Selenium supplements for healthy hair and nails 'can increase risk of type-2 diabetes'
Risk: Selenium has been found to harm people who already have enough of the mineral in their diets
Selenium supplements may be harmful to people who already have enough of the mineral in their diet, a study has found.
Possible effects of having too much selenium include an increased risk of type-2 diabetes.
Selenium, a trace mineral found in soil and foods, is essential in small amounts. Low selenium levels have been linked to an increased risk of death, poor immune function and mental decline.
There is also evidence that selenium may enhance male fertility and protect against some cancers.
But a new review of clinical trials in different populations has shown mixed results from selenium supplements.
In some cases, additional selenium appeared to have adverse effects.
This indicates that the supplements only benefit people with too little selenium in their diet, according to Professor Margaret Rayman, from the University of Surrey in Guildford.
She 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.'
The supplements should not be taken by people whose blood levels of selenium are already 122 micrograms per litre or higher, she argued.
The average level in American men was 134 micrograms.
Study: Professor Margaret Rayman said the supplement was not of risk to anyone who already had low levels
Prof Rayman added: 'However, there are various health benefits, and no extra risk, for people of lower selenium status (blood level less than 122 micrograms per litre) who could benefit from raising their status to 130-150 micrograms per litre, a level associated with low mortality.'
The review, published in an early online edition of The Lancet medical journal, also suggests that genetic background may be important. People could be more or less genetically receptive to the benefits of selenium, said Prof Rayman.
The Health Food Manufacturers' Association pointed out that average selenium intake for British men was just 54 micrograms and for women 43 micrograms, significantly below recommended levels.
Executive director Graham Keen said: 'This shows that for many in the UK there is a strong need to increase selenium in their diet, particularly with this study also reporting that low selenium status is linked with an increased risk of death, poor immune function and cognitive decline.
'Vitamins and minerals are essential for good health and well-being. In an ideal world our diet would provide us with all the vitamins and minerals that our body needs for good health. But evidence shows that a significant proportion of the UK population simply doesn't achieve nutritional sufficiency through diet alone.'