The findings suggest that people should hold off on taking selenium supplements for the sake of their hearts, say researchers.
A number of studies have found that people with higher body stores of selenium might have a lower risk of heart disease. But such studies, known as observational studies, are not enough to prove cause-and-effect.
For that, researchers have to conduct clinical trials where participants are randomly assigned to take a supplement or a placebo and then have their heart health followed over time.
And clinical trials of selenium supplements have yielded mixed results, the new analysis finds. In general, the smaller, lower-quality trials of have suggested a heart benefit, while the larger, better-designed trials have not.
"Overall, the evidence is inconclusive," study co-author Dr. Ana Navas-Acien said in an interview. "We can't say there is (a benefit), and we can't say there isn't."
She and her colleagues at Johns Hopkins School of Public Health in Baltimore based their findings on an analysis of 25 observational studies and six clinical trials.
The observational studies looked at the association between people's blood and toenail levels of selenium and their risk of developing heart disease. The clinical trials tested selenium supplements, sometimes in combination with other vitamins or minerals, for the prevention of heart disease.
Overall, the analysis found a disconnect between the two types of studies.
When the researchers pooled the data from the observational studies, they found that a 50 percent increase in the body's selenium levels was linked to a 24 percent reduction in heart disease risk.
But across the clinical trials, there was no clear evidence of heart disease protection.
It's possible, according to Navas-Acien, that in the observational studies, high selenium stores were a marker of something else that affects heart health. People with high levels might have been of higher socioeconomic status, for example; similarly, certain medical conditions and lifestyle habits -- such as diabetes and smoking -- can affect selenium levels in the body.
Given what is known about selenium -- that the body requires only a small amount, and that too much can be toxic -- Navas-Acien said she would not recommend taking supplements.
Other antioxidants and vitamins, such as vitamin E, beta-carotene and various B vitamins, have looked promising for heart disease prevention, but fallen short when put to the test in clinical trials. Some studies have even found that beta-carotene may increase heart risks, Navas-Acien said.
Selenium has been less extensively studied than these other supplements, she noted, and ongoing clinical trials should bring more-definitive evidence.
"We know from other antioxidant trials that we need to be careful," she said.
SOURCE: American Journal of Clinical Nutrition, November 2006.