Selenium supplements may be harmful for people who already get enough of the mineral in their diets — which is most people in the U.S. — and could increase the risk for type 2 diabetes, according to a new review.

 

Use of selenium supplements has become widespread over the past 10 years, largely due to the belief that selenium can reduce the risk of cancer and other diseases. But "excessive zeal for increasing selenium intake has at times had adverse consequences," study author Margaret Rayman, a professor of nutritional medicine at the University of Surrey in England, wrote in her findings.

 

Those who get enough selenium in their diets should not take selenium supplements, Rayman concluded. People already get that mineral from grains, seafood and other common elements of the American diet.

 

The review was published online on Feb. 29 in the Lancet.

 

"Excessive zeal"

Though selenium supplements have been marketed for a multitude of conditions, this largely has been based on the results of observational studies, according to the paper. However, findings from clinical trials looking to confirm the supplements' effectiveness have been mixed.

 

Rayman reviewed selenium studies conducted since 1990. She said the mixed findings probably stem from the fact that supplements offer benefits only when the amount of selenium in a person's diet is inadequate.

 

Research has linked low selenium intake or levels in the blood with an increased risk of dying over a given period, poor immune function and cognitive decline. And higher selenium intake or blood levels have been linked to enhanced male fertility, antiviral effects, and protection against some cancers.

 

But the new review shows that levels that are too high can bring harmful effects.

 

Specifically, Rayman found people with high levels had an increased risk of type 2 diabetes. The link might be explained by the fact that selenium is incorporated into a protein, called GpX1, that affects the way insulin works in cells, she wrote.

 

Selenium in the diet

Selenium is a naturally occurring trace mineral found in soil and water and taken up by plants. The foods with the highest concentrations of selenium are organ meats and seafood, but the mineral is also found in cereals and grains, muscle meats and, to a lesser extent, dairy products, fruit and vegetables, according to the paper.

 

Recommendations for selenium intake average 60 micrograms per day for men, and 53 micrograms per day for women, according to the paper.

 

"The implications are clear: People whose serum or plasma selenium concentration is already 122 µg/L or higher — a large proportion of the U.S. population — should not supplement with selenium," Rayman wrote, pointing to data from blood samples taken as part of National Health and Nutrition Examination Survey, a large, ongoing study conducted by the Centers for Disease Control and Prevention.

 

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