Men taking daily vitamin E were more likely to get prostate cancer than those not taking the dietary supplement in a new study of close to 35,000 North Americans.
Over a decade, an additional one or two men out of 100 taking vitamin E would be expected to get prostate cancer, researchers found.
"If you have enough of these vitamins in your system... extra doesn't help you any, and too much of something like this can be harmful," Dr. Eric Klein from the Cleveland Clinic, one of the study's authors, told Reuters Health.
The findings, released today in the Journal of the American Medical Association, come on the heels of a study suggesting older women who take multivitamins have slightly increased death rates than those who don't.
"There's a theme here that taking vitamins is not only not helpful but could be harmful" in people who aren't deficient, Klein said.
Still, one researcher who wasn't part of the new study said he doubts it means vitamin E causes prostate cancer.
According to the National Cancer Institute, about 241,000 men in the US will be diagnosed with prostate cancer in 2011, and close to 34,000 will die from the disease.
Although men are typically screened regularly for prostate cancer with prostate-specific antigen (PSA) tests, a government-funded task force recently put out draft recommendations saying that the screening doesn't prevent men from dying of cancer, but may cause undue harm through unnecessary procedures.
For the current study, men in the US, Canada and Puerto Rico were randomly assigned to one of four groups. Starting between 2001 and 2004, about 9,000 men each took daily supplements of 400 international units (IU) of vitamin E, 200 micrograms of selenium, vitamin E and selenium together or a vitamin-free placebo pill.
The study was halted in late 2008 when the researchers saw a hint of an increased risk of prostate cancer in men taking vitamin E, but they kept monitoring men for cancer after they stopped taking the supplements. And it turned out that extra risk became clearer over time.
By mid-2011, about seven per cent of men who had taken vitamin E only had gotten prostate cancer, compared to six per cent of those assigned to the placebo pills.
The researchers didn't find an extra risk of prostate cancer in men who took only selenium or vitamin E together with selenium.
Klein and his colleagues say it's not clear how vitamin E would increase the risk of prostate cancer, and that not all past studies have shown it does any harm to the prostate. Some have even found a lower prostate-cancer risk with vitamin E.
He said the new findings aren't definite proof that vitamin E causes extra prostate cancers, but that there wasn't anything else that could explain why men taking the vitamin were more likely to be diagnosed with cancer -- for example, they weren't screened more frequently.
The supplement doses, he added, are much higher than what's in most over-the-counter multivitamins, which typically contain 15 to 25 IU of vitamin E.
Prostate cancer researcher Dr. Neil Fleshner, from the University of Toronto, was doubtful that vitamin E does in fact increase the risk of prostate cancer, and said the result may have been a chance finding, or a "false positive."
"It's an interesting finding. I'm not sure I believe it," he told Reuters Health. Either way, he said, vitamin E doesn't seem to be beneficial for prostate health.
"There's certainly no major evidence that vitamin E helps," he said. "So, why bother?"
Vitamin supplements are known to prevent disease in people who have vitamin deficiencies, Klein said, but so far studies generally haven't found much extra benefit in people who already get enough vitamins through their diet. Specifically, vitamin E has not been shown to protect against heart disease, colon cancer or lung cancer.
On the other hand, there is a growing body of evidence suggesting supplements may be harmful in high doses.
"Vitamins are not innocuous substances," Klein concluded.