Rayman found several studies linking low selenium intake to a higher risk of dying from all causes as well as from cancer.
There's also some evidence that selenium levels can affect immune system function. Rayman found studies that suggest that selenium supplementation decreased hospital admissions due to infection for people who have HIV.
Selenium also plays an important role in brain function, according to the review. In a study of adults older than 65, performance assessments of coordination were worse in people who had low selenium levels. There was also an increased incidence of Parkinson's disease in people with low selenium. Too little selenium may also increase the risk of dementia, the review found.
"Low selenium status has been associated with higher risk of mortality, poorer immune function and cognitive [brain] decline," Rayman said. "Increasing selenium intake can help our ability to handle viruses, increase successful male and female reproduction, and reduce the risk of autoimmune thyroid disease. There is also some evidence that selenium may reduce the risk of cancer."
But, higher levels of selenium don't come without risk. People with the highest levels of selenium intake may have a greater risk of type 2 diabetes, non-melanoma skin cancers, hair loss and skin rashes, according to Rayman.
A supplement industry spokesman weighed in on the findings.
"There are many established benefits of selenium, and if you don't get adequate intake, you may be forgoing those benefits. There's a small amount of evidence that too much of anything may have a risk, but there's a U-shaped curve, which means with too little, there are clear risk
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