Selenium Supplements: Diabetes Risk?

Study: Taking Selenium Supplements May Raise Type 2 Diabetes Risk

Medically Reviewed by Louise Chang, MD on July 09, 2007
From the WebMD Archives

July 9, 2007 -- Taking selenium supplements may, over time, make type 2 diabetes more likely, a new study shows.

The study, published in the Annals of Internal Medicine's advance online edition, doesn't prove that selenium supplements cause type 2 diabetes.

But the researchers write that selenium supplements don't appear to prevent type 2 diabetes and "may increase risk for the disease."

Selenium is a mineral found in the soil and in plants.

The body needs small amounts of selenium. Too much selenium can cause health problems including stomach upset, hair loss, nail problems, and nerve damage.

The new study comes from researchers including Saverio Stranges, MD, PhD, who worked on the study while at the State University of New York at Buffalo. Stranges now works at Warwick Medical School in the U.K.

Selenium and Type 2 Diabetes

Stranges' team studied data on some 1,200 U.S. adults enrolled in a cancer prevention study.

The participants were 63 years old, on average. They lived in areas where the soil had low levels of selenium.

All participants had previously had nonmelanoma skin cancer. But none reported having type 2 diabetes when the study started.

The study mainly focused on selenium and cancer. The researchers also tracked new cases of type 2 diabetes, since experiments on animals had suggested that selenium supplements might help prevent type 2 diabetes.

Selenium Supplement Study

The researchers randomly split participants into two groups.

One group was assigned to take selenium supplements in a daily dose of 200 micrograms.

That’s higher than the Institute of Medicine's recommended dietary intake of 55 micrograms per day of selenium for men and for women who aren't pregnant or breastfeeding. But it's lower than the Institute of Medicine's upper limit of 400 micrograms of selenium per day.

For comparison, the other group took placebo pills containing no selenium. Participants didn’t know whether they were taking the selenium supplements or the placebo.

Participants were followed for nearly eight years, on average. During that time, they reported any new diabetes diagnoses and got their blood levels of selenium checked twice yearly.

Selenium Study's Results

During the study, 58 participants in the selenium group reported being diagnosed with type 2 diabetes, compared with 39 people taking the placebo.

The possible link between type 2 diabetes and selenium supplementation held when the researchers considered participants' age, sex, BMI (body mass index, which relates height to weight), and smoking habits.

But the study had a pretty narrow group of participants -- older adults with a history of nonmelanoma skin cancer -- and wasn't designed with diabetes in mind. So further studies are needed to see if selenium supplements truly raise diabetes risk, note the researchers.

Selenium Study Editorial

Stranges' study is accompanied by an editorial by doctors including Joachim Bleys, MD, MPH, of Johns Hopkins University Bloomberg School of Public Health.

The editorialists note that obesity and lack of physical activity are the main causes of diabetes. They also point out that most people in the U.S. get enough selenium from their diet.

"By taking selenium supplements on top of an adequate dietary intake, people may increase their risk for diabetes," write Bleys and colleagues.

That's just a theory at this point, but that theory "may be biologically plausible," write the editorialists.

Show Sources

SOURCES: Stranges, S. Annals of Internal Medicine, July 9, 2007; advance online edition. Bleys, J. Annals of Internal Medicine , July 9, 2007; advance online edition. Office of Dietary Supplements: "Dietary Supplement Fact Sheet: Selenium." News release, American College of Physicians.

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