Selenium is a mineral that is essential for humans, and is also present in soil and rocks.
While selenium recommendations from the United States and World Health Organisation vary between 30 and 55 micrograms per day for adults, the authors note, companies that sell the supplements claim that higher doses have a range of health benefits, including cancer prevention.
However, "we still do not have an exact picture of what selenium is doing to human health", Dr Marco Vinceti, one of the authors of the new review from the University of Modena and Reggio Emilia in Italy, told Reuters Health.
To try to get a better focus on that picture for cancer in particular, Vinceti and his colleagues analysed 55 studies on the link between selenium and different types of cancer.
Most of those were so-called "observational studies". 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. These kinds of studies are thought to be better at accounting for outside factors that may affect cancer risk.
The observational studies suggested that talking selenium may 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 - prostate cancer and skin cancer, in particular - 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.
The review is published in the Cochrane Library, a publication of the Cochrane Collaboration, an international organisation that evaluates medical research.
"If you put it all together, it's not a good story for patients," Dr Neil Fleshner, a researcher at University Health Network in Toronto, who has studied the effect of selenium on prostate cancer risk, told Reuters Health.
No good evidence of benefits
He said that it's still possible that selenium may help prevent cancer over a very long time period, or that certain kinds of selenium might work while others don't. But right now "there's no good evidence at all that selenium is beneficial", said Fleshner, who was not involved in the review.
James Marshall, of the Roswell Park Cancer Institute in Buffalo, said that the excitement about selenium began with evidence that animals or people with very low selenium in their diet are at an increased risk for cancer.
While those individuals might benefit from a selenium supplement, that's where the mineral's anti-cancer effects appear to end.
"Once you get an animal or a person up to a place where selenium intake is adequate, then, this is where the evidence has proven disappointing," Marshall, who was not linked to the new research, told Reuters Health.
"Putting them on more selenium is not going to do them any good, and it may in fact harm them," Marshall said.
Vinceti said that while few people in Europe take selenium as a supplement, it's common in the United States.
Selenium supplements are about $2 for a month's supply.
Vinceti added that 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. In that regard, "we are not at the end of the story", he said.
But when it comes to preventing cancer, at least for now, the evidence is not there.
"There are many ways of preventing cancer, but putting more selenium... on your breakfast cereal, is probably not one of them," Marshall concluded. - (Genevra Pittman/Reuters Health, May 2011)
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