Mineral Metabolism.—A study of the organic foodstuffs reveals the fact that many of the mineral salts concerned in nutrition enter the body in organic combination with those constituents. Certain of the mineral salts, however, enter, exist in and leave the body in the same organic form in which they occur in the food materials. This is true of chlorine, which for the most part, functions in and leaves the body in the form of chlorides (chiefly sodium chloride). A small part of the chlorine is used in the production of the hydrochloric acid of the gastric juice.
Sulphur and iron, both enter the body as essential constituents of proteins, and their metabolism occurs with that of these foodstuffs; the sulphur being converted largely into sulphuric acid must be neutralized at once, and it leaves the body by way of the urine as inorganic sulphates. Part of the sulphates are excreted as ethereal (conjugated) sulphates; the amount excreted in this form depending largely upon the extent of purification in the intestinal tract.[62]
Phosphorus.—This mineral salt is found to be present as an essential constituent of certain proteins, fats and carbohydrates. It also enters the body in the form of inorganic phosphates. During the digestion and metabolism of the organic foodstuffs the phosphoric acid radical is split off and eventually nearly all of the phosphorus leaves the body in inorganic form (inorganic phosphates).
Calcium.—Being the chief constituent of the bones, large quantities of calcium salts are stored in the skeleton of the child both before and after birth. The functions of calcium have already been discussed. That part not stored, which has finished its work in the body is excreted through the intestinal wall and leaves the body by way of the feces, only a small part of the calcium concerned in metabolism being excreted in the urine.
The Process of Osmosis in the Body.—The influence exerted upon the process of osmosis in the body is one of the most important parts played by the mineral salts in metabolism. The fact that these chemical substances are indispensable to the metabolic processes makes it necessary for the nurse to know where they can be found in food and how best to use them.
Metabolism of Body Tissues.—The constant breaking down and building up of the tissues of the body and the evolution of heat as a by-product of the energy expended may be summed up in the term “metabolism.” The metabolism of the body is normally supported by the food ingested. However, it is a known fact that were no food eaten the processes would continue just the same, the difference only being the use of the body structure instead of food materials. According to Sherman, the chemical changes and energy transformations are of course inseparable. It has become customary to speak of the metabolism of matter and the metabolism of energy, and to regard the extent of the metabolism of any material substance as measured by the amount of its end products eliminated, and the extent of the energy metabolism as measured by the amount of heat or of heat and external muscular work which the body gives off.
In summarizing the important factors in the utilization of food by the body several distinct points are made evident:
1. The composition of the body, and the composition of food, are practically the same.
2. Food must make good the losses resulting from metabolic processes and physical activity.
3. Utilization of food by the body necessitates: