Selenium supplements may increase the risk of illnesses such as type-2 diabetes, new research suggests.
A clinical review, published online in The Lancet medical journal, suggests that the supplements may be harmful to those who already have enough selenium in their diets.
Small amounts of selenium are essential to health, and some evidence suggests that the mineral can enhance male fertility and protect against some forms of cancer.
Previous studies have suggested that a lack of selenium is linked to poor immune function, mental decline and death.
In the latest review, researchers looked at the effects of selenium supplements in different populations. They found that in some cases, extra selenium created adverse affects.
The team concluded that selenium supplements only benefit people who are lacking selenium in their diet.
Lead researcher Professor Margaret Rayman, from the University of Surrey in Guildford, said: “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% of the population takes dietary supplements.”