The body is analogous to an engine. The working engine needs fuel, whose oxidation supplies the heat required to set the engine in motion. In a similar way the working body requires fuel, whose oxidation furnishes the energy for the body movements. Engine fuels are the easily oxidized coal, wood, or oil. Body fuels are the easily oxidized starch, sugar, and fat. The oxidation of engine fuel liberates heat, which either directly or indirectly sets the parts of the engine into action. The oxidation of body fuel liberates heat, and at the same instant produces the energy peculiar to the active tissues of the body - in muscle tissue, motor energy; in glandular tissue, chemical energy; in nervous tissue, ' nervous energy," closely related to if not identical with electrical energy.

Thus, we find that the body needs foods similar in composition to its own substance for growth and repair, while it needs fuel foods from which to produce the active energy for the working body.

We will find later that these two phases of body requirements - growth and repair on one side and work on the other - make the basis for the classification of foods into two great classes, each class serving one of these fundamental requirements of the body.