Selenium is a trace element needed in very small quantities in the body, which can get the required amount through a varied and balanced diet and a healthy lifestyle. It is essential for the synthesis of certain proteins called selenoproteins (which therefore contain selenium). Some of these are key components of the antioxidant system, which protects the body from free radicals. These are responsible for adverse oxidant effects of endogenous (produced by normal biological reactions in cells) or exogenous (generated by the external environment, such as pollution or UV rays) origin. Selenium plays a major role in combating oxidative stress; it is also involved in a number of biological reactions including immune responses, thyroid metabolism and muscle function. Vitamins B9 and B12 are cofactors for its activity, i.e. they are necessary for the normal metabolism and optimal activity of selenium.