What is selenium?
Selenium is a trace mineral (a nutrient that is essential to humans in tiny amounts). Selenium is found in certain proteins that are active in many body functions, including reproduction and immunity. Food sources of selenium include meat, vegetables, and nuts. The amount of selenium found in the food depends on the selenium content of the soil where the food grows. Selenium is stored in the thyroid gland, liver, pancreas, pituitary gland, and kidneys.
Selenium is found in an enzyme called glutathione peroxidase which acts as an antioxidant. However, in high amounts, selenium may act as a pro-oxidant (a substance that can make oxygen byproducts that may damage cells).
Selenium may play a role in many diseases, including cancer. Animal and population studies have suggested that supplementing the diet with selenium may lower the risk of cancer. Results from the Nutritional Prevention of Cancer Trial (NPC) showed that, although selenium supplements did not affect the risk of skin cancer, they markedly lowered the rates of lung, colorectal, and prostate cancer. However, studies of how selenium levels in the blood affect the risk of developing of prostate cancer have shown mixed results.
The Selenium and Vitamin E Cancer Prevention Trial (SELECT) was begun by the National Institutes of Health in 2001 to study the effects of selenium and/or vitamin E on the development of prostate cancer.
How is selenium administered or consumed?
Selenium may be consumed in the diet or taken in dietary supplements. For adults, the recommended daily allowance for selenium is 55 µg/day.
Have any preclinical (laboratory or animal) studies been conducted using selenium?
Laboratory studies to find out if selenium may be useful in preventing or treating prostate cancer have shown the following:
- Different forms of selenium have been shown to slow the growth and spread of prostate cancer cells.
- Selenium nanoparticles may be less toxic to normal tissues than other selenium compounds.
Studies of selenium in animal models of prostate cancer have shown the following:
- A study in mice looked at the effect of dietary selenium on prostate cancer prevention starting at different ages. Adult mice and younger mice were fed selenium-enriched diets or diets with no selenium for 6 months or 4 weeks and then injected with human prostate cancer cells. Adult mice with selenium in their diets developed fewer tumors than adult mice with diets lacking in selenium. However, in younger mice, dietary selenium had no effect on tumor development.
- Strains of mice which developed prostate cancer that acts like human cancer were treated with 2 forms of selenium, MSeA and methylselenocysteine (MSeC), or water only. In the selenium-treated mice, growth of precancerous lesions was slowed and cancer cell death was increased compared to the water-treated mice. MSeA treatment also increased survival time of the study mice. The mice that received MSeA treatment starting at 10 weeks of age had less aggressive prostate cancer than did mice starting treatment at 16 weeks of age, suggesting early treatment with MSeA may be more effective than later treatment.
Have any population studies or clinical trials (research studies with people) of selenium been conducted?
Population studies and clinical trials have been done to find out if selenium may be useful in preventing or treating prostate cancer.
Population studies look for risk factors and ways to control disease in large groups of people.
Studies of how selenium levels in the blood affect the risk of developing of prostate cancer have shown mixed results. One study tracking subjects for up to 10 years found that men with higher levels of selenium in their blood had a lower risk of prostate cancer. Another study found that prostate cancer patients had lower whole blood selenium levels than did healthy men. However, a 2009 study of prostate cancer patients found that men with higher selenium levels in their blood were at greater risk of being diagnosed with aggressive prostate cancer. These differences may be due to genetic variations among individual patients.
Clinical trials of preventing/ treating prostate cancer
Clinical trials of the effects of selenium on prostate specific antigen (PSA) levels or the development of prostate cancer have shown mixed results, including the following:
- In a study reported in 2013, men at high risk for prostate cancer were given either daily doses of high-selenium yeast (200 µg or 400 µg) or a placebo for up to 5 years. There were no differences in prostate cancer rates or PSA velocity in men who took the selenium supplement compared to those who took the placebo.
- In an earlier study, men with high-grade prostatic intraepithelial neoplasia (HGPIN) were given either a selenium supplement (200 µg/ day) or a placebo for 3 years or until they were diagnosed with prostate cancer. The results suggested that selenium supplements had no effect on prostate cancer risk.
- Sixty men were given either a selenium glycinate supplement (200 µg/ day) or a placebo for 6 weeks. Blood samples were collected at the start and the end of the study. Compared to the placebo group, men who received selenium supplements showed higher activity of two selenium enzymes in their blood and lower levels of PSA at the end of the study.
The Selenium and Vitamin E Cancer Prevention Trial (SELECT)
The Selenium and Vitamin E Cancer Prevention Trial (SELECT) was a large clinical trial begun by the National Institutes of Health in 2001 to study the effects of selenium and/or vitamin E on the development of prostate cancer. Over 35,000 men, aged 50 years and older, were randomly assigned to receive one of the following combinations daily for 7-12 years:
- Vitamin E (alpha-tocopherol acetate, 400 IU / day) and a placebo;
- Selenium (L-selenomethionine, 200 mcg/ day) and a placebo;
- Vitamin E and selenium; or
- Two placebos.
First results of SELECT reported in 2009 found no meaningful difference in the rate of prostate cancer among the 4 groups. In the Vitamin E alone group, there was a slight increase in the rate of prostate cancer and in the selenium alone group, there was a slight increase in the rate of diabetes. Even though these changes were not clearly shown to be due to the supplement, the men in the study were advised to stop taking the study supplements.
Updated results of SELECT in 2011 showed that selenium supplements had no meaningful effect on prostate cancer risk; however, men taking vitamin E alone had a 17% increase in prostate cancer risk compared to men in the placebo group.
In 2014, further results of SELECT showed that selenium supplements in men with low selenium levels at the start of the trial had no effect on prostate cancer risk; however, selenium supplements in men who had high levels of selenium at the start of the trial increased the risk of high-grade prostate cancer.
Several factors may have affected study results, including the dose of vitamin E chosen and the form of selenium used.
Have any side effects or risks been reported from selenium?
Selenium supplements were well tolerated in many clinical trials. In two published trials, there were no differences reported in adverse effects between placebo or treatment groups. However, in the SELECT trial, selenium supplements were linked with a slight increase in the rate of diabetes mellitus.
Is selenium approved by the U.S. Food and Drug Administration (FDA) for use as a cancer treatment in the United States?
The U.S. Food and Drug Administration has not approved the use of selenium supplements for the treatment or prevention of cancer or any other medical condition.
Selenium is available in the United States in food products and dietary supplements. Because dietary supplements are regulated as foods, not as drugs, FDA approval is not required unless specific claims about disease prevention or treatment are made.