Selenium Linked to Lower Diabetes Risk by 24%

The findings, from 7,000 male and female health care professionals followed for decades, add to a mixed bag of evidence on the protective effects of selenium, a known antioxidant, when it comes to diabetes. 

Dr. Dariush Mozaffarian, senior author of the new report, from the Harvard School of Public Health in Boston, said, "I wouldn't suggest, based on the findings from this study, that people start taking selenium supplements." For one, he said, there are different types of selenium, which may have varying effects -- and supplements contain only a single type.

In some places, selenium 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 collected from 3,630 women in the Nurses' Health Study and 3,535 men in Health Professionals Follow-Up Study between 1982 and 1987.

None had diabetes or heart disease at the beginning of the study. And just over 10% developed type 2 diabetes in subsequent years. The researchers noted that this rate is likely to be lower than in the general population, since the study participants were all health professionals.

For both men and women, the researchers found the risk of developing diabetes was 24% lower among people in the highest quintile of toenail selenium content, compared to those in the lowest quintile. The inverse relationship between diabetes risk and selenium levels appeared to be linear, according to the report. Risks in the second, third, and fourth quintiles were 9%, 22% and 23% lower, respectively, than in the lowest quintile (P for trend = 0.01).

Still, the authors emphasized that the study reinforces the need for a healthy diet, while discouraging the use of supplements to get more selenium. If anything, said Dr. 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 mcg of selenium each day. Three ounces of canned tuna in water contains about 68 mcg, for example, and one egg 15 mcg, according to the Office of Dietary Supplements of the National Institutes of Health.

Selenium toxicity is rare, but health officials suggest an upper limit of 400 mcg/day for adults to avoid side-effects. High levels of selenium in the blood can lead to selenosis, with symptoms including stomach problems, hair loss and mild nerve damage. (Brazil nuts sometimes contain more than 500 mcg of selenium per ounce, leading the Office of Dietary Supplements to say "it is wise to eat Brazil nuts only occasionally.")

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, stated that, "The difference between the beneficial effects and the harmful effects of selenium is very narrow." "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.

Published online before print May 22, 2012, doi: 10.2337/dc11-2136 Diabetes Care May 22, 2012