Selenium is an essential nutrient, which we require in tiny quantities - 70 micrograms for an adult male, 55 for an adult female, less for children. It was discovered to be a toxin causing skin lesions, diarrhoea and death almost twenty-five years before people realised that it is a nutrient; doses only ten times the RDA can have toxic effects, while selenium deficiency is relatively uncommon. When deficiency does occur it can cause Keshan Disease, a reversable condition characterised by an enlarged heart and poor heart function which may be the result of an opportunistic virus; it can also cause arthritic Kashin-Beck disease, and may impair thyroid function and make one more susceptible to cancer.

Selenium forms part of the important antioxidant enzyme glutathione peroxidase, making the element significant in the fight against cancer and other diseases caused or aggravated by free oxygen radicals.

The selenium content of food depends on the soil where it was grown, and cases of both under- and over-exposure to selenium have been blamed largely on the makeup of the soil in the regions they have occurred. We get selenium from various grains and nuts - brazil nuts and walnuts are particularly potent sources - as well as from fish, meat and cheese.