Taking extra selenium supplements offers no protection against heart disease - at least among people who already get enough of the mineral in their diets, according to a new analysis of past research.
In the review of 12 studies that included close to 20,000 people, there was no difference in the number of strokes and heart attacks, heart disease-related deaths, or deaths from any cause among participants randomly assigned to take selenium or nothing.
"We need to, in a sense, erase some of the misconception around the indiscriminate and widespread use of supplements containing selenium," said Dr. Saverio Stranges, from the University of Warwick Medical School in Coventry, UK, who worked on the review.
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Still, he added, "We do need evidence from clinical trials from less well-nourished populations where dietary intakes of selenium are lower."
Selenium is found in meat, bread, and some nuts. Plant foods also contain varying amounts of selenium, depending on how much is naturally present in the soil where they were grown. In supplement form, it costs about $2 for a month's supply.
The Institute of Medicine recommends most U.S. adults consume 55 micrograms of selenium per day.
Past nutrition surveys suggest some 99 percent of Americans have plenty of the mineral in their blood.
Most of the data in the new analysis came from men in the U.S., with smaller trials from Europe,
Australia, and
China also included.
Participants were randomly assigned to take a daily selenium supplement - anywhere from 100 to 800 micrograms, depending on the study - or a placebo pill or nothing.
Those trials were initially designed to look at whether selenium affects the risk of cancer or other conditions, but also included data on how many study volunteers went on to have heart disease. Studies lasted anywhere from a few months to five to eight years.
On every heart-related measure - as well as cholesterol and blood pressure readings - there was no effect of supplementation, according to the review published in the Cochrane Library.
However, the studies did show a slightly higher risk of hair loss and skin reactions among people taking selenium.
Nutrition scientist José Ordovás, who has studied selenium at Tufts University in Boston, said it's also possible selenium may increase the risk of diabetes or high blood pressure at large doses.
Like other vitamins and minerals, he told Reuters Health, it may be best to get a moderate amount of selenium through diet - not too little, not too much.
Selenium is incorporated into so-called selenoproteins, a type of antioxidant. For that reason, researchers have believed selenium supplements might help prevent a range of chronic diseases, including cardiovascular disease and cancer.
It's possible supplements may still help people in countries where the soil is low in selenium, or those who are sick or elderly and nutrient-deficient, Stranges told Reuters Health.
SPECIAL: These 4 Things Happen Right Before a Heart Attack — Read More.
But at least in the U.S., the majority of people seem to get enough selenium naturally to optimize any antioxidant effects of selenoproteins.
"It's difficult to justify additional benefits of boosting selenium levels with supplements, in terms of antioxidant actions and therefore prevention of chronic disease," he said.
Ordovás, has collaborated with Stranges but wasn't involved in the new review, agreed.
"For people that are well-nourished and don't have any known disease, I don't see any reason to recommend supplementation with selenium, at least for the prevention of cardiovascular disease," he said.