Although food has been adapted to the needs of the tissues by-digestion, it must undergo still further chemical modification when it is selected by cells for their repair, is stored in them, or by contact with them is decomposed to generate heat or muscular power.

Food is not consumed in the stomach as coal in a furnace - to produce heat and energy. It must be prepared by digestion; it must be absorbed; it must be modified by the liver, by the thyroid gland or its secretions, and by other glands; it must be selected by such cells as need it; it must be modified chemically and physically each time before it serves its purpose of contributing to growth and strength, to the production of heat and muscular and nervous energy, to the formation of secretions, and to reproduction. Finally, remnants of food that are not utilized may in part have to undergo additional changes to prepare them for elimination from the body by the skin, the lungs, the liver, the intestines, or the kidneys.

As the utilization of food involves such numerous modifications of its form and character before it is finally adapted to the needs of the various tissues of the body, it is evident that very many breaks may take place in the series of changes that it must undergo, destroying its value or even making it harmful. Food must be adapted not only to the needs of the digestive organs, but to the capabilities of other tissues to elaborate it still more, and finally to appropriate it. When the digestive organs are diseased, food must evidently be adapted to the digestive disturbances that have been produced. But even when primary digestion is well performed, some other metabolic failure may call for consideration in the adaptation of a diet. For example, in diabetes food must be adapted to the ability of the liver to retain sugar or of the tissues to utilize it. In this malady diet is not regulated by the ability to digest foods.

In pathologic states of the human body various kinds of food, or for the time, all food, may be harmful. For example, when elimination by the kidneys is greatly lessened or suppressed, nitrogenous food, potassium, extractives, etc., may provoke uremic poisoning. When the stomach fails to secrete sufficient gastric juice and performs its normal churning movements infrequently and imperfectly, food will be digested in it slowly and disintegrated imperfectly; decomposition will almost surely take place and give rise to chemical compounds that are unnatural and noxious to the viscus. Even when all the bodily functions are being normally performed, too great a quantity of food of a certain kind may do harm. In certain conditions, as when the stomach is acutely inflamed or when vomiting is persistent, it may be necessary to abstain from food for a time in order to rest the stomach.

These statements are made to emphasize a fact often lost sight of in the prescription of diet: that it is not sufficient to adapt food to the power of the stomach and the intestines to digest and absorb it; the complex processes of utilization after digestion and of elimination of waste must likewise be kept in mind. Classification

Foods may be variously classified. For instance, they may be divided into organic and inorganic; those belonging to the former, into animal and vegetable groups. Physiologists find the following chemical classification the most convenient:

Water

Proteins

Fats

Carbohydrates

Mineral Salts

It is necessary to trace the passage of each of these groups of foods through the system. The therapeutist, however, cannot administer simple chemicals such as these, but is obliged to give them in mixtures of various composition. For illustration, the mineral salts that the human body needs are partly obtained in spring-water and partly in meat, in vegetables, and in cereals. Although proteins predominate in meat, fats and salts are also found in it. Milk contains water, proteins, fats, carbohydrates, and mineral salts. It is therefore necessary to consider the complex composition of each article of diet. For convenience, however, we may classify the various kinds of foods by the ingredient that predominates in them, as water, protein, fat, or carbohydrate. The therapeutist must study not only the chemical composition of foods, but also their palatability, digestibility, assimilability, and capability of elimination; and give attention likewise to the best methods of preparation and administration. All these qualities and conditions affect the dietetic value of individual articles.