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Selenium Supplementation May Increase Type 2 Diabetes Risk by 50%

Posted: Thursday, August 23, 2007

More than half of diabetes cases in the National Prevention of Cancer trial occurred in individuals receiving selenium.  

Long-term selenium supplementation may place individuals at a 50% increased risk for type 2 diabetes,
according to results of the National Prevention of Cancer trial.

This large, randomized, multicenter clinical trial was designed to evaluate whether selenium supplementation prevents skin cancer. Secondary endpoint findings suggested that selenium does not prevent type 2 diabetes but rather increases the risk for the disease.

Saverio Stranges, MD, PhD, associate professor of cardiovascular epidemiology, Clinical Sciences Research Institute, Warwick Medical School, United Kingdom, stated that, “We found an increased risk for type 2 diabetes in people who took 200 mcg of selenium per day for almost eight years compared with people who took placebo.”

However, Stranges said that these data are from a single study and there is little understanding of the mechanisms by which selenium increases the risk for type 2 diabetes.

The National Prevention of Cancer (NPC) trial included 1,202 patients who were seen in dermatology clinics, did not have type 2 diabetes at baseline and lived in a low-selenium area of the United States. Each individual was assigned to either 200 mcg of selenium (n=600) or placebo (n=602).

After 7.7 years of follow-up, there were 97 new cases of diagnosed type 2 diabetes. Fifty-eight of these cases were in individuals who were assigned to selenium supplementation, and 39 occurred in individuals who were assigned to placebo (12.6 cases per 1,000 person-years vs. 8.4 cases per 1,000 person-years). The incidence of disease persisted even when the researchers stratified the analyses by age, sex, BMI and smoking status.

“It is important to inform the general population that long-term use of these supplements can bring about some detrimental effects,” he said, recommending larger clinical trials to examine this reported increased risk.

Selenium is a trace mineral used by the body to aid in metabolism. It has antioxidant properties that help prevent cellular damage from free radicals. Animal studies have shown a potential benefit of selenium in glucose metabolism and insulin resistance, and other analyses suggest benefits in preventing prostate cancer, lung cancer and overall cancer incidence; however, this evidence is not conclusive.

Levels of selenium vary according to the area. Soil in Nebraska and North and South Dakota is rich in selenium, whereas China and Russia have low amounts of selenium, and some people there have selenium deficiency.

“In the United States, the general population already has sufficient levels of selenium from the normal diet to optimize metabolism. There is no need for Americans to take more selenium — they can get it already from different dietary items,” including plant-based foods, like Brazil nuts, meat and seafood, Stranges said.

Most Americans consume between 30 mcg and 200 mcg of selenium per day. The recommended daily intake is 55 mcg for adults aged 19 years and older, according to the NIH.

Source: Diabetes In Control: Stranges S, Marshall JR, Natarajan R, et al. Effects of long-term selenium supplementation on the incidence of type 2 diabetes. Ann Intern Med. 2007;147:217-223

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