Selenium supplements increase diabetes risk, review study confirms

Researchers say the common supplement’s benefits are unproven, while risks are clear. 

A new analysis of data from multiple studies has linked selenium, a mineral found in the soil as well as in many multivitamins and other supplements, with an increased risk of diabetes. Scientists at the University of Modena in Italy looked at the results of 55 studies over the past 30 years. They found patients taking selenium supplements had an 11 percent higher risk of diabetes.

That’s a relatively small risk, the authors admit, but one that’s important because of the high and growing numbers of diabetes patients worldwide and selenium’s overall positive reputation as a supplement. Older observational studies reported the nutrient reduced cancer risk, but the new review results add to a growing body of evidence that selenium supplements might not be a healthy choice on your next pharmacy run.

Selenium is a nutrient we need but can’t produce ourselves. We ingest it with our diet, and seafood and organ meats are the richest source of selenium. The National Institutes of Health (NIH) recommend 55 microgram of selenium per day for adults. Most Americans get enough selenium from their food. “Although selenium is an essential element for human life due to its anti-oxidant effects, more isn’t better in this case,” Tommaso Filippini, one of the study’s authors, says.

With their review, the research team confirmed the results of a 2007 study that had started out suspecting selenium supplements may actually prevent type 2 diabetes due to their antioxidant effects. To the scientists’ surprise, they found the opposite: supplements may have increased the risk among their over 1,200 study participants.

Why selenium increases diabetes risk isn’t clear yet. Experiments in animals have shown that selenium unhinges the balance of free radicals and antioxidants in the liver and leads to an imbalance in blood insulin levels. The Modena authors also think that excess selenium’s pro-oxidant effect and toxicity may be involved in the development of other diseases, including neurodegenerative diseases. Another study at the University of Modena recently suggested a connection between inorganic selenium and the progression of mild cognitive impairment to Alzheimer’s disease.

Selenium supplements became popular after observational studies in the 1960s reported that people with a diet rich in selenium, or who had high selenium levels in their tissue, had lower cancer risks. Some studies had backed up these results by showing that selenium could slow the growth of cancer cells in the lab. But a recent Cochrane review of newer, randomized controlled trials showed no effect of selenium on cancer risk.

The Modena team won’t pursue further human trials with selenium because of the risks they found. Except in cases of a diagnosed deficiency, they also recommend staying clear of selenium supplements. Filippini says: “It would be unethical to suggest supplementation because the benefits couldn’t be confirmed in trials, and the risks for other diseases, including diabetes 2, are clear.”

Feature image: Selenium via Wikimedia