Selenium is an essential mineral needed for proper health. It's found widely in plant foods, although the amount of selenium in food can vary depending on the selenium content of soil where it is grown. It's also found in seafood and poultry.
Past studies have suggested a link between high selenium blood levels and a decreased risk of cancer, including prostate cancer.
To investigate, researchers analyzed 55 studies on the link between selenium and different types of cancer.
Most of the studies were observational studies, where scientists measured how much selenium people ate every day or how much they had in their blood or toenails, then tracked who got cancer over the next few years.
The remaining 6 studies were done through more rigorous trials, in which researchers randomly divided participants into one group that took selenium supplements for a month or more, and another that took a drug-free placebo pill or nothing, then followed them for cancer.
The observational studies suggested that talking selenium might be linked to a slightly lower risk of cancer, more so in men than women.
But in the randomized trials, people assigned to take selenium at doses at least four times higher than the daily recommendation were not less likely to get cancer, including prostate or skin cancer, than those not taking selenium.
And some of those trials raised the question of whether high doses of selenium might be dangerous, such as by increasing the risk of diabetes.
Researchers say more research is needed to determine the range of daily selenium that is beneficial to human health, as both too much and too little appear to have health risks.
The study was published in The Cochrane Library.
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