US researchers warned Tuesday of an alarming link between vitamin E supplements and a 17 percent increased risk of prostate cancer, describing the findings as an "important public health concern."
Ten years after the start of a randomized trial of more than 35,000 men, researchers discovered the spike in prostate cancer among those assigned to take vitamin E rather than selenium or a placebo.
"Dietary supplementation with vitamin E significantly increased the risk of prostate cancer among healthy men," said the study in the Journal of the American Medical Association.
Selenium, a trace mineral found in foods like Brazil nuts, tuna and beef, is often deficient in areas such as China and Russia where it is lacking in the soil.
The study was launched based on previous research that had suggested that selenium or vitamin E might reduce the risk of developing prostate cancer.
The latest data emerged three years after a preliminary study of the findings, published in 2008, showed a slightly higher but statistically insignificant risk of prostate cancer among those taking vitamin E.
However, since the risk was approaching statistical significance, a safety committee called for a halt to the randomized Selenium and Vitamin E Cancer Prevention Trial (SELECT) in the United States, Canada and Puerto Rico in 2008.
A longer-term follow up, concluded in July of this year, has revealed the higher cancer incidence in men assigned to the vitamin E portion of the trial.
"Based on these results and the results of large cardiovascular studies using vitamin E, there is no reason for men in the general population to take the dose of vitamin E used in SELECT as the supplements have shown no benefit and some very real risks," said Eric Klein, a study co-chair for SELECT, and a physician at the Cleveland Clinic.
"For now, men who were part of SELECT should continue to see their primary care physician or urologist and bring these results to their attention for further consideration."
The study began in 2001 and broke participants into four groups: one would receive selenium, another would get 400 international units of daily vitamin E, another group would take both, and the fourth was prescribed a placebo.
A total of 620 men in the vitamin E group developed prostate cancer, as did 555 in the combined selenium and vitamin E group.
Those taking selenium only saw 575 develop prostate cancer, compared to 529 on the sugar pill.
"The observed 17 percent increase in prostate cancer incidence demonstrates the potential for seemingly innocuous yet biologically active substances such as vitamins to cause harm," said the study.
Men entering the trial had no signs of prostate cancer and were considered to exhibit average risk of developing the disease, which is the second most common cancer among US men, after skin cancer.
According to the American Cancer Society, 240,890 new cases of prostate cancer will be diagnosed in 2011 and 33,720 men will die of prostate cancer in the United States.
The study found no biological explanation for why vitamin E was driving the risk higher, but warned that the effects of the pills may continue even after the patient stops taking them.
"The fact that the increased risk of prostate cancer in the vitamin E group of participants in SELECT was only apparent after extended follow-up... suggests that health effects from these agents may continue even after the intervention is stopped," it said.
The findings also "underscore the need for consumers to be skeptical of health claims for unregulated over-the-counter products in the absence of strong evidence of benefit demonstrated in clinical trials," it said.
The trial was funded by the National Cancer Institute, the National Center for Complementary and Alternative Medicine, the National Heart, Lung, and Blood Institute, the National Institute of Aging and the National Eye Institute.