A measure postulated to prevent prostate cancer may do the opposite, says a study published on Tuesday, October 11, 2011 in the Oct. 12 issue of JAMA. Vitamin E supplements, rather than reducing the risk of prostate cancer, have been found to increase the risk of developing the disease. The findings are a 3-year follow-up to the Selenium and Vitamin E Cancer Prevention Trial, or the SELECT trial.
The trial had aimed to show that taking vitamin E, selenium, or both could prevent prostate cancer. The SELECT trial was stopped after a pre-planned interim analysis 3 years ago when an independent review of the data showed no reduction in the risk of prostate cancer with either selenium or vitamin E supplements—and surprisingly, a statistically nonsignificant increase in the risk of prostate cancer in those taking vitamin E.
The trial was initiated based on animal studies, epidemiological evidence, and secondary findings. However, after a median of 5.5 years of follow-up, the original trial results showed an increased risk of prostate cancer for men taking vitamin E supplements. There were 473 men in the vitamin E group diagnosed with prostate cancer, 432 in the selenium group, 437 in the combination group, and 416 in the placebo group. The difference between the vitamin E and placebo groups had approached statistical significance (P = .06) and the trial was stopped.
A total of 35,533 men from the United States, Canada, and Puerto Rico had entered the study, and approximately half have been followed for the last 4 years to allow for observation of additional prostate cancer events. The data released this week found a 17% increase in prostate cancer for those who took vitamin E compared to placebo. For every 1,000 men in the study, 76 who took vitamin E supplements vs 65 who took placebo developed prostate cancer. The elevated risk was consistent for both low and high-grade disease.No biological explanation was put forth by the authors to explain why vitamin E may increase the incidence of prostate cancer.
A total of 147 prostate cancers occurred in the vitamin E–only group, 113 occurred in the men who took neither supplement, 143 occurred in the selenium group, and 118 occurred in the combination group. The current follow-up discounted an initial signal seen in the study data 4 years ago that showed a statistically nonsignificant increase in the incidence of type 2 diabetes mellitus. No increase in risk was seen after another 4 years of follow-up.
There was a statistically significant interaction between selenium and vitamin E (P = .02): no increase in cancer risk was seen when vitamin E and selenium were taken together. The study authors suggest that selenium may dampen the increased risk of prostate cancer associated with vitamin E and thus has a protective effect in this context. Because of these results, the authors suggest that caution be taken when designing factorial prevention trials, since interactions may make it difficult to assess the effect of each individual component on the outcome.
The researchers believe that supplements should be taken with caution, especially because the health effect of taking a supplement may persist even after discontinuation, as occurred in this study. Health claims about supplements and other over-the-counter products not backed by sound clinical data should be viewed with caution and skepticism, they conclude.
With a 16% chance of developing prostate cancer, a high rate of incontinence, impotence, and other effects from treatment, as well as high treatment costs, preventive measures would be very well received by clinicians and patients. For now, they will have to wait.