But now, follow-up findings from a study involving 35,000 men offer evidence that the opposite may be true.
Compared to men who did not take vitamin E, those who took 400 international units (IU) of the vitamin every day in the study funded by the National Cancer Institute were 17% more likely to develop prostate cancer over an average of seven years of follow-up.
Researcher Eric A. Klein, MD, of the Cleveland Clinic, says it is unlikely that the association is due to chance.
He adds that taking large doses of vitamin E may be dangerous.
The study, which appears in the Oct. 12 issue of The Journal of the American Medical Association, is not the first to suggest that antioxidant vitamin supplements may promote the cancers that they are intended to prevent.
Earlier studies in smokers found that beta-carotene supplements increased lung cancer risk and the trace mineral selenium has been linked to an increased risk of non-melanoma skin cancer in women and men.
"In the absence of vitamin deficiency, there are no compelling data to indicate that these dietary supplements are beneficial, and some may be harmful," Klein tells WebMD.
Impact of Vitamin E
About 240,000 men in the U.S. will be diagnosed with prostate cancer this year and 33,000 men are projected to die of the disease.
The Selenium and Vitamin E Cancer Prevention Trial (SELECT) began in 2001 to test the theory that taking the antioxidant vitamins reduced the risk of certain cancers.
The 35,533 men in the study -- recruited from the U.S., Puerto Rico, and Canada -- were assigned to one of four treatment groups:
- 200 micrograms of selenium alone daily
- 400 IU of vitamin E
- daily supplementation with both vitamins
- no supplementation (placebo group)
"There has never been a trial of this size and magnitude to examine an intervention to prevent prostate cancer," study co-researcher Laurence H. Baker, DO, of the University of Michigan, tells WebMD.
In the fall of 2008, about 5.5 years after beginning the supplements, the men taking vitamin E and selenium were told to stop taking them when it became clear that the goal of a 25% reduction in prostate cancers would not be met.
At the time, there was a suggestion that men who took vitamin E alone actually had an increased prostate cancer risk but the association did not reach statistical significance.
Researchers continued to follow the men until July of this year, during which time 521 additional prostate cancers were diagnosed.
A total of 147 prostate cancers occurred in the vitamin E-only group, while 113 occurred in the men who took neither supplement.
"The expectation was that we would see less difference in prostate cancer rates among the two groups when we followed the men longer. But the opposite proved to be true," National Cancer Institute acting director for cancer prevention Lori M. Minasian, MD, tells WebMD.
Risks of Excessive Vitamin Use
For every 1,000 men who took vitamin E supplements for seven years, there were 76 prostate cancers in the vitamin E group and 65 cancers in the placebo group.
The 11 additional cancers represented a 17% increase in risk.
American Cancer Society Chief Medical Officer Otis W. Brawley, MD, says the surprising findings from the SELECT trial should serve as a cautionary tale.
"It is now clear that we need to be very cautious in our advocacy of taking excessive amounts of any vitamin," he tells WebMD. "Taking a multivitamin every day may be OK, but we now know that taking excessive amounts of vitamins can be dangerous."