Selenium supplements could be harmful for people whose levels are already high enough, experts warn today.
Low levels of selenium have been linked to a raised risk of death together with a lowered immune function, and a drop in cognitive skills. High levels on the other hand may help fight viruses and cancer, and are crucial for reproduction.
Professor Margaret Rayman of Surrey University, UK, writes in the Lancet that selenium trial results have been mixed, "which probably emphasises the fact that supplementation will confer benefit only if intake of a nutrient is inadequate".
Whereas "supplementation of people who already have adequate intake might increase their risk of type-2 diabetes".
Professor Rayman investigated this U-shaped link in which additional selenium may benefit people with low levels and those with adequate-to-high levels might be affected adversely.
She writes: "The intake of selenium varies hugely worldwide. Intakes are high in Venezuela, Canada, the USA, and Japan, but lower in Europe. Selenium-containing supplements add to these intakes, especially in the USA where 50 per cent of the population takes dietary supplements."
She points out that selenium supplements have been marketed for a multitude of conditions, but findings from clinical trials are mixed, possibly because supplementation is only beneficial when intake is inadequate.
More trials are needed in populations with low selenium status, she believes, as well as groups with different genetic backgrounds as people vary in how genetically receptive they are to the benefits of selenium supplements.
Selenium and human health. Rayman, M. P. The Lancet February 29 2012 doi:10.1016/S0140-6736(11)61452-9
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