CHAPTER XVII 



ANIMAL FOODS AND FEEDING 



FOOD VALUE 



In the study of the physiological processes of plants and 

 animals we have brought out the following general facts. By 

 utilizing the energy of the sunlight, living plants manufacture 

 carbohydrates out of the simple substances carbon dioxide and 

 water, which are present in the atmosphere and the soil. These 

 photosynthesized carbohydrates by further metabolic changes in 

 the plant are transformed, in part, into the vegetable fats and, 

 with the assistance of nitrogen compounds obtained in most 

 cases from the soil in the form of nitrates, are in part transformed 

 into proteins. In this way the plant builds up its own food 

 supply and stores up a reserve supply of food for its offspring. 

 This reserve food may also become a source of food for animals. 

 These essential organic food constituents all possess potential 

 energy which, through the processes of animal metabolism, be- 

 comes converted into the kinetic energy of animal heat or 

 muscular work. This intimate relation between plants and 

 animals is the important one of food and nutrition. 



In our study of animal physiology we discussed the processes 

 by which these food constituents — carbohydrates, fats and pro- 

 teins — were digested and then metabolized in order to yield the 

 energy of animal life. The chemical reaction involved in the 

 liberation of this energy is that of oxidation brought about 

 by the process of respiration, by which the oxygen of the air 

 is brought into contact with the digested, assimilated and 

 partially metabolized food materials. 



Quantitative Relation of Food to Energy. — We have now to 

 consider the quantitative relation of this food to the energy of the 



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