The combustible parts of the food are built up together with oxygen into the living protoplasm of the cell to form a highly unstable molecule with large potential energy. In the breaking down of this molecule there is a rearrangement of atoms to form more stable compounds, the carbon and oxygen combining into carbon dioxide with the evolution of energy; which may be displayed either as heat or work. The earliest scientific observations concerning nutrition were founded upon the commonly noted fact that in spite of large quantities of food eaten, a normal man did not vary greatly in size from year to year. The weight added by the ingestion of food and drink is lost in the urine and faeces and in the excretions of the lungs and skin. The expenditure of the body must balance its income; it must lose as much nitrogen as it takes in, otherwise it would put on flesh; it must lose as much carbon as it takes in, otherwise it would put on fat. Again, the body may be losing or gaining fat, giving off more or less carbon than it takes in, while its "flesh" - its protein constituents - remain constant in amount, the expenditure of nitrogen being exactly equal to the income. In both cases we say that the body is in nitrogenous equilibrium. A starving animal or a fever patient, on the other hand, is living upon capital - the former entirely, the latter in part; the expenditure of nitrogen is greater than the income. A growing child is living below its income, and is increasing its capital of flesh. In neither case is nitrogenous equilibrium present. Where fats or carbohydrates are given in addition to proteins, nitrogenous equilibrium is attained with a smaller quantity of the latter. Fats and carbohydrates therefore economise proteins to a certain extent, and are spoken of as protein-sparers.
The greater part of the living structure of the body is composed of proteins, or of more complex nitrogenous bodies in the building-up of which the proteins play a preponderating part. In the young and growing animal these tissues are constantly being added, and the raw material for growth can be supplied by proteins, and by proteins only. Moreover, in the period of adult life when the body is neither gaining nor losing weight, every vital act is associated with a certain degree of what we may term "wear and tear " of the living structure. Most of the cells are continually dying and being replaced by fresh ones. For this nutritional metabolism a supply of proteins in the food is an absolute necessity. Every diet therefore must contain a certain minimum amount of protein to supply the nutritional needs of the organism, while the energy requirements can be supplied at the expense of either proteins, fats, or carbohydrates. Feeding an animal with excess of protein only leads to increased excretion of urea. If, for instance, a man were taking 10 grammes of nitrogen in the form of protein in the day, with a sufficiency of fats and carbohydrates to maintain his weight constant, he would probably excrete also 10 grammes (9 grammes in the urine, 1 gramme in the faeces), and would therefore be in a state of nitrogenous equilibrium. On doubling the protein intake, the nitro-genous excretions would rise in proportion, and the man would remain in a state of nitrogenous equilibrium, however much his protein intake were increased. The increased protein diet would, however, raise his energy income above his daily requirements. A certain amount of the fat and carbohydrates would therefore escape oxidation, and would be stored up in the form of fat, and the man would therefore increase in weight. In these circumstances there has also to be considered the extent of the wear and tear of the tissues involved in eliminating the excess of proteins.
Gelatine is a special type of protein, with interesting and valuable properties. When used alone it has little nutritive power, but in proper combination with other foods it is a most useful aliment. It has been shown that by the addition of gelatine very large quantities of albumin can be spared in the body or devoted to increase of bulk, just as by the supply of fats and carbohydrates.
The fats absorbed as food may subserve several purposes. Through its oxidation it is a source of heat energy; it may be stored in the tissues as part of the body fat; it may be synthetised with other substances to form more complex constituents of the body, such as lecithin; and in view of the fact that fats economise proteins to a certain extent, they play an important part as protein-sparers. The final fate of fat in the economy is its transformation through oxidation into carbon dioxide and water. The exclusion of fat from the dietary leads to the development of cold extremities, notably the fingers, feet, and ears; also a liability to chilblains.
The carbohydrates of the food may be stored up as glycogen or be converted into fat. As in the case of fat, the final end-products resulting from its oxidation are carbon dioxide and water. In the metabolism of carbohydrates the internal secretion of the pancreas plays an important part. The function of carbohydrates in the economy are -
(a) A source of energy for muscular work.
It has long been taught that carbohydrates, like fats, are of no value as tissue-builders. Some doubt has of late been thrown on the correctness of this view by recent researches, which go to show that there is a carbohydrate radicle in the protein molecule. These researches seem to point to the necessity of additional importance being attached to the carbohydrates of the food as tissue-builders, as well as sources of energy and heat production. When carbohydrates are given in excess, actual or relative, their metabolism is incomplete, and sugar appears in the urine.