An analysis of data from a large national study found that people who took a 200 microgram selenium supplement each day for almost eight years had an increased risk of developing type 2 diabetes than those who took a placebo or dummy pill.

The data came from the Nutritional Prevention of Cancer Trial (NPC), a large randomized, multi-center, clinical trial from the eastern United States, designed to evaluate whether selenium supplements prevent skin cancer.

In the study, researchers selected 1,202 participants who did not have diabetes when they were enrolled in the NPC Trial. Half received a 200 microgram selenium supplement and half received a placebo pill for an average of 7.7 years.

Saverio Stranges, MD, PhD, of the Warwick Medical School in the UK, said the findings 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," said Stranges, lead author of the study. "We have observed an increased risk for diabetes over the long term in the group of participants who took selenium supplements.”

Selenium is a naturally occurring trace mineral present in soil and foods. The body needs selenium in very small amounts to aid in metabolism. 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.

In the study, 58 out of 600 participants in the selenium group and 39 out of 602 participants in the placebo group developed type 2 diabetes. After 7.7 years, the risk rate was about 50 percent higher for those taking selenium compared to those in the placebo group.

The results showed higher risks of disease among participants regardless of their age, gender, and smoking status. However, the selenium supplements had no impact on the most overweight participants and the risk of developing diabetes was higher in people who had higher blood selenium levels at the start of the study.

“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," said Stranges. "Nevertheless, I would not advise patients to take selenium supplements greater than those in multiple vitamins.”

Stranges said that selenium levels in soil in United States are higher than the minimum needed to optimize metabolism, so people in the United States should not need to take selenium supplements greater than those in multivitamin supplements.