smoking does to cells, according to the report. The suggested upper limit, however, is 2,000 mg daily of vitamin C for adults. Fruits and vegetables are a good source of C.

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