Taking selenium supplements may increase the risk for developing diabetes among those whose levels of the nutrient are already adequate, a Lancet review suggests.

Although maintaining appropriate selenium levels is thought to confer multiple health benefits, including providing some protection against prostate, lung, colorectal and bladder cancers, taking too much could be harmful.

“The crucial factor that needs to be emphasized is that people whose blood plasma selenium is already 122 µg/L or higher – a large proportion of the U.S. population – should not take selenium supplements,” study researchers Margaret P. Rayman, DPhil, of the University of Surrey in Guildford, England, said in a press release.

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National Health and Nutrition Examination Survey data have shown an association between high selenium levels and increased risk for type 2 diabetes (Diabetes Care 2007; 30: 829-834), and a smaller French studied showed an association between plasma selenium levels and fasting plasma glucose (Am J Clin Nutr 2006; 84: 395-399). Other randomized clinical trials evaluating type 2 diabetes as a secondary outcome have had mixed results.

Conversely, numerous observational studies have associated adequate selenium levels with health benefits such as reduced mortality risk, enhanced immunity, and improved cognitive function and fertility. But few clinical trials have confirmed these benefits, according to Rayman.  Most recently the large Selenium and Vitamin E Cancer Trial (SELECT), failed to confirm the supplement’s benefit on prostate cancer risk.

These conflicting study results may be attributable to the fact that selenium supplementation is only beneficial when intake is inadequate, Rayman noted. The largest selenium trials to date have been performed in countries like the United States, in which intake and status is good. Therefore, future trials should include only patients with low selenium levels.

Rayman also pointed out that researchers studying the health affects of selenium should genotype participants, as polymorphisms in selenoproteins affect selenium status and disease risk.

Rayman MP. Lancet. 2012; doi: 10.1016/S0140-6736(11)61452-9.