We easily realize that muscular force or energy cannot be maintained without nutriment in proper quality and amount. An underfed or starving man has not the strength of a well-fed person. He cannot lift the same weight, cannot walk as far, cannot work as hard. We do not as easily comprehend the nervous organism, and generally have less sympathy with worn-out or ill-nourished nerves than muscles, but the sensibilities and the intellectual faculties, of which the nerves and brain are but the instruments, depend upon the right nutrition of the whole system for their proper and healthful exercise.

So many factors enter into the make-up of a thought that it cannot be said that any particular kind of food will ultimately produce a poem; but of this we may be sure, that the best work, the noblest thoughts, the most original ideas, will not come from a dyspeptic, underfed, or in any way ill-nourished individual.

The classification of foods has been usually based upon the deductions of Prout that milk contains all the necessary nutrients in the best form and proportions, viz., the nitrogenous matters, fat, sugar, water, and salts; the latter being combinations of magnesium, calcium, potassium, sodium, and iron, with chlorin, phosphoric acid, and, in smaller quantities, sulphuric acid.

These different classes seem to serve different purposes in the body, and are all necessary for perfect nutrition. Some of them closely resemble each other in composition, but are quite different in their physiological properties, and in the ends which they serve. For instance, starch (C6H10O5) has almost the same chemical formula as sugar (C12 H12 O11), and yet the one cannot replace the other to its entire exclusion. The Protein Compounds. In general it may be said that the carbohydrates are changed into fats, and are used for the production of force, and that the fats are stored in the body as fat and used as fuel. The protein compounds do all that can be done by the fats and carbohydrates, and in addition something more; that is, they form the basis of blood, muscle, sinew, skin, and bone. They are, therefore, the most important of all the food compounds. The terms "power-givers" and "energy-formers" are sometimes applied to them, because wherever power and energy are developed they are present, though not by any means the only substances involved in the evolution of energy. Probably the fats and carbohydrates give most of the material for heat and the various other forces of the body. In case of emergency, where these are deficient, the proteins are used; but protein alone forms the basis of muscle, tendons, skin, and other tissues. This the fats and carbohydrates cannot do (Atwater). The different tissues are known from analysis to contain this complex nitrogenous compound, protein. Now, since the body cannot construct this substance out of the simpler chemical compounds which come to it, it becomes perfectly evident that the diet must have a due proportion of protein in order to maintain the strength of the body. We get most of our proteins from the flesh of animals, and they in turn get it from plants, which construct it from the crude materials of earth and air.

The Extractives, usually classed with the protein compounds, such as meat extract, beef tea, etc., are not generally regarded as direct nutrients, but, like tea and coffee, are valuable as accessory foods, lending savor to other foods and aiding their digestion by pleasantly exciting the flow of the digestive fluids. They also act as brain and nerve stimulants, and perhaps also in some slight degree as nutrients.

The principal proteins or nitrogenous substances are albumen in various forms, casein both animal and vegetable, blood fibrin, muscle fibrin, and gelatin. All except the last are very much alike, and probably can replace one another in nutrition.

Modern chemists agree that nitrogen is a necessary element in the various chemical and physiological actions which take place in the body to produce heat, muscular energy, and the other powers. Every structure in the body in which any form of energy is manifested is nitrogenous. The nerves, muscles, glands, and the floating cells1 in the various liquids are nitrogenous. That nitrogen is necessary to the different processes of the system, is shown by the fact that if it be cut off, these processes languish. This may not occur immediately, for the body always has a store of nitrogen laid by for emergencies which will be consumed first, but it will occur as soon as these have been consumed. The energy of the body is measured by its consumption of oxygen. Motion and heat may be owing to the oxidation of fat, or of starch, or of nitrogenous substances; but whatever the source, the direction is given by the nitrogenous structure - in other words, nitrogen is necessary to all energy generated in the body.

Protein matter nourishes the organic framework, takes part in the generation of energy, and may be converted into non-nitrogenous substances.1 The necessity of the protein compounds is emphasized when we realize that about one half of the body is composed of muscle, one fifth of which is protein, and the nitrogen in this protein can be furnished only by protein, since neither fats nor carbohydrates contain it. It is therefore evident that the protein-containing foods, such as beef, mutton, fish, eggs, milk, and others, are our most valued nutrients. Our daily diet must contain a due proportion.

1 Hemoglobin, the red coloring matter of the blood, contains albumen.

The proteins are all complex chemical compounds, which in nutrition become reduced to simple forms, and are then built up again into flesh. The animal foods are in the main the best of the protein compounds, for they are rich in nitrogenous matter, are easily digested, and from their composition and adaptability are most valuable in maintaining the life of the body.

A diet of lean meat alone serves to build up tissue. If nothing else be taken, the stored-up fat of the body will be consumed, and the person will become thin.2 Athletes while in training take advantage of this fact, and are allowed to eat only such food as shall furnish the greatest amount of strength and muscular energy with a minimum of fat. The lean of beef and mutton, with a certain amount of bread, constitute the foundation of the diet.