The preceding pages have been devoted to a discussion of the origin of human foods: what they are in substance, how their nutrients are made available, and how used. So far no attempt has been made to summarize into a systematic statement what may be called the fundamental principles or laws of nutrition, some of which we have not yet directly formulated, but which are inferences from the facts presented. It is desirable to do this, however, before passing to the consideration of the practical side of human nutrition.

176. Food Source Of All Energy And Building Material

All energy and building material applied to the maintenance and growth of the human body come from the food, water and oxygen being included in this term. The human organism originates neither matter nor force.

177. Only Digested Food Available

Only that portion of the food which is digested, i.e., that which is dissolved by the digestive fluids and rendered soluble and diffusible so that it passes into the blood, is available for any use whatever. This fact is especially important in view of the greatly varying digestibility of different foods.

178. Avenues Of Excretion

The undigested food and the wastes from the digested food pass from the body in some direction. The undigested part appears in the solid excrement or feces. The urea and other nitrogenous compounds which are the unoxidized portion of the digested protein, pass out wholly in the urine. All digested nitrogen not stored is found here. The carbon dioxid is eliminated through the skin and lungs, chiefly the latter, and water is disposed of through the kidneys, skin, and lungs.

179. Uses Of Digested Food

The digested food is used in two general directions: (a) for the production of energy, and (6) for constructive purposes, (a) The food energy is made available through combustion, i.e., the oxidation of the carbon compounds of the food to simpler substances, carbon dioxid and water, thus liberating the energy stored in the plant during its growth. Protein is never fully oxidized, but carbohydrates and fats may be. All the organic nutrients may be oxidized to produce energy, the available heat values of protein, carbohydrates, and fats being approximately as 1, 1, 2.25. This liberated energy finds expression in the animal organism in various ways, as heat, mechanical energy or motion, and chemical transformations. The total energy of food is never all available for use because of a loss in the excreta and gases. Moreover, the net energy gain seems not to be proportional to the available energy, but is dependent upon the work of digestion, which varies somewhat with different foods.

(6) The food compounds are used for constructive purposes, either without changing their general character, as, for instance, the building of muscular tissue from the plant proteins, or they may be reorganized into bodies of a very different character, as in the formation of animal fats from starch and sugar. The proteins are used to construct muscular tissue; in fact, all the nitrogenous parts of the human body, and they are perhaps a source of fat. Carbohydrates can only be used constructively for the formation of fat, and the same is true of food fats or oils. Mineral matter is needed for the formation of bone, is distributed through the soft tissues, and has important functions in digestion.