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Selenium, one of many products manufactured by 5N Plus Inc.

Supplement poppers beware: Taking too much selenium could be harmful, a new review study cautions.

Small amounts of selenium, a trace mineral found in soil and foods, are needed for maintaining good health.

Over the past decade, use of selenium supplements has become widespread, fuelled, in part, by a few studies that suggested it may reduce the risk of certain cancers and other ailments. It also has antiviral properties and can apparently boost fertility.

However, the body uses selenium in "a very narrow therapeutic range," according to Margaret Rayman from the University of Surrey in Britain. In her review of the medical literature, published this week in The Lancet, she reports that low levels of selenium are associated with poor immune function and cognitive decline. Yet high levels can cause dermatitis, hair loss and perhaps type-2 diabetes.

"The intake of selenium varies hugely worldwide," noted Dr. Rayman. People living in Canada and the United States tend to get sufficient levels of selenium through their diet. That's why taking supplements increases the likelihood of consuming too much.

If the amount of selenium in your bloodstream is at or about 122 micrograms per litre, you don't require any more, she said. You'd need a lab test to determine your specific level. But that extra effort is not necessary for most people. "Personally, if I lived in Canada, I wouldn't supplement with selenium," Dr. Rayman said.

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