Q & A Library

Selenium, a mineral found in soil, water, and some foods, is required in trace amounts. It has antioxidant effects that help the body address cellular damage from free radicals. One of its most valuable roles is as a cofactor of an important antioxidant enzyme in the body called glutathione peroxidase. Selenium helps support a strong immune system, regulates thyroid function, and may help reduce the risk of prostate and other cancers. It also plays a role in the prevention of cataracts and heart disease.

I doubt that your selenium supplementation is responsible for your medical test results. But a study published in the August 21, 2007 issue of the Annals of Internal Medicine did suggest that over time, selenium supplementation may increase the risk of type 2 diabetes The study compared two groups of adults whose average age was 63. All had a history of nonmelanoma skin cancer. One group took a daily dose of 200 mcg of selenium while the other took a placebo. Neither group knew whether the pills were the supplements or the placebo. The participants were followed for an average of almost eight years, during which they had their blood levels of selenium checked twice a year and reported any new diagnoses of skin cancer or other disorders.

Over the course of the study, 58 participants in the selenium group were diagnosed with type 2 diabetes, compared with 39 people taking the placebo. The selenium connection remained even after the researchers considered other risk factors including the participants' age, sex, BMI (body mass index, which relates height to weight), and smoking habits. However, because the participants had been selected on the basis of their age and skin cancer history and because the study wasn't designed to look for a selenium/diabetes connection, the investigators concluded that more research is needed to see if selenium supplementation really does influence diabetes risk.

The greatest risk factors for type-2 diabetes are excess weight and lack of exercise. In an editorial accompanying the study, a group of physicians from Johns Hopkins' Bloomberg School of Public Health wrote that since most people in the United States get adequate selenium in their daily diets, adding to that with selenium supplements might have an effect.

I take 200 mcg of selenium a day as part of my antioxidant formula and continue to recommend it. As an experiment you might try discontinuing it to see if there is any change in your fasting blood sugar or other markers of prediabetes.

Andrew Weil, M.D.