Others seek selenium supplementation to combat heart disease and arthritis. Most recently, the mineral has been reported to show treatment benefits in AIDS patients. And the supplements are widely promoted on the Internet for a host of other reasons -- such as the prevention of cold sores, anti-aging, detoxification and fertility enhancement.
But the fact that selenium may be so widely available could make it even more of a diabetes threat. In the study, the 200-microgram pills taken by participants have comparable levels of selenium seen in many common multivitamin pills. And Marshall said the additional selenium may hold an even greater risk for those who already have high enough levels in their bodies.
"What's more worrisome to us is that among the people who are the most selenium-replete -- those with the baseline highest levels -- the relative risk of selenium supplementation is actually higher yet," he said.
Dr. Eliseo Guallar of Johns Hopkins University Bloomberg School of Public Health, who wrote an accompanying editorial to the study, described the findings as "more bad news for supplements."
And he says the findings underscore an even larger problem.
"I think it's a good reflection of what's happening with many other supplements as well," Guallar said. "We have millions of people who are taking these supplements, and we don't really understand the consequences … It's a bit scary."
Dan Hurley, medical journalist and author of the book Natural Causes: Death, Lies and Politics in America's Vitamin and Herbal Supplement Industry, agreed.
"This is another important piece of evidence showing that the media and the supplement industry for years have been making much ado about preliminary studies that have suggested benefits from antioxidants, but have never proven them," he said.
However, Hurley added, consumers must be careful to view this development in the proper context.
"People shouldn't misinterpret this study to mean that all vitamins are bad or that all doctors are crazy because they keep changing their minds," he said. "What it does mean is that people may want to be more careful, and they may want to touch base with their doctors about any supplements they are taking."
And Guallar said that it may not yet be time to close the book on selenium's possible anti-cancer benefits.
"There is still a possibility that selenium prevents some types of cancers; if this is the case, then we need to figure out what is the risk and what is the benefit," Guallar said.
But, he added, unless upcoming research reveals a definite advantage to extra selenium, "I don't think that U.S. consumers should be thinking about selenium supplementation at all. We should strike it off our health concerns for now."