Selenium and Vitamin E Cancer Prevention

by National Cancer Institute

Key Points

SELECT stands for the Selenium and Vitamin E Cancer Prevention Trial, a clinical trial to see if one or both of these dietary supplements prevent prostate cancer.

Selenium and vitamin E are antioxidants. Earlier studies suggested that these supplements might prevent prostate cancer.

In order to participate in SELECT, men had to meet certain eligibility requirements.

Men who participate in this study were randomly assigned to take two capsules a day - selenium and vitamin E, selenium and a placebo, vitamin E and a placebo, or two placebos.

Participants are asked to provide toenail and blood samples, respond to an annual questionnaire, and pick up capsules every 6 months.

1. What is SELECT?

SELECT stands for the Selenium and Vitamin E Cancer Prevention Trial, a clinical trial to see if one or both of these dietary supplements prevent prostate cancer. The trial is funded by the National Cancer Institute (NCI) and is being coordinated by the Southwest Oncology Group (SWOG), an international network of research institutions that receives NCI funding. Enrollment began in 2001 and ended in 2004, 2 years ahead of the originally planned 5 years. The study will continue for 7 years after the last man enrolled, meaning that each man will participate for 7 years or more, depending on when he joined the study. More than 400 sites in the United States, Puerto Rico, and Canada are taking part in the study. Over 35,000 men are participating in SELECT.

2. What are a man's chances of developing prostate cancer?

Except for skin cancer, prostate cancer is the most common type of cancer in men in the United States. In 2005, there will be an estimated 232,090 new cases of prostate cancer and 30,350 deaths from this disease in the United States.

Approximately 1 in 6 men in the United States (or 17.8 percent) will develop prostate cancer during his lifetime. All men are at risk, but those at highest risk fall into one or more of the following categories: age 55 or older; black; or have a father or brother with prostate cancer.

3. What is selenium? Why study it for prostate cancer prevention?

Our bodies need selenium, a nonmetallic trace element that we get from food - especially plant foods like rice and wheat, seafood, meat, and Brazil nuts. Selenium is an antioxidant that might help control cell damage that can lead to cancer.

The Nutritional Prevention of Cancer Trial, published in 1996, included 1,312 men and women who had skin cancer. Men who took selenium to prevent nonmelanoma skin cancer received no benefit from selenium in preventing skin cancer. However, men who had taken selenium for 6½ years had approximately 60 percent fewer new cases of prostate cancer than men who took the placebo. In 2002, study data showed that men who took selenium for more than 7½ years had about 52 percent fewer new cases of prostate cancer than men who took the placebo. This study is one of the reasons selenium is being studied in SELECT. (See Question 11 for more information about selenium.)

4. What is vitamin E? Why study it for prostate cancer prevention?

We get vitamin E in a wide range of foods, especially vegetables, vegetable oils, nuts, and egg yolks. Vitamin E, like selenium, is an antioxidant, which might help control cell damage that can lead to cancer.

In a 1998 study of 29,133 male smokers in Finland, men who took vitamin E to prevent lung cancer had 32 percent fewer new cases of prostate cancer than men who took the placebo. Some men also took beta carotene, but neither substance helped prevent lung cancer. (See Question 12 for more information about vitamin E.)

5. What do researchers hope to learn from SELECT?

SELECT is the first study to look directly at the effects of selenium and vitamin E on the risk of prostate cancer. This study is designed to find out if selenium, vitamin E, or both prevent prostate cancer.

A large trial of selenium and vitamin E is needed to substantiate earlier, separate findings from studies in which prostate cancer was not the primary outcome. Although two earlier studies suggested that these two supplements might prevent prostate cancer, this cancer was not the focus of those studies. The primary goal of SELECT is to assess the effect of these substances on the number of new cases of prostate cancer diagnosed during routine clinical practice. Other objectives are to assess the impact of selenium and vitamin E on the incidence of lung and colon cancer, as well as on survival. SELECT will study the molecular genetics of cancer risk and associations between diet and cancer. Additionally, SELECT will examine quality of life.

6. Who was eligible to participate in SELECT? Were there restrictions on eligibility?

Many diseases, including prostate cancer, occur more frequently in older persons. The risk of developing prostate cancer increases with age. More than 92 percent of prostate cancer cases occur in men age 55 and older.

Black men had to be age 50 or older to participate, and men of other races and ethnicities had to be 55 or older. The age for eligibility was lower for black men because, on average, they get the disease at an earlier age. In addition, black men in the United States have the highest incidence of prostate cancer in the world. Thus, the SELECT age requirement ensured that men at risk for the disease were the ones who could enroll.

Men who had taken, or were taking, vitamin E and/or selenium on their own had to stop using these supplements and any multivitamins and use only what is provided by SELECT. These supplements are provided to participants free of charge, including a multivitamin that does not contain vitamin E or selenium. Past use of these supplements did not disqualify men from joining SELECT.

Participants must have had no history of prostate cancer. Men who joined SELECT also must not have had any other cancer, except nonmelanoma skin cancer, in the last 5 years. They must have been in generally good health. Many of the 18,000 men who participated in another prevention study sponsored by NCI and run by SWOG, known as the Prostate Cancer Prevention Trial (PCPT), were eligible to participate in SELECT if they had no evidence of prostate cancer.

Some SELECT study sites are located at Veterans Administration hospitals run by the Department of Veterans Affairs. Veterans could enroll at these sites even if they did not receive their routine medical care at that hospital.

7. Could men with benign prostatic hyperplasia (BPH) join SELECT?

Men with BPH, an abnormal growth of benign prostate cells, could join SELECT because BPH is not a cancerous or precancerous condition. In BPH, the prostate grows larger and presses against the urethra and bladder, interfering with the normal flow of urine. More than half of the men in the United States between the ages of 60 and 70, and as many as 90 percent of men between the ages of 70 and 90, have symptoms of BPH.

BPH can be treated with four different FDA-approved drugs: finasteride (Proscar®), terazosin (Hytrin®), doxazosin (Cardura®), and tamsulosin (Flomax®). Most men taking one of these drugs were eligible to participate in SELECT.

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