Vitamin E and Selenium Do Not Prevent Prostate Cancer; Use of the Supplements Stopped in Large-Scale Study

Nick Mulcahy

October 29, 2008

October 29, 2008 — The SELECT (Selenium and Vitamin E Cancer Prevention Trial) prostate cancer prevention study is instructing its 35,000-plus participants to stop taking the 2 nutritional supplements because of an apparent lack of benefit and a possibility of harm.

"The Data and Safety Monitoring Committee made the decision to stop use of the supplements, not to stop the trial. We will follow participants for 3 more years to determine if there is any benefit or any harm," said Larry Baker, MD, chairman of the Southwest Oncology Group, which coordinated the trial, and professor of medicine at University of Michigan Medical School in Ann Arbor.

The Data and Safety Monitoring Committee said that "the data could not exclude a small chance that the study supplements might have effects later in the men's lives." However, the antioxidants selenium and vitamin E, taken alone or together for an average of 5 years, did not prevent prostate cancer, according to the committee.

The antioxidants selenium and vitamin E, taken alone or together for an average of 5 years, did not prevent prostate cancer.

"We went back to the biologists, and they said that 8 months was sufficient exposure to see benefit," Dr. Baker explained.

The data from SELECT also show 2 trends that were of concern but not statistically significant: in men taking only vitamin E, there were slightly more cases of prostate cancer; and, in men taking only selenium, there were slightly more cases of diabetes. Neither of these findings means an increased risk from the supplements, and they could be due to chance, according to the National Cancer Institute (NCI), which funded the trial.

The Data and Safety Monitoring Committee also determined that it was unlikely that selenium and vitamin E supplementation would ever produce a 25% reduction in prostate cancer, which was the study's goal.

Dr. Baker said that the results to date speak for themselves, and that SELECT is a much larger trial than previous trials that suggested benefit. "This is the definitive study and anyone who argues that is ignoring the facts."

SELECT Details

Participants were randomized to 1 of 4 groups (2 capsules a day): selenium (200 μg) and vitamin E (400 mg); selenium and placebo; vitamin E and placebo; or 2 placebos. The 35,000-plus participants consisted of African-American men who were 50 years or older, and men of other races and ethnicities who were 55 years or older. The eligibility age was lower for African Americans because of the earlier average onset of the disease.

Selenium is a nonmetallic trace element found especially in plant foods, such as rice, wheat, seafood, meat, and Brazil nuts. Selenium is an antioxidant that may help control the cell damage that can lead to cancer, according to the NCI.

Vitamin E is found in a wide range of foods, especially vegetables, vegetable oils, nuts, and egg yolks. Like selenium, vitamin E is an antioxidant.

According to the NCI, selenium was selected for study in part because of results from the Nutritional Prevention of Cancer Trial, which was a skin cancer trial of 1312 men and women that incidentally revealed prostate cancer data; the results showed that men taking selenium for more than 7.5 years had about 52% fewer new cases of prostate cancer than men taking placebo.

Vitamin E was selected for the trial in part because of results from a 1998 study of 29,133 male smokers in Finland; men in the study who took vitamin E to prevent lung cancer had 32% fewer new cases of prostate cancer and 40% fewer deaths from prostate cancer than men taking placebo.

Other Objectives and Related Research

SELECT is not limited to an analysis of the impact of selenium and vitamin Eon prostate cancer. The other objectives of SELECT include assessments of the impact of the 2 supplements on the incidence of lung cancer and colon cancer, and on total cancer incidence and survival. SELECT will also provide a basis for studying the molecular genetics of cancer risk and associations between diet and cancer, according to the NCI. A biorepository of blood samples obtained from SELECT participants at their entry into the trial and at 5 years was created for use in molecular and mechanistic studies of cancers in men and other diseases of male aging. Additionally, SELECT has been examining the impact of selenium and vitamin E supplementation on quality of life.

According to the NCI, the Institute is the primary funding agency for SELECT, awarding about $114 million to Southwest Oncology Group from 1999 to 2008. An additional $4.5 million was contributed by the National Center for Complementary and Alternative Medicine. NCI has also funded a substudy of SELECT to learn whether the supplements affect colon polyp growth.

Additionally, 3 ancillary studies related to SELECT are ongoing and are evaluating selenium and vitamin E for different conditions in the SELECT participant population. The Prevention of Alzheimer's Disease with Vitamin E and Selenium (PREADVISE) trial, cosponsored by the National Institute on Aging, is evaluating whether these supplements can help prevent memory loss and dementia, such as that found in Alzheimer's disease. The SELECT Eye Endpoints Study (SEE), cosponsored by the National Eye Institute, is evaluating the dietary supplements in the prevention of age-related macular degeneration and cataract. The Respiratory Ancillary Study (RAS), cosponsored by the National Heart, Lung and Blood Institute, is evaluating whether the supplements can affect the loss of lung function experienced with aging, which is higher in people who smoke cigarettes.

More than 400 sites in the United States, Puerto Rico, and Canada are involved in the SELECT study.

The researchers have disclosed no relevant financial relationships.

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