Although mineral elements constitute only a small proportion (40/0) of the body tissue, they are essential as structural components and in man.)' vital processes. Some form hard tissues, such as bones and teeth; some are in the fluids and soft tissues. There are functions in which the balance of mineral ions is important-for example, for bone formation, the amount and the ratio of calcium and phosphorus, and for normal muscular activity, the ratio between potassium and calcium in the extracellular fluid. Electrolytes, of which sonium and potassium salts are the most important, are the major factors in the osmotic control of water metabolism as discussed in the preceding chapter. Other minerals may act as catalysts in enzyme systems or as integral parts of organic compounds in the body, such as iron in hemoglobin, iodine in thyroxine, cobalt in vitamin B12, and sulfur in thiamin and biotin.
Plant life and animals, as well as bacteria and other one-celled organisms, all require proper concentrations of certain minerals to make life possible. In fact, changes in concentration of minerals, small in themselves, can be fatal to various forms of life. Common salt, which in dilute solution is necessary for most forms of animal life, becomes a pre::;erva tive when foods are salted or kept in brine because the salt concentration kills bacteria. On the other hand, marine forrns (fish and shellfish) quickly die when subjected to fresh water. In the human body, also, the maintenance ofa normal concentration of minerals in body fluids is essential.
The mineral dements that the body requires are frequently classed as either macro- or micronutrients, depending on the amount of each that is needed in the diet. Calcium, phosphorus, potassium, sulfur, chlorine, sodium, and magnesium are considered macronutrients. Iron, iodine, fluorine, zinc, copper, chromium, selenium, cobalt, manganese, molybdenum, vanadium, tin, silicon, and nickel are often called micronutrients or trace elements. A comparison of the relative amounts of some of these minerals in the body is shown in Table 6-1. Cadmium, lead, mercury, arsenic, boron, lithium, aluminum, and other minerals may also be present in animal tissue as environmental contaminants, but at this time they have no known essential nutritional role.
