In the body of animals there are approximately 20 minerals that are essential for maintenance and normal functioning of the body. Lack or insufficient amounts of these minerals result in deficiency symptoms leading to reduced performance. Excess amounts, on the other hand, may also lead to a reduction in performance and toxicity may occur.
Minerals are present in all organs and tissues in the body. The functions of the inorganic minerals are extremely diverse. They range from structural functions in some tissues to a wide variety of regulatory functions in other tissues (NRC, 1980; Underwood, 1981; McDowell, 1992; Underwood and Suttle, 1999). The major minerals calcium, magnesium, sodium, potassium, phosphorus, sulphur, and chlorine are present in the body in relatively larger amounts than the trace minerals, such as iron, copper, zinc, cobalt, molybdenum, manganese, iodine, and selenium.
Absorption and utilization of major minerals may be, apart from passive transport through the gut wall, mediated by hormonal control, that is primarily based on their concentration in the extracellular fluid. Animals are able to maintain a homeostasis in the extracellular and intracellular fluids by means of several regulatory mechanisms. In consequence, large differences in absorption and utilization of minerals can be found, which can depend on a number of factors, including the nutritional status of the animal. As requirements for major minerals should be based on absorbable or utilisable/available minerals rather than on total amount, more attention should be directed to factors that affect mineral absorption. Actually, this holds true for trace minerals, although assessing their absorbability is much more difficult. It is obvious that a sufficient amount of (available) minerals should be supplied, because an insufficient supply impairs efficiency of animal production.
In this introduction, an outline is given on factors that affect the bioavailability of minerals. Furthermore, evaluation methods and terms are discussed that are used to assess and express the bioavailability of minerals in mineral sources and feeds for livestock.
