Experiments on animals suggested that selenium might protect against type 2 diabetes, but a randomised trial now suggests that, rather than being protective, selenium may increase the risk of diabetes. A secondary analysis of a parent trial where diabetes was a secondary outcome comprised 1202 people without diabetes at baseline, who were randomised to 200 µg of selenium daily, or placebo.
After an average follow-up of 7.7 years, 58 people taking selenium had type 2 diabetes, compared with 39 people in the placebo group, giving a hazard ratio of 1.55 (95% CI 1.03 to 2.33). Furthermore, higher plasma concentrations of selenium at baseline were associated with an increased risk for developing diabetes. People whose plasma concentration of selenium was in the highest third had a 2.7-fold increased risk of developing type 2 diabetes during follow-up compared with those in the lowest third.
Before selenium was found to be essential for immune function and thyroid function, and to protect against oxidative stress, it was thought to be highly toxic to animals and humans, says the linked editorial (p 271). Most people who live in the United States get enough selenium from food, and supplementation seems to be harmful. The editorial reminds us that no dietary supplements have yet been shown to prevent cardiovascular disease or cancer in Americans.