What does Selenium do?

Selenium is an essential trace mineral.

Selenium activates an antioxidant enzyme called glutathione peroxidase, which may help protect the body from cancer. Yeast-derived forms of selenium have induced "apoptosis" (programmed cell death) in cancer cells in test tubes and in animals.^{1 2 3} A double-blind trial that included over 1,300 people found those given 200 mcg of yeast-based selenium per day for 4.5 years had a 50% drop in the cancer death rate compared with the placebo group.⁴ In that same study, however, selenium supplementation was associated with a significant increase in the risk of developing one type of skin cancer (squamous cell carcinoma).⁵ Another study found that men consuming the most dietary selenium (assessed indirectly by measuring toenail selenium levels) developed 65% fewer cases of advanced prostate cancer than did men with the lowest levels of selenium intake.⁶

Selenium is also essential for healthy immune functioning. Selenium supplementation has reduced the incidence of viral hepatitis in selenium-deficient populations, presumably by enhancing immune function.⁷ Even in a non-deficient population of elderly people, selenium supplementation has been found to stimulate the activity of white blood cells—primary components of the immune system.⁸ Selenium is also needed to activate thyroid hormones.

In a placebo-controlled study, supplementation with 200 mcg per day of selenium for three months reduced anti-thyroid antibody levels (indicating a reduction in disease activity) in people with autoimmune thyroiditis (inflammation of the thyroid gland).⁹

In a double-blind trial, selenium supplementation of infertile men improved the motility of sperm cells and increased the chance of conception.¹⁰