Does high selenium exposure or supplementation in selenium-replete people worsen glycemic control and raise type 2 diabetes risk?

Research indicates that the relationship between selenium and metabolic health is characterized by a "U-shaped" curve, where both deficiency and excess can lead to dysfunction. For populations that are already selenium-replete (common in the United States), additional exposure or supplementation has been consistently linked to adverse glycemic outcomes.

Clinical and epidemiological evidence

Large-scale clinical trials and observational studies have identified a significant association between high selenium status and metabolic impairment:

• Diabetes risk: Meta-analyses of randomized controlled trials (RCTs) show a pooled relative risk for type 2 diabetes (T2D) of approximately 1.11 (95% CI 1.01–1.22) associated with selenium supplementation.
• The SELECT Trial: This massive study of over 35,000 men found that 200 μg/day of selenium was associated with a statistically significant increase in T2D risk (HR 1.07).
• The NPC Trial: A secondary analysis showed that supplementation significantly increased T2D risk specifically in participants who were already in the highest tertile of baseline selenium levels.
• Observational thresholds: Cross-sectional data from NHANES indicated that individuals in the highest quintile of serum selenium had a significantly higher prevalence of T2D (OR 7.64) compared to the lowest. The risk appears to escalate notably when blood selenium exceeds 160 μg/L or dietary intake surpasses 80 μg/day.

Mechanistic explanations

The biological basis for selenium-induced insulin resistance involves the disruption of essential signaling pathways:

• Antioxidant interference: While selenium is required for glutathione peroxidase (GPx1) activity, excessive selenium can lead to GPx1 over-expression. This over-neutralizes reactive oxygen species (ROS) like hydrogen peroxide (H2O2). While often viewed as "bad," H2O2 acts as a secondary messenger in insulin signaling; its excessive removal inhibits the phosphorylation of AKT/protein kinase B, directly causing insulin resistance.
• Hormonal and enzyme regulation: High selenium levels upregulate Selenoprotein P (SeP) and can influence hepatic gluconeogenic enzymes, potentially increasing glucose production in the liver and worsening overall glycemic control.

Bottom line

For individuals who are already selenium-replete, high selenium exposure or supplementation is strongly linked to an increased risk of type 2 diabetes and impaired insulin signaling. While supplementation may benefit deficient populations, it appears to be metabolically detrimental for those with adequate baseline levels.