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More bad supplement news: Vitamin E may be risky for prostate

Yet another dietary supplement has backfired.

A daily dose of vitamin E could actually increase a man's risk of prostate cancer, a study out today shows. Researchers discovered the disturbing link while studying the effects of antioxidants on men's health.

The vitamin E study comes on the heels of yesterday's report that common daily supplements, including multivitamins, iron, B6 and magnesium, appear to raise the death rate of older women.

There are big differences between these two studies, but both point to the inevitable conclusion that vitamins and supplements are not the magic potions that many people hope, even though more than two-thirds of the U.S. population takes them and they make up a $28 billion-a-year industry. 

The vitamin E study, called the SELECT trial, began in 2001, used the highest standard of scientific evidence -- a double-blind, placebo-controlled trial -- with the goal of proving vitamin E and selenium reduced the risk of prostate cancer. The rationale was sound. Small studies had suggested that the antioxidant effects of the two substances might reduce a man's cancer risk.

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As part of this major trial, funded by the National Cancer Institute, 35,000 men were recruited to test one or both of the compounds, or a placebo. By 2008, the study was halted because the evidence was clearly showing no benefit. 

Meanwhile, researchers continued to monitor the men. After four more years of follow-up, they found that vitamin E actually increased the risk for prostate cancer by 17 percent among men who took a daily dose of 400 IU, according to the report's conclusion published in the Journal of the American Medical Association. While the verdict on selenium is still not in, the researchers fear it might show negative effects, too, as they continue to follow the participants.

"I was surprised by the results of this trial," says Dr. Eric Klein, a urologist and the study leader from the Cleveland Clinic in Ohio. "There was a substantial amount of evidence going into SELECT when it was designed in the early 2000s to suggest that vitamin E or selenium might prevent prostate cancer and that's why we did the trial."

Because the government rules for the sales of vitamins and supplements are so much more lax than those for pharmaceuticals, you can still walk into a health food store today and find a bottle of Vitamin E with a label that reads: "Supports prostate health."

"Consumers should be skeptical about claims that are made on bottles and elsewhere unless there is solid scientific evidence," Klein says.

In the multivitamin study, researchers tracked 3,800 older women in Iowa for nearly two decades. The study found women taking vitamin B6, folic acid, magnesium, zinc, copper and iron had higher death rates. Those taking calcium had lower death rates.

It is important to note the multivitamin study was an observational trial where women simply filled out questionnaires regularly about their vitamin and supplement intake. The authors of this study say it is only suggestive and more research is needed to establish cause and effect.

Common vitamins could increase death risk

Despite all those supplements on the shelves, all too often, the evidence is just not there. The two studies together do raise the issue that Americans may be taking far more supplements and vitamins than they need and sometimes it can be harmful. As the Cleveland Clinic study shows, the products may not just waste your money, they could harm your health.

The take-home message: Most Americans get the nutrients they eat from food. As boring as that mantra sounds, you are better off eating fruits and vegetables, exercising regularly and maintaining a normal weight. A pill will seldom substitute.

Robert Bazell is NBC's Chief Science and Medical Correspondent.