Vitamin E supplements significantly increase the risk of prostate cancer among healthy men - even after they stopped taking them, according to a new study published Tuesday in the Journal of the American Medical Association.

In the study, randomly assigned men took a 400-unit capsule of vitamin E every day for about five years. These men were 17 percent more likely to get prostate cancer, compared to the men taking dummy pills.

Although researchers were not clear on how vitamin E can harm the prostate, experts believe the best sources of vitamins remain foods. In this case, nuts, seeds and vegetable oils are rich in vitamin E.

Researchers said that consumers should be skeptical of health claims in over-the-counter supplements, if they are not supported by strong evidence based on clinical trials. The study found a lack of benefit from dietary supplementation with vitamin E or other agents in preventing common health conditions and cancers or improving overall survival.

"People tend to think of vitamins as innocuous substances, almost like chicken soup - take a little and it can't hurt," said lead author Dr. Eric Klein of the Cleveland Clinic. "If you have normal levels, the vitamin is probably of no benefit, and if you take too much, you can be harmed", he said according to the Associated Press.

The implications of the study "are substantial" given that more than 50 percent of individuals 60 years or older are taking supplements containing vitamin E and 23 percent of them are taking at least 400 IU/d despite a recommended daily dietary allowance of only 22.4 IU for adult men, researchers noted.

The 17 percent of men with increased prostate cancer risk meant that for every 1,000 men who took vitamin E, there were 11 additional cases of prostate cancer, compared with men taking dummy pills.