_Nothing is lost in Nature’s distribution of force and energy. Everything accomplished in life, either in the physical handling of material, the brain work in planning the constructions, the mental movements of thought in art, literature, or science, are all representatives of the heat and energy released from the body, and it is the effort of every man and woman to make the body yield as large an income as possible in the expression of this energy. In order that it may do so, it must be used with intelligence, just as any other great machine must be used intelligently; it must be fed, exercised, and rested judiciously._ [Sidenote: Repair and Elimination of Waste (Metabolism)] Every part of the body is constantly changing. Its work never stops. If kept in thorough repair it must be torn down and rebuilt incessantly. These chemical changes are called collectively _metabolism_. They are divided into two groups: the chemical process of building up complex substances from simple ones is known as _anabolism_; the chemical process of oxidizing and breaking down the complex substances into simple ones, so that they are in a state to be excreted, is _catabolism_. While the process of oxidation in catabolism is going on, heat and energy are set free. Most of the chemical changes in the body are catabolic in character. This work of tearing down and rebuilding body tissues never ceases—even in sleep. It is not enough that the proper foods be furnished the body in kind and quantity. The essential thing is that the system be kept in condition to _assimilate_ the foods to its needs and to promptly eliminate the waste. Few people assimilate all of the foods eaten; nearly every one eats more than necessary for the body needs. By _assimilation_ is meant the digestive process by which foodstuffs are made soluble and diffusible, so that they can pass into the blood; also, the metabolic activity by which the food is converted into cells and tissues. Nature provides for an incomplete knowledge of the amount of re-supply necessary, by enabling the system to carry off a limited amount of surplus food above the bodily requirements. The distinct steps in anabolism are discussed in the following chapter describing the work of different organs and the chemical changes of foods as they come in contact with the elements in the digestive juices. CLASSIFICATION OF FOOD ELEMENTS