New evidence about the effect of vitamin E on prostate cancer risk may make some men think twice before they pop a daily supplement. Researchers at the National Cancer Institute found that men who took a high daily dose of vitamin E had a 17 percent greater risk of developing prostate cancer.
The report, published today in the Journal of the American Medical Association, adds to a growing list of studies suggesting that supplemental vitamins have few benefits and could even be harmful.
"Many people are taking supplements because they view it as a health insurance," said Dr. Lori Minasian, one of the study's authors. "This would be an indication that it's probably not just neutral, but there is some level of harm."
Data from previous studies suggested that taking vitamin E might help protect men against prostate cancer, so Minasian and her colleagues at the NCI decided to take a closer look at the connection between the two. Beginning in 2001, they started the Select trial, studying more than 35,000 men age 55 and older in the U.S., Canada and Puerto Rico. They divided the men into four groups, each with different kinds of daily diet supplements: vitamin E, selenium, both vitamin E and selenium, or a placebo pill.
"We expected that the numbers of prostate cancer would be smaller in men taking vitamin E, but we found larger numbers instead," Minasian said.
After seven years, the researchers detected more prostate cancer in men taking one or both of the supplements, but the men taking vitamin E showed the most significant increase in their rates of prostate cancer.
And the numbers of prostate cancer diagnoses kept climbing after the men stopped taking the vitamins. Of the 8,737 men taking vitamin E, 620 of them got prostate cancer, compared to 529 of the 8,896 men taking a placebo – a 17-percent increased risk for men taking the supplement.
The authors of the study noted the findings were particularly concerning, considering how many people take supplemental vitamin E every day. Fifth percent of people over age 60 reported taking daily supplements containing vitamin E, and 23 percent of them took supplements with more than 13 times the recommended amount of vitamin E.
"We're very careful about how we take medications. We need to be equally careful about how we take supplements," Minasian said.
Some experts say that the findings from the Select trial are interesting, but the results don't necessarily mean there is an association between vitamin E and prostate cancer. Two previous studies looked at a large number of men taking the supplement and each reached different conclusions on how vitamin E affects prostate cancer.
In 2003, data from the Alpha-Tocopherol, Beta –Carotene Cancer Prevention ) trial showed that smokers taking 50 mg of vitamin E each day had a 35 percent reduction in prostate cancer. In the Physicians Health Study II, participants took the same amount of vitamin E as the men in the Select trial – 400 IU, but it had no effect on their risk of prostate cancer.
Dr. Tim Byers, the associate director for cancer prevention and control at the University of Colorado Cancer Center, said scientists should weigh the findings of all of these studies when thinking about the link between prostate cancer and vitamin E.