Aspects of mineral metabolism
Although mineral elements constitute only a small proportion (4%) of the body tissue, they are essential as structural components and in many 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 preservative 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. 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.
Mineral Content of Foods
In unrefined foods, minerals are present in various forms mixed or combined with proteins, fats, and carbohydrates. Processed or refined foods, such as fats, oils, sugar, and cornstarch, contain almost no minerals. The total mineral content of a food is determined by burning the organic or combustible part of a known amount of a food and weighing the resulting ash. The ash then is analyzed for individual mineral elements. Most foods have been analyzed for ten or more mineral elements, but in dietary practice the figures most commonly used are those for calcium, phosphorus, and iron and, for therapeutic purposes, sodium, potassium and magnesium.
Minerals such as iodide, fluoride, copper, and other trace elements that are essential for life may be found abundantly in drinking water in certain areas or in foods grown in the soil of those areas, whereas in other parts of the country the same minerals are deficient in both soil a.nd wa.ter . Still ather mineral elements) such as sodium) potassium, chlorine, and sulfur-all necessary in human nutrition-are so universally present in foods that there is no need to worry about deficiencies.
Questions about the relative availability of mineral elements for physiologic processes continue to stimulate new investigations in this field. Fifty or more years ago, the opinion prevailed that the organic forms of minerals found in plant and animal foods were utilized better than the inorganic forms. However, modern research has disproved this theory. Today we are aware that many minerals occur in inorganic forms in natural foods and are absorbed from the digestive tract.
