Selenium, named in 1818 by Berzelius after the Greek moon-goddess Selene, tends to be forgotten among the riot of elements in our environment in favour of iron, copper, calcium, manganese and the rest. Nevertheless, the humble selenium is essential to human health as an element in the amino acid selenocysteine. Indeed, as Margaret Rayman explains in an extensive review in the Lancet for July 15, the trace element is "an essential nutrient of fundamental importance to human biology."
Selenoproteins carry out important enzymic functions in the sphere of redox reactions. Selenium enters the food chain through plants which take it up from the soil, and areas of the world where acidity and complexing elements reduce its uptake suffer serious diseases among their livestock. Geological differences also affect human dietary intake of selenium, and bring about deficiency diseases, often accompanied by loss of immunocompetence.
Selenium is normally present in immune tissues such as liver, spleen and lymph nodes. Where there is a deficiency, and even where there is not, selenium supplementation induces marked immunostimulation, including activated
T-cell proliferation. Not surprisingly, selenium deficiency is linked to the occurrence, virulence and progression of some viral infections. This effect might explain the emergence of new strains of influenza virus in selenium-deficient belts of China, and the progress of human immunodeficiency virus throughout the world. Selenium also protects against progressive changes in people infected with hepatitis viruses.
In animal husbandry, selenium has long been accepted as essential for successful breeding. It is required for testosterone biosynthesis and the formation and development of spermatozoa. There are indications that it is important for brain function through its antioxidant activity. Low selenium status may be associated with depression, anxiety, confusion and hostility. Selenium supplementation may improve mood. Increased oxidative stress or inflammation, in such conditions as cardiovascular disease, pancreatitis and asthma, may be reduced by selenium, and there is evidence of an inverse relation between selenium intake and cancer mortality. This last effect may be due to the ability to enhance the immune response, or that of inducing active antitumorigenic metabolises.
The desirable intake of selenium in the United Kingdom has been determined as 75mcg daily for men and 60mcg daily for women. In practice, its intake has been only about half that, and has declined over the past 25 years. Bread and cereals make a substantial contribution, but meat, poultry and fish provide more. Selenium is taken in the form of selenomethionine in plant and animal foods and dietary supplements, selenocysteine mainly in animal foods, and selenate and selenite in supplements.
Care must be taken not to encourage overconsumption of selenium supplements. A daily consumption of 15mcg per kg body weight seems to have no prolonged effect upon human health, but the element is toxic and in some sensitive individuals the maximum safe intake may be as low as 600mcg per day. It is recommended that adult intake be limited to 400-450mcg daily.