Selenium supplements are widely promoted on the Internet for conditions ranging from cold sores and shingles to arthritis and multiple sclerosis. They are sold to prevent aging, enhance fertility, prevent cancer and get rid of toxic minerals such as mercury, lead and cadmium.
Selenium supplements have shown some promise in preventing prostate cancer. Because of selenium's antioxidant activities, some scientists feel it might be effective against diabetes.
But now researchers say that selenium supplements don't appear to prevent type 2 diabetes and "may increase risk for the disease."
The study is published in the Annals of Internal Medicine's advance online edition.
Dr. Saverio Stranges, lead author of the study, says it's not clear why selenium would be associated with an increased risk of developing diabetes.
"No single study can provide the answer to a scientific question, but at this time, selenium supplementation does not appear to prevent type 2 diabetes, and it may increase risk of the disease. However, our understanding of the mechanisms whereby selenium would increase risk of diabetes is very limited at this time and this issue needs to be further explored.
"Nevertheless, I would not advise patients to take selenium supplements greater than those in multiple vitamins," he said.
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.
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.
All participants in the study had previously had non-melanoma 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.
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.
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 non-melanoma 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.
In an accompanying editorial, Dr. Eliseo Guallar from Johns Hopkins University Bloomberg School of Public Health, agrees that selenium seems to offer few benefits and at high levels, may be toxic.
Dr. Guallar says most people have adequate selenium in their diet.
"Moreover, taking selenium supplements on top of an adequate dietary intake may cause diabetes."