Selenium supplements could be harmful

Most people in Canada and the U.S. get enough selenium in their diet and taking supplements could be too much of a good thing, a review suggests.

A review of the medical literature published in Wednesday's online issue of The Lancet concludes that high blood levels of selenium might have harmful effects such as raising the risk of Type 2 diabetes.

Selenium is a naturally occurring mineral found in soil and foods such as Brazil nuts, fish, poultry and wheat.

Low blood levels of selenium have been linkedto an increased risk of death and poor immune system function.

"The crucial factor that needs to be emphasized with regard to the health effects of selenium iswhereas additional selenium intake may benefit people with low status, those with adequate-to-high status might be affected adversely and should not take selenium supplements," concluded Margaret Rayman, a professor of nutritional medicine at the University of Surrey in England.

Intake of selenium is high in Venezuela, Canada, the United States and Japan and lower in Eastern Europe and areas of China, Rayman noted.

In the U.S., average daily intake recommendations for selenium are 60 micrograms per day for men and 53 micrograms per day for women.

The review included several studies linking low selenium intake to a higher risk of dying from all causes as well as from some cancers.

At the other end of the spectrum, there was also evidence that selenium levels affect immune function, and selenium supplements may help patients hospitalized with HIV.

The mineral is also crucial to the brain, Rayman said. A study of people over 65 found poorer co-ordination scores among those with low levels and it may also play a role in Parkinsons disease and cognitive decline.

A large U.S. study pointed to a pivotal point where selenium levels in the blood change from negative to positive in the risk of cancer, non-melanoma skin cancer and Type 2 diabetes: 122 micrograms per litre.

People with concentrations at that level or higher should not supplement with selenium, she concluded.

The Washington-based Council for Responsible Nutrition, which represents the supplement industry, said there are benefits to selenium.

"People in the 'sweet spot' for getting these benefits do so through a combination of diet and supplements, likely with a multivitamin containing selenium. Without supplementation, we'll likely see a decrease in the number of people reaching optimal levels. Too much of anything is not good; however selenium toxicity is extremely rare," the council said in a statement to CBC News.