The basis of all training progress is an optimally functioning immune system, which is able to quickly identify a danger and immediately overcome it. Selenium is an element that is part of glutathione peroxidase - one of the most powerful endogenous antioxidant compounds. In addition, it stimulates the compounds which optimize immune function. It monitors the proper synthesis of thyroid hormones, which play key roles in metabolism. The suitable concentration of selenium protects against hepatic steatosis, aflatoxins and is likely to have a beneficial effect on reducing the renewal of tumors.