Selenium is a trace mineral found in soil, water and certain foods. Selenium promotes antioxidant activity in the body via glutathione peroxidase (GPX), a selenium dependent enzyme. As an antioxidant, selenium helps guard cells against free radical damage within the body and as such supports healthy vision, lungs and other organs that may be subject to oxidative stress. Selenium also contributes to healthy functioning of both the immune system and the thyroid gland. In addition, selenium may promote breast health in women and prostate health in men.