Long-term selenium supplementation may have adverse effects on glucose metabolism and risk of type-2 diabetes in selenium-replete populations, such as the US. There is limited trial evidence on the effect of selenium supplementation on glucose metabolism in European populations whose selenium status is much lower than that of the US. We investigated the effect of selenium supplementation at different dose levels on changes in HbA1c after 6 months and 2 years in a population of relatively low selenium status.

The Denmark PRECISE study was a single-centre, randomized, double-blinded, placebo-controlled, multi-arm, parallel clinical trial with four groups. In total, 491 volunteers aged 60-74 years were randomly assigned to treatment with 100, 200, or 300 µg selenium/d as selenium-enriched-yeast or placebo-yeast. HbA1c measurements were available for 489 participants at baseline, 435 at 6 months, and 369 after 2 years of selenium supplementation. Analyses were performed by intention to treat.

The mean (SD) age, plasma-selenium concentration, and blood HbA1c at baseline were 66.1 (4.1) years, 86.5 (16.3) ng/g, and 36.6 (7.0) mmol/mol, respectively. During the initial 6- month intervention period, mean HbA1c (95% CIs) decreased by 1.5 (-2.8 to -0.2) mmol/mol for 100 µg/d of selenium supplementation and by 0.7 (-2.0 to 0.6) mmol/mol for the 200 and 300 µg/d groups compared with placebo (P = 0.16 for homogeneity of changes across the four groups). After 2 years of selenium supplementation, HbA1c had decreased significantly in all treatment groups, with no difference between active treatment and placebo.

Selenium supplementation in an elderly European population of relatively low selenium status did not significantly affect HbA1c levels after 2 years. Our findings corroborate a possible U-shaped response of selenium supplementation on glucose metabolism.