Is selenate a diabetogenic form of selenium? Evidence from a natural experiment in Northern Italy

Background Consistent evidence from randomized controlled trials has shown that overexposure to selenium induces type 2 diabetes. However, uncertainties remain about the specific species and doses of selenium that trigger such diabetogenic effect. Methods We investigated the long-term effects of selenium exposure on diabetes risk using data from a natural experiment in Northern Italy. During 1974–1985, a small cohort of residents had been consuming drinking water with an unusually high content of inorganic hexavalent selenium, selenate (8–10 μg/L), close to the standard of the European Union and other countries of 10 μg/L. Using data from a population-based registry, we compared the prevalence of type 2 diabetes in 2013 in selenate-exposed (n = 1310) and unexposed residents (n = 56,251). Results In December 2013, the prevalence of diabetes in exposed and unexposed cohorts was 9.85 % and 10.29 %, respectively, with a multivariable-adjusted prevalence ratio of 0.95 (95 % confidence interval 0.81–1.12). Results were similar after stratifying by sex, age, and country of birth. Conclusions Overall, these results do not support the hypothesis that consumption of water with inorganic hexavalent selenium levels close to the European limit increases the risk of type 2 diabetes. Null results could be due to non-differential outcome misclassification, other sources of bias, or the fact that selenate is a non-diabetogenic selenium species or that the dose of exposure was too low to elicit an adverse effect.