Although the rate of prostate cancer was greater among men taking supplements compared to a placebo, it was statistically significant only among men taking vitamin E alone, with a 17 percent increased risk of the disease, Klein's group found.
Among all the men in the trial, 529 men taking placebo developed prostate cancer as did 620 men taking vitamin E, 575 men taking selenium and 555 men taking both supplements, the researchers noted.
The effect of vitamin E only became clear during the third year of the trial and was seen in men with both low- and high-grade prostate cancer. This finding indicates that the risk of prostate cancer from vitamin E may continue even after the men stop taking it, the study authors added.
"There is not a lot of evidence that, for healthy people who eat a normal diet, taking supplementary vitamins is beneficial," Klein said. "There is no compelling evidence for taking vitamin E at this dose. It doesn't provide any health benefits and SELECT strongly suggests it increases your risk for getting prostate cancer."
Because more than 50 percent of men 60 and older take supplements containing vitamin E and 23 percent take as much as 400 international units (IU) a day despite the recommended daily dietary allowance of only 22.4 IU, the implications of this finding are "substantial," the study authors said.
Speaking for the supplement industry, Duffy MacKay, vice president for scientific and regulatory affairs at the Council for Responsible Nutrition, questioned the importance of the findings.
"Interestingly, when vitamin E was combined with selenium, the risk was reduced to a non-significant statistic, perhaps even the result of chance. This reinforces the theory that vitamins work synergistically and that drug-like trials of nutrients, when used in isolation from other nutrients, may not be the most appropriate way to study them," MacKay said in a council press release.
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