It has been learned through science that the food taken into the human body is broken up by digestive agencies. It is then made over for body-use and body-tissue for repair or growth. Energy is provided by the heat generated as the body-tissue breaks down in working and as the food unites with the oxygen of the air breathed in by the body. In some respects this action is similar to the production of heat as fuel burns. As fuel burns it unites with the oxygen of the air in the stove; heat is thus produced. This heat in the body supplies the body-heat and is converted into the energy that the body uses as it works, as heat in the stove may boil water.

But no machine, it must be remembered, has the power of self-repair through simply the energy fuel gives it. Self-repair and growth come only with life, so the body has in its power of self-repair and growth what all machines lack.

It has also been learned that food and the human body are composed of the same constituents, five in all. Though most foods contain all five constituents in some quantity, all are not present in the same quantity in the same food, nor are the quantities of the different constituents in any food the same as in any other.

It is the chief constituent of a food which gives it its principal use in the diet; but all that is edible in a food is used in the body.

The food constituents that build and repair (protein and mineral salts) are in largest quantity in eggs, milk, cheese, meats, grains. Those that give heat and energy (carbohydrates and fats) are principally in starchy vegetables, sugar, fats, oils. Those that especially aid the body in keeping itself in condition to use its food are green vegetables and fruits.