FOOD AND DIGESTION 91 



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 con- 

 tain 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 min- 

 eral). 'Oatmeal contains starch, oil, gluten, and water. 



USES OF THE NUTRIENTS, OR FOOD STUFFS 



r. Proteids. The tissue-building foods (also of value as fuel). 



2. Starches (and sugars) 1 Energy and heat (fuel) and 



3. Fats (and oils) J 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 Ib. of proteid or 2.3 Ib. 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 sugat 



