Men taking daily vitamin E were more likely to get prostate cancer than those not taking the dietary supplement, according to a study of close to 35,000 North Americans.

This means that over a decade, one or two men out of 100 taking vitamin E would be expected to get prostate cancer, said the researchers, whose findings were published in the Journal of the American Medical Association.

"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," said Eric Klein from the Cleveland Clinic, one of the study's authors, to Reuters Health.

The findings come on the heels of a study suggesting that older women who take multivitamins have slightly increased death rates compared to those who don't.  

"There's a theme here that taking vitamins is not only not helpful but could be harmful" in people who are not deficient in vitamins, Klein added.

But one researcher who wasn't part of the new study said he doubted it means vitamin E causes prostate cancer.

"It's an interesting finding. I'm not sure I believe it," said Neil Fleshner, from the University of Toronto, adding that the result may have been a chance finding or false positive.

For the study, men in the United States, Canada and Puerto Rico were randomly assigned to one of four groups. Starting between 2001 and 2004, about 9,000 men in each group 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 the men taking vitamin E. But they kept monitoring men for cancer after they stopped taking the supplements. It turned out that the extra risk became clearer over time.  

By mid-2011, about seven percent of men who had taken vitamin E only had gotten prostate cancer, compared to six percent of those assigned to placebo pills.  

The researchers did not find an extra risk of prostate cancer in men who took only selenium or vitamin E together with selenium.

Klein and his colleagues said 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 added that 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.  

Fleshner said that either way, vitamin E doesn't seem to be beneficial for prostate health.

"There's certainly no major evidence that vitamin E helps, so why bother?" he said.

While vitamin supplements are known to prevent disease in people who have vitamin deficiencies, so far studies 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 said.