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.