Selenium is an essential trace element, but it can also be toxic at high doses. Most Americans have high levels of selenium intake as compared to people in some other countries, due to the higher levels of selenium in U.S. soils as well as the use of dietary supplements containing selenium (Laclaustra et al. 2009). High levels of selenium have been found in streams subject to mountaintop mining and valley fills in central Appalachia. In some streams, selenium has bioaccumulated to four times the toxic level in the food chain, a level that can cause harm in fish and birds. Groundwater wells are also affected, and state advisories are in effect for consumption of fish due to high selenium levels (Palmer et al. 2010). There is a high prevalence of diabetes in many counties in central Appalachia (Centers for Disease Control and Prevention 2009).
Selenium has been associated with type 2 diabetes in some studies of people in the U.S. Laclaustra et al. (2009) found that in U.S. adults exposed to background selenium levels, the prevalence of diabetes increased with increasing levels of selenium. Fasting glucose levels and hemoglobin A1C levels (a measure of long-term glucose control) increased with increasing selenium levels as well. Another study using the same dataset but from an earlier time period also found selenium levels to be associated with diabetes (Bleys et al. 2007). The dataset used in these studies does not distinguish between type 1 and type 2 diabetes, although most participants would have had type 2. In countries with lower selenium levels, including Singapore and France, studies have generally not found associations between selenium and diabetes or glucose levels (Laclaustra et al. 2009).
Yet a long-term study found that toenail levels of selenium were associated with a lower risk of type 2 diabetes in U.S. adults, at dietary levels of intake (Park et al. 2012).
Since selenium is an essential element, it is sometimes found in dietary supplements. Interestingly, two trials of selenium supplementation suggest that selenium may increase the risk of type 2 diabetes. Previous research had suggested that selenium supplementation might improve glucose metabolism. Yet a randomized, double-blind, placebo controlled study found that selenium supplementation instead may increase the risk of developing type 2 diabetes (Stranges et al. 2007). Another large, randomized, double-blind, placebo-controlled study, aimed to determine whether selenium (and/or vitamin E) supplements could prevent prostate and other cancers. It found instead that those who took selenium supplements alone had higher rates of type 2 diabetes (although the increased risk was not significant). The study was discontinued early (Lippman et al. 2009).
Some selenium compounds can induce oxidative stress, perhaps a mechanism involved in diabetes development (Laclaustra et al. 2009).
High levels of selenium may increase the risk of type 2 diabetes. Selenium supplements should be avoided, especially in the U.S.