(HealthDay News) – While selenium is necessary for good health, levels that are too high can be harmful, and people whose serum selenium levels are already at least 122µ/L should not take supplements, according to a review published online Feb. 29 in The Lancet.

Margaret P. Rayman, DPhil, from the University of Surrey in Guilford, UK, reviewed the literature to investigate the worldwide intake of selenium and its status in relation to health.

The author found that selenium intake varied worldwide, ranging from deficient to toxic concentrations. Dietary intake ranged from 7µg–4,990µg per day, and selenium supplements added to these intakes. Low plasma selenium levels were associated with a higher risk of death, poor immune function, and cognitive decline, while high selenium levels had antiviral effects, were essential for male and female reproduction, and reduced the risk of autoimmune thyroid disease. High selenium status was also associated with some reduction in the risk of prostate, lung, colorectal, and bladder cancers, but findings from trials were inconclusive. Selenium supplementation may have increased the risk of type 2 diabetes in people whose intake was already adequate. Supplements were not recommended for individuals whose serum or plasma selenium concentration was 122µg/L or higher.

“The crucial factor that needs to be emphasized is the inextricable U-shaped link with selenium status: additional selenium intake (e.g., from food fortification or supplements) may well benefit people with low status,” Rayman concludes. “However, people of adequate or high status could be affected adversely and should not take selenium supplements.”

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