Comparing responses of animals fed a particular mineral in purified or practical diets with those fed a reference mineral is a routine method for determining mineral bioavailability. As an important economic trait, growth performance is often used to measure mineral bioavailability. Young chicks have been frequently chosen, as larger animals are somewhat less desirable for this purpose. Milk production and reproduction can also be bioavailability indicators. Bone ash and bone breaking strength are considered two of the most reliable response criteria for estimating bioavailability of calcium and phosphorus. Tibia in chicken and metacarpals and metatarsals in swine are the commonly sampled bones for these assays. Accumulation of the mineral elements such as copper and iron in target organs has also been found to be well-related to their bioavailability. Tissue uptake of the element can be measured following dietary supplementation of physiological doses for a long period or supplementation of high (pharmacological) levels for a short period. Both methods may provide similar tissue concentrations of the test elements. The pharmacological type of assay can be performed with high dietary mineral levels formulated with natural ingredients and relatively short period of time. The physiological type of assay may require semipurified diets.
Many minerals are essential components of catalytic enzymes or hormones. Examples include iron in hemoglobin and selenium in glutathione peroxidases. Thus, effects of minerals on the gene, protein, and activity expressions of those factors are good bioavailability measures for these minerals.
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