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With Selenium, a Little Goes a Long Way

Could too much of even a good thing have serious health effects? That appears to be the message of late, when it comes to taking some supplements and multivitamins.

A study published in the August 21 issue of Annals of Internal Medicine concluded that selenium supplementation may increase the risk of type 2 diabetes. The study compared selenium use at a controlled dosage (a 200-microgram daily supplement for nearly eight years) against use of a placebo.

Most people in the United States already get adequate amounts of selenium in their diets (in the range of 35 to 200 micrograms), and cases of deficiencies are rare, notes Eliseo Guallar, MD, DrPH, associate professor of Epidemiology at the Bloomberg School and co-author of an editorial that accompanied the study.

While antioxidants like selenium have been promoted as helping to prevent chronic disease, "there is also increasing evidence that vitamins and minerals such as selenium have a narrow therapeutic range and that at high levels can be toxic," says Guallar, who was senior author of a 2004 study associating the use of high-dose vitamin E supplements (in excess of 400 IUs) with a higher overall risk of dying.

In the Annals editorial, Guallar and colleagues went on to note that the potential harmful effects of selenium supplementation could extend beyond diabetes. A secondary analysis of the trial found a possible small increase in total cancer risk among participants with higher levels of the trace mineral, which is found in foods such as fish, whole grains, nuts and vegetables, as well as in soil and water.

Guallar says that the results of the recent study raise legitimate concerns for a supplement- and vitamin-captivated populace. Currently, more than 35 percent of Americans take multivitamin and multi-mineral supplements that often contain selenium and vitamin E.

Lawrence Cheskin, MD, director and founder of the Johns Hopkins Weight Management Center, cautions that the selenium study should not be an open call to throw vitamin and supplement bottles into the trash can. "I'm certainly reluctant to tell people who do not eat a well-balanced diet to stop doing something that could be positive for them," says Cheskin, an associate professor in International Health.

"I think the message here is to be cautious across the board with supplements, and avoid mega-doses of everything," he says. "A little bit is good; we often don't get the full amount of vitamins and nutrients that we need. So multivitamins and supplements can be useful. I take them myself."

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