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 oxygen in the cells of the body gives the heat and motion (energy) of the body. Only substances which will oxidize, or burn, are true foods. Water, salt, and carbon dioxid will not burn; hence, they cannot give rise to energy in the body. But the sun energy, acting in the green leaf, tears apart the carbon from the oxygen (Plant Biology, Chap. XIII), sets free the oxygen, and the carbon is stored in starch for future burning. Sunshine is energy (light and heat). The sun sustains the life of plants and through them the life of animals. The oxidation in the body is so slow that it can hardly be called a burning, but it is faster than the oxidation of iron in rusting or of wood in rotting, and is about equal to the continual burning of two candles.

The Four Kinds of Nutrients, or Food Stuffs.—The kinds of food which we eat seem to be numberless, but they contain only four kinds of food stuffs,—starches, fats, proteids, and minerals. Many foods contain all four classes of food stuffs. Milk contains sugar (a changed form of starch), cream (a fat), curd (a proteid), and water (a mineral). Oatmeal contains starch, oil, gluten, and water. 1. Proteids. The tissue-building foods (also of value as fuel). 2. Starches (and sugars) } Energy and heat (fuel) and 3. Fats (and oils)	    }   fat producing foods. 4. Minerals (water, salt). Important aids in using other foods. Relative Fuel Value.—A pound of fat produces as much heat in the body as 2.3 lb. of proteid or 2.3 lb. of starch, the last two having equal fuel value in the body. Starch and the sugars are closely related; starch readily changes into sugar. They contain much carbon and are called carbohydrates. Starch is especially abundant in grains, seeds, and fleshy roots (Fig. 88). The sugar in ripe fruit and in honey is called fruit sugar. Milk sugar