Selenium

Selenium is an antioxidant trace element and a component of the glutathione peroxidase enzymes, which neutralise damaging peroxides and shield cells from oxidative stress. It works in close partnership with [vitamin E](/glossary/vitamin-e), each able to partly cover a moderate shortfall of the other, so the two are usually considered together. Selenium also contributes to thyroid metabolism, since it forms part of the deiodinase enzymes that convert thyroxine into its active form, and it supports immune function. A defining feature of selenium is its narrow safety margin: the gap between the amount needed and the amount that becomes toxic is relatively small, which demands careful formulation. As a concrete benchmark, a dietary requirement of about 0.15 mg per kg of dry matter has been reported for growing kittens (NRC, 2006). Deficiency can cause muscular disorders and weakened immunity, often made worse by a concurrent vitamin E shortfall, while chronic excess, called selenosis, is toxic. Dietary selenium comes from meat, fish and organ tissues, with content reflecting in part the selenium in the soil where the source was raised. In a complete food it is supplied either in inorganic form or as organic selenomethionine, the latter regarded as better absorbed. On a label it appears among the trace elements. See the [Petipedia glossary](/glossary).

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