Selenium deficiency leads to lowered glutathione peroxidase activity and it is implicated with a higher risk for cancer of the liver, lungs, breast, prostate, skin, rectum, and colon. It is unclear whether the lowered risk of developing certain cancers from taking about 200 mcg of selenium per day also applies to individuals who previously exhibited normal levels of selenium, or only to those with lower levels before supplementing that amount... Full Text