Selenium, a trace element naturally-occurring in several organic and inorganic forms, is ubiquitous in the environment. Its main source of exposure in the human is diet. Selenium has nutritional and toxicological properties, and it has an extremely narrow safe range of exposure, though it boundaries are still uncertain and controversial. Both selenium deficiency and overexposure have been associated with adverse health effects, based on laboratory observations and epidemiologic studies with experimental and nonexperimental design. However, recent reviews and dose-response meta-analysis of epidemiologic studies, particular large randomized trials carried out in the US, have shown that no beneficial effect of selenium on cancer risk exists, contrary to previous expectations.
