Foods or food principles (Chapter I, Part I) supply us with means to carry on the work of the body - i.e., the different processes incident to the existence of a living being and the work carried on outside of the body, or "productive work."

It is estimated that about one third of the energy contained in the food we assimilate can be utilized in productive work, the other two thirds are needed internally in carrying on the processes of life. The several food principles serve the body in different ways. Take the two great principles, protein and carbohydrate; study their composition and the reason for this is obvious. All food contains carbon; oxygen taken in through the lungs unites with carbon, a chemical change results, heat and energy are produced, and carbon dioxide, a waste product, is formed. Without a supply of oxygen the carbon could not be burned and the life's process would cease. Carbon, slowly oxidized in the body, gives off just as much heat and supplies just as much force as in case it were burned outside the body.

Of the carbohydrates, fats and oils (often called hydrocarbons) have greater fuel value than starch. The chemical elements are the same in all, but in starch the proportions are six - or a multiple of six - parts of carbon to hydrogen and oxygen in the proportions found in water, viz., two parts hydrogen to one of oxygen. In fats and oils, the relative proportion of the oxygen is much smaller, and the combustion of these calls for a greater quantity of oxygen from the air and, in consequence, a greater amount of heat and muscular power is evolved. The body also changes the carbohydrates of food into fat. This fat and that stored directly from the fat in food are kept in the body as a reserve supply of fuel for use in times of need.

The proteids also contain carbon, oxygen and hydrogen, but they contain as well an element not found in the carbohydrates, nitrogen. Oxygen from the air unites with the nitrogen in the food to form compounds which build and repair bodily tissues. This, then, is the primal office of the proteids, and it cannot be relegated to foods from the group, carbohydrates. The gelatinoids are useful in promoting nutrition and save the waste of the real tissue builders. They do not, however, build tissue. Broth of meat to which nothing is added is not a tissue builder.

Of course, if the best fuel be not provided, the system can utilize proteid to produce heat and energy, but it is not the part of wisdom or economy to call upon the more expensive foods, which contain this element, to do the work that may be quite as satisfactorily done by the cheaper carbohydrates; for health itself depends upon securing the right proportions of these principles.

Not all of the food taken into the system is made use of and assimilated. A part passes away as waste matter. The kidneys are the organs whose office is to eliminate this waste from nitrogenous or proteid foods, and if these be overtaxed disorders of serious nature follow. The evil results of a diet deficient in proteid, during the period of growth, are often even more marked. As a rule, the rich are in poor health because food rich in this principle is ingested too freely, while the children of the middle classes and the poor are warped and stunted mentally and physically for lack of material suitable to proper growth. An adult can get along very well if a meal once in a while does not come up in all respects to the required standard. He has fat stored in tissues which can be called upon in time of need and burned or oxidized in place of food. Not so in the case of a young and growing child. Too often lean and tall, he has little reserve force, and needs the full complement of food at each of his three or four meals per day. When he reaches the age of about twelve or thirteen, the factor of flavor comes in; and too much care cannot be exercised in selecting palatable dishes.

Let us learn to secure flavor in the finished dish, not by adding condiments, spices and the like, but by so cooking our food that the natural flavors are brought out and intensified, not lost. Many fruits and vegetables abound in flavors that are entirely lost if they be subjected to careless cookery. The high flavor of expensive boiled and roasted meats are much relished, and when cheaper meats are supplied, vegetables or fruits can be carefully employed to make up for the absence of the appetizing flavor found in caramelized meats.