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. One study found that men consuming the most dietary selenium developed 65% fewer cases of advanced prostate cancer than did men with low levels of selenium intake.

Selenium is also essential for healthy immune functioning. Even in a nondeficient population of elderly people, selenium supplementation has been found to stimulate the activity of white blood cells. Selenium is also needed to activate thyroid hormones.

In a placebo-controlled study, selenium supplementation indicated a reduction in disease activity in people with autoimmune thyroiditis (thyroid inflammation). In a double-blind trial, selenium supplementation of infertile men improved the motility of sperm cells and increased the chance of conception.