Selenium has a variety of functions. The main one is its role as an antioxidant in the enzyme selenium-glutathione-peroxidase. Selenium also seems to stimulate anti-body formation in response to vaccines. It also may provide protection from the toxic effects of heavy metals and other substances. It may assist in the synthesis of protein, in growth and development, and in fertility, especially in men. Selenium has been shown to improve the production of sperm and sperm motility.