Background And Relevant Pharmacokinetics

Selenium is an essential trace element that enters the food chain through incorporation into plants from the soil. Selenium is mainly present in the form of selenite in acid soils, which is poorly assimilated by crops, whereas for alkaline soils, it is in the form of selenate, which is more soluble and assimilated by crops. When taken in supplement form, animal and human trials demonstrate that bioavailability of organic forms of selenium (Se-methionine and Se-cysteine) is higher than that obtained for inorganic forms (selenite and selenate) (Navarro-Alarcon & Lopez-Martinez 2000).

The variation in selenium content of adult humans living in different parts of the world is testimony to the influence of the natural environment on the selenium content of soils, crops and human tissues. According to a WHO report, adults in New Zealand have approximately 3 mg selenium in their bodies compared with 14 mg in some Americans (WHO 2002).

Selenium is readily absorbed, especially in the duodenum but also in the caecum and colon. Vitamins A, E, and C can modulate selenium absorption, and there is a complex relationship between selenium and vitamin E that has not been entirely Selenium 1079

elucidated for humans (Bates 2005). Selenium enters the body in two major forms:

Se-methionine, which is derived from plants, and Se-cysteine, which is mainly derived from animal selenoproteins (Shils et al 2006). Metabolism is complex and occurs via several routes for the different selenoproteins. Se-methionine enters the methionine pool where it undergoes the same fate as methionine until catabolised. Once the selenium from Se-methionine is liberated by the trans-sulfuration pathway in the liver or kidney, it is able to be used by peripheral cells. Ingested selenite, selenate and selenocysteine are metabolised to selenide. Urinary excretion accounts for 50-60% of total excretion of selenium and homeostasis is achieved through regulation in the kidney. Volatile forms of selenium are exhaled when intake is very high and presents a significant route of excretion at this level.

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