Biochemical and physiological indicators of selenium status in animals.

Selenium (Se) concentrations in animal tissues vary with the tissue and with the amount and chemical form of Se in the diet. In cattle, sheep and swine, Se concentrations rank in kidney greater than liver greater than heart greater than skeletal muscle greater than adipose tissue. Selenium concentrations (wet basis) in skeletal muscle of swine (.03 to .52 ppm) reflect natural dietary Se concentrations ranging from .03 to .49 ppm. Inorganic Se additions to diets low in natural Se (.05 ppm) increase skeletal muscle Se concentrations until dietary Se levels are adequate. After a period of Se repletion, skeletal muscle Se concentrations should be at least .08 ppm on a wet basis. Selenium concentrations in plasma, serum or whole blood are also related to inorganic Se intake and rise in direct relation to each other in the deficient to adequate range. Plasma or serum Se concentrations of .08 to .12 ppm are consistent with dietary adequacy. Selenium-dependent glutathione peroxidase (GSH-Px) activity of plasma or whole blood may also be used to assess Se status in some animals since plasma or whole blood Se concentrations are positively correlated with GSH-Px activity in animals that are low to adequate in dietary inorganic Se. However, inter-laboratory variation in GSH-Px values is large, and it is doubtful that limits of normalcy developed in one laboratory are applicable in others. In certain tissues it is important to distinguish between GSH-Px and glutathione (GSH) S-transferases, which can reduce organic hydroperoxides but which are not Se-dependent. It is also important that the instability of GSH-Px be considered so that losses in activity during handling and storage may be minimized. Urinary Se excretion and Se retention as percentages of Se intake may be helpful in assessing Se status when facilities for metabolism studies are available.