The analogy between the animal body and a machine has been pointed out so often that it is almost a platitude, nevertheless it is particularly apt. Like a machine, the body must be provided with fuel, which it converts into heat and mechanical energy. Like a machine, the efficiency of the body depends largely upon the character and quantity of the fuel supplied. If the fuel is scanty there is loss of power; if it is piled on recklessly the system becomes choked with unburned material and waste products. As with machines, there is a maximum efficiency for each individual which can be secured only by careful and intelligent management, but too often, for want of management, the body, like the machine, is running below par.

For the nourishment of an animal there are six different classes of materials required, which are frequently spoken of as foodstuffs. These are fats, carbohydrates, proteins, water, mineral salts, and vitamins; the first three making up what are known as the organic foodstuffs. The great variety of edible materials are usually mixtures of these, generally containing all six in varying proportions. Each foodstuff has its special function in nutrition, and although the relative amounts of each can be varied very considerably without ill-effect, a well-balanced diet must contain all six.

Water, while not a food in the sense of being a source of nutriment in itself, is nevertheless most essential to the organism, as is proved by the fact that most starving animals die much more quickly if deprived of water than when water is obtainable. Water is required to dissolve the solid foods so that they can be absorbed by the tissues and carried from place to place by the blood, and also to dissolve and carry off waste products from the tissues. Since water is continually being eliminated from the body in the urine and the perspiration it is necessary to replace the loss. Many of our foods, especially fruits and vegetables, contain a large proportion of water, but in the ordinary diet additional amounts must be obtained by drinking water. Dr. Hawk and his collaborators at Jefferson Medical College have conducted a long series of experiments from which it would appear that copious water-drinking to the extent of a quart or more with each meal has many beneficial features. In one subject such procedure resulted in a gain of two pounds in five days.

The minerals of the diet are necessary for the building and repair of the bony structure of the body, and also serve in less obvious ways to maintain the proper concentration of the body fluids and the tone of the cells. Moreover, certain mineral elements enter into the composition of the body substance, as iron, which forms a part of the haemoglobin of the blood, and others have specific roles, like calcium which plays an important part in regulating the beating of the heart, and sodium which exercises a profound influence on the irritability of the muscles. Fruits, vegetables, and milk are all excellent sources of mineral matter in the diet.

The organic foodstuffs, fats, carbohydrates, and proteins, are all . complex chemical compounds which must be broken down by the process of digestion before they can be utilized by the system.

Fats are broken down in the intestine into fatty acids and glycerine, which are 'absorbed through the intestinal wall and apparently re-combined into the particular body fat characteristic of the species of animal utilizing it. This fat can be stored up in the tissues until needed by the organism as a source of energy.

Carbohydrates include sugar, starch, and allied compounds. They also must be broken down by digestion into simple compounds before absorption can take place, and these digestion products may be used at once as a source of energy, or built up into a new form of carbohydrate for storage, or converted into fat and stored in the tissues in that form.

Proteins differ from the other foodstuffs in containing nitrogen in the only form in which this element can be utilized by the animal body. The most diverse substances are protein in nature. The muscle tissues of all animals, white of egg, and the curd of milk are the principal animal proteins used as food. All vegetables and fruits contain more or less protein, but peas, beans, and nuts are the chief vegetable sources. Inasmuch as nitrogen is an essential part of animal tissues and the animal can obtain the necessary nitrogen only in the form of protein it is obvious that this class of foodstuffs is of peculiar importance. The relative proportions of fat and carbohydrate in the diet can be varied almost at will, since either one can be utilized as a source of energy or converted into reserve fat, but there is a minimum amount of protein necessary for every organism without which the necessary nitrogen for tissue building or repair will be lacking. If excess of protein above the requirement is eaten a small portion may be converted into fat, but most of it is decomposed and excreted as waste.

Proteins are combinations of smaller units known as amino-acids. Mankind requires a certain amount of specific amino acids for proper nourishment. The various proteins have been found to possess different biological values according to their power to supply these constructive food elements essential to the maintenance of the body in proper nitrogenous equilibrium. There are eighteen principal amino acids, and a few more of lesser importance. The principal amino acids make a formidable list. These are glycine, alanine, valine, leucine, isoleucine, phenylalanine, tyrosine, serine, cystine, aspartic acid, glutamic acid, ornithine, arginine, lysine, histidine, proline, oxyproline, tryptophane.

Amino acids are combined in different proportions to form the enormous variety of proteins, somewhat as the twenty-six letters of the alphabet are combined into an almost unlimited number of words. The specific character of the individual proteins is due to the relative proportions in which the amino acids are present. In the course of digestion the proteins are decomposed into amino acids, are absorbed in that form and are carried by the blood to the various tissues, which select what they need for building and repair. The residue is then carried to the liver, decomposed there, and the waste excreted.

Careful observations have shown that the nutritive value of a protein depends on its content of amino acids, and since this has become established, enormous strides have been made in the science of nutrition, for it is possible to find out by experiment which proteins provide the best combination of amino acids for a given species of animals and how to supplement one protein which is poor in a certain amino acid by another in which it is present in excess. (See Chap. III).

The energy of the body is supplied by the slow burning (oxidation) of the food. If the food supply is insufficient, the tissues will be used up instead. This energy appears partly as muscular force, and partly as heat. The more work done by the muscles, the more energy is required and the more food must be burned. It has been found that the energy set free when food is burned inside the body is exactly the same as that set free when it is burned outside the body, and consequently the energy-value of a food can be measured by burning it in a special form of apparatus, the calorimeter. The unit of energy used in nutrition is the calorie, and is the amount of heat which would be required to heat one pound of water about 4° F., or the equivalent of this. The calorific value of a given amount of a particular food is the number of units of energy which it will set free when it is burned.

It is also possible to measure the energy used up in performing any kind of muscular work and to express this in calories. The relation between the energy output of the body and the available energy from the food eaten can therefore be readily calculated. Clearly, if an animal performs a piece of work which requires one hundred calories of energy it will require to consume an amount of food which will supply one hundred calories or else it must use up a portion of its own tissues to make good the deficit. Even when no voluntary work is being done and the body is resting as completely as possible, energy is being used in the beating of the heart, the movements of the stomach and intestines, the rise and fall of the chest in respirations, and so on. This irreducible minimum of energy is called the basal metabolism of the individual and varies somewhat with the body weight, surface, age and sex. A vast number of measurements have been made on human beings of various ages and averages for different types have been ascertained, which may safely be taken as close enough to the actual basal metabolism of any given individual.

By keeping account of the time spent in various activities in the course of twenty-four hours, calculating the energy required for each, and adding the values thus obtained to the basal metabolism, a measure of the daily energy requirement of the individual can be found. From this observation the amount of food required to supply that energy can be determined. A slight excess over and above this demand will be required by adults to provide for the repair of the tissue, and a larger allowance is needful for the growth of the young.

Fig 1. - This chart shows that butter-fat contains something essential for normal growth. The curves indicate that after feeding a diet of purified foodstuffs, the fat being lard, the animals after growing normally for several weeks suddenly began to lose weight. When a part of the lard was replaced by butter-fat (shown by the beaded line), they immediately recovered. These animals would have died in .a few days if this change had not been made.

Courtesy of the Connecticut Agricultural Experiment Station (Bulletin 215).