Selenium is a natural antioxidant and appears to preserve the elasticity of tissues by delaying oxidation of  polyunsaturated fatty acids. It supports the immune system, protects against cancer, is a factor in fertility, and is  necessary for the production of prostaglandin, a substance that affects blood pressure.
Selenium content of foods depends on the extent of its presence in soil whether directly as plant food or indirectly as animal products when selenium levels are derived from feed. Sulfur content in commercial fertilizers inhibits plant absorption of the mineral. Refining, processing, and cooking of foods reduce selenium levels.
High doses are toxic and no more than 300 mcg a day are recommended.