They can't -- at least not in the formulations and dosages used in the study, called SELECT (SELenium and vitamin ECancer prevention Trial). The safety panel for the 35,000-man study called for a halt when an early look at the data showed no benefit for the treatment.
There were slightly more prostate cancers in men taking vitamin E alone, and slightly more diabetes in men taking only selenium. But neither finding was statistically significant, meaning they were likely due to chance.
"The data to date suggest, but do not prove, that vitamin E may slightly increase the chance of getting prostate cancer, and that selenium may increase the chance of getting diabetes mellitus," warns a letter sent to study participants by the Southwest Oncology Group, which ran the NCI-funded study.
Study participants were told to stop taking the two pills they'd been taking every day since the trial opened in 2001. The men received either vitamin E (400 milligrams) and selenium (200 micrograms), vitamin E and placebo, selenium and placebo, or placebos alone.
The men can now ask to be told which treatment they received. And because the study was designed to look at more than just prostate cancer (it received an additional $16.5 million from various NIH agencies), study participants will continue to receive regular checkups.
"SELECT was always designed as a study that would answer more than a single question about prostate cancer," Cleveland Clinic researcher and study co-chair Eric Klein, MD, says in a news release. "As we continue to monitor the health of these 35,000 men, this information may help us understand why two nutrients that showed strong initial evidence to be able to prevent prostate cancer did not do so."
That evidence included a 1998 Finnish study of whether vitamin E could prevent lung cancer in some 30,000 smokers. It didn't, but men taking vitamin E had 32% fewer prostate cancers.
And a 1996 study failed to show that selenium could prevent skin cancer -- but found 60% fewer new cases of prostate cancer in men who took selenium for over six years.
The SELECT findings dash hopes raised by these earlier studies, says Edward M. Messing, MD, professor and chairman of urology and deputy director of the Cancer Center at the University of Rochester, N.Y. Messing serves as a SELECT study investigator.
"I am afraid it will be the end of the story for large trials of vitamin E and selenium to prevent prostate cancer," Messing tells WebMD. "For vitamin E, that is unfortunate. Probably if given in a more effective form, it would be a protective or even therapeutic agent."
Men enrolled in the SELECT study may call the NCI's Cancer Information Service at 800-4-CANCER.