Selenium supplements appear to increase risk of developing type 2 diabetes

A new 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 being published, 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, lead author of the study, says that the findings from this study 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. We have observed an increased risk for diabetes over the long term in the group of participants who took selenium supplements."

Dr. Stranges is currently working at Warwick Medical School, UK, but previously worked at the State University of New York at Buffalo. Other authors of the article include Mary E. Reid, PhD, and James R. Marshall, PhD, researchers at the Roswell Park Cancer Institute in Buffalo.

Selenium is a naturally occurring trace mineral present in soil and foods. The body need selenium in minute 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.

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.

In the current 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 of follow-up, the relative risk rate was approximately 50 percent higher among those randomly selected for the selenium group than among those randomly placed in the placebo group.

The results consistently showed higher risks of disease among participants receiving selenium across subgroups of baseline age, gender, and smoking status. However, the selenium supplements had no impact on the most overweight participants. The risk of developing diabetes tended to be higher in people who had higher blood selenium levels at the start of the study.

Dr. Stranges said, "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."

About 60 percent of Americans take multivitamin pills, many of which contain between 33 and 200 micrograms of selenium, in addition to the selenium taken in from food and the air. The RDA (recommended dietary allowance) for selenium varies by age. For people aged 14 and over, 55 micrograms per day is recommended for the body to function normally.

Dr. 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.

In an accompanying editorial, Eliseo Guallar, MD, DrPH, from Johns Hopkins University Bloomberg School of Public Health, says the article is ,more bad news for supplements." He says that the NPC trial is the largest and longest experimental study available comparing selenium supplements to placebo, that selenium has a narrow therapeutic range and that at high levels, it can be toxic.

"What the U.S. public needs to know," Dr. Guallar says, "is that most people in the United States have adequate selenium in their diet. Moreover, taking selenium supplements on top of an adequate dietary intake may cause diabetes."