The findings, published in the journal Diabetes Care, are based on 7,000 male and female health care professionals followed for decades. But they add to a mixed bag of evidence on the protective effects of selenium, a known antioxidant, when it comes to diabetes.

"I wouldn't suggest, based on the findings from this study, that people start taking selenium supplements," said Dr. Dariush Mozaffarian, senior author of the new report, from the Harvard School of Public Health in Boston.

For one, he said, there are multiple different types of selenium, which may have different effects -- and supplements contain only a single type.

Antioxidants are thought to offer some protection against chronic diseases, including diabetes, and selenium has become a popular supplement in recent years for that reason. The mineral is also found naturally in foods like bread, meat and nuts.

In some places, it occurs in high concentrations in soil, affecting the direct exposure of people who live nearby and the selenium content of foods grown in the region.

To look at the long-term effect of selenium exposure on diabetes risk in otherwise healthy people, Mozaffarian and his colleagues analyzed toenail clippings from the 1980s.

A little over 7,000 women and men participating in the long-term Nurses' Health Study and Health Professionals Follow-Up Study had contributed the samples between 1982 and 1987 and answered extensive questionnaires about their diets, lifestyles and illnesses over the next two decades.

None had diabetes or heart disease at the beginning of the study. And just over 10 percent developed Type 2 diabetes in subsequent years (a rate that is likely to be lower than in the general population, according to the researchers, since the study participants were all health professionals).

Though questionnaires about diet rely on memory, which can be faulty, the accumulation of selenium in toenails accurately reflects exposure to the mineral over about a year's time.

For both men and women, the researchers found the risk of developing diabetes was 24 percent lower among people with the highest levels of selenium in their toenails, compared to those with the lowest levels.

Still, they emphasized that the study reinforces the need for a healthy diet, while discouraging the use of supplements to get more selenium. If anything, said Mozaffarian, people should be choosing healthy foods like whole grains and fish, which are rich in the mineral.

The Institute of Medicine, an advisory panel to the U.S. government, recommends most adults get 55 micrograms of selenium each day. One bagel contains about 27 micrograms, for example, and one egg 15 micrograms.

Selenium toxicity is rare, but health officials suggest an upper limit of 400 micrograms per day for adults to avoid side effects. High levels of selenium in the blood can lead to a condition called selenosis, with symptoms including stomach problems, hair loss and mild nerve damage.

"The difference between the beneficial effects and the harmful effects of selenium is very narrow," said Dr. Eliseo Guallar of Johns Hopkins University in Baltimore, who has studied the mineral's health effects but was not involved in the new study. "A little bit can be very good, but once you go above a certain level there can be side effects."

Selenium levels in the U.S. population are already quite high because of high selenium content of soils in some parts of the country, the study authors noted.

In addition, they haven't ruled out the possibility that high selenium levels in the study participants was a sign of other lifestyle factors that could partly explain their lower diabetes risk.

Participants with higher levels of selenium also ate more whole grains and consumed less saturated fat, coffee and alcohol and were less likely to be smokers than those with lower levels of the mineral.

SOURCE: bit.ly/LpEV7x Diabetes Care, online May 22, 2011.

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