Findings from animal models suggest that selenium supplementation improves glucose metabolism.

To examine the effect of long-term selenium supplementation on the incidence of type 2 diabetes.

Secondary analysis of a randomized, double-blind, placebo-controlled trial.

Areas of low selenium consumption of the eastern United States.

1202 persons seen in dermatology clinics who did not have type 2 diabetes at baseline.

Oral administration of selenium, 200 μg/d, or placebo.

Incidence of type 2 diabetes.

During an average follow-up of 7.7 years (SD, 2.7), type 2 diabetes developed in 58 selenium recipients and 39 placebo recipients (incidence, 12.6 cases per 1000 person-years vs. 8.4 cases per 1000 person-years, respectively; hazard ratio, 1.55 [95% CI, 1.03 to 2.33]). The lack of benefit of selenium supplementation on the incidence of type 2 diabetes persisted in analyses stratified by age, sex, body mass index, and smoking status. An exposure–response gradient was found across tertiles of baseline plasma selenium level, with a statistically significantly increased risk for type 2 diabetes in the highest tertile of baseline plasma selenium level (hazard ratio, 2.70 [CI, 1.30 to 5.61]).

Diabetes was a secondary outcome in the parent trial. Diagnoses of diabetes were self-reported but were validated in most participants. The sample was mostly older and white.

Selenium supplementation does not seem to prevent type 2 diabetes, and it may increase risk for the disease.