While selenium supplements have been touted as a preventative measure for conditions ranging from cold sores to cancer, those who take the pills daily may be getting more than they bargained for when it comes to diabetes.
Specifically, people taking selenium supplements daily over a period of years may be putting themselves at a 50 percent higher risk of developing type II diabetes than those who do not, new research suggests.
The analysis, published on the Annals of Internal Medicine Web site Tuesday, used data from the Nutritional Prevention of Cancer Trial (NPC), a large clinical trial designed to determine whether selenium supplements prevent skin cancer.
Researchers selected more than 1,200 participants from the study who were initially diabetes-free. Half took a 200-microgram selenium supplement daily for an average of nearly eight years, while half received a placebo pill over the same duration.
What researchers found was that those taking the actual selenium supplements actually increased their risk of developing type II diabetes by about 50 percent.
Lead study author Dr. Saverio Stranges of Warwick Medical School, UK, said that the findings from this study suggest that selenium supplements do not prevent diabetes and that they might be harmful.
"At this time, the evidence that people should take selenium supplements is extremely limited," Stranges said in a press release issued Monday. "We have observed an increased risk for diabetes over the long term in the group of participants who took selenium supplements."
Study co-author James Marshall, senior vice president for cancer prevention and professor of urology at the Roswell Park Cancer Institute in Buffalo, N.Y., told ABCNews.com that the finding is of particular interest because of a widespread belief that loading up on antioxidants such as selenium invariably protects against a host of ills.
"The idea that people should just go out and start taking selenium in large amounts, willy-nilly, seems not to be sensible," he said. "Fifty percent risk is not trivial."
Selenium Supplementation Often Unnecessary
Like all nutrients, selenium is an essential ingredient in some of the body's most important functions. Specifically, the mineral is thought to contribute to metabolism, and there is evidence that not enough selenium can bring about a certain form of heart disease, hypothyroidism, and a weakened immune system.
In the United States, however, selenium deficiency is very rare; most Americans are able to get the recommended 55-to-70 micrograms per day of the mineral in their normal diets through eating foods as common as corn and wheat.
But the most common reasons that Americans opt for selenium supplements usually have little to do with deficiency concerns. In 1996, a paper published in the Journal of the American Medical Association suggested that selenium supplementation may reduce the risk of prostate cancer.
Additionally, selenium's antioxidant effects have been shown in some past studies to reduce the occurrence of other cancers as well, including lung, colorectal, and prostate cancer.
Research suggests that selenium may affect cancer risk in two ways -- both as an antioxidant and as an agent to prevent or slow tumor growth. However, not all studies have shown a relationship between selenium status and cancer.
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."