Food is the matter that is taken into the body to supply nourishment or to replace tissue-waste. Every physical act consumes a part of the force that has been derived from food. The maintenance of the body-heat consumes another part, and in growing individuals a certain amount is utilized in building up the new tissues.

Food as it is taken into the body differs very much in composition from the material that can be utilized in cell-growth and in replacing the tissue-waste. The function of digestion is so to alter the food that it may be absorbed by the blood, and prepare it for assimilation and utilization by the various tissues. The food of mankind is most varied in nature, differing with the seasons, and with climates, races and countries.

The study of foods is a most complex one, and until recently few scientific investigations along this line had been made. Fortunately, however, experiments are now being carried on the world over, and it is to be hoped that the subject of diet in health and in disease will soon be lifted out of the vale of empiricism where it has so long rested.

Water enters into the composition of every tissue in the body and forms more than 60 per cent, of the entire body weight of a full-grown man. As it is not burned up in the metabolic processes, it does not furnish any energy.

The earthy salts, which form about 6 per cent, of the body weight of an adult man, furnish little if any energy. They are most abundant in the bones and teeth, but they also enter into the composition of other tissues and fluids of the body. The principal salts of the body are calcium phosphate and the various compounds of potassium, sodium, magnesium, and iron. The mineral salts are very necessary to life and health.

Proteins are substances which contain nitrogen, are essential to life, and are regarded as combinations of the various amino-acids. In addition to carbon, hydrogen, oxygen, and nitrogen protein generally contains sulphur and some of them phosphorus, iron, copper, iodine, manganese and zinc.

The proteins are variously classified and two elassfications, based on the solubility, have been suggested, one by the English Society of Physiologists and one by the American Society of Biochemists.

Proteins are essential to life and the body is constantly metabolizing it, whether any is being taken in or not. In ordinary life the body is in protein (or nitrogen) equilibrium and as much protein as is ingested is metabolized. It is difficult to get a positive nitrogen balance, except after prolonged fasting or after recovery from wasting diseases or during the period of body growth.

A negative nitrogen balance is seen in starvation where more is used up than is taken in and in all wasting diseases, such as tuberculosis, in fevers, and hyperthyroidism. In pathological states such as nephritis there may be retention of nitrogen compounds in the body due to the failure of the kidney to excrete them, and if the amount exceeds a certain amount a condition of poisoning and uremia is brought about.

Some food proteins are better suited for human food than others, because when broken up into their elementary parts or amino-acids more of these can be utilized in forming the various body tissues than those derived from other foods. For this reason the proteins of milk, meat, eggs and fish are most valuable, those of rice and potatoes next in value, while those of wheat, maize and beans are distinctly inferior.