CHAPTER XXXVI 

 THE NOURISHMENT OF THE BODY 



214. The Need of Food. The human body is like all other 

 machines in that it cannot work without energy. It also 

 requires more or less material for growth and repair (179). 

 The material that supplies these needs are comprised under 

 the general name of foods. Water, oxygen, and various salts 

 are sometimes included in the list of foods, since they are 

 needed by the body, but they are not foods in the usual sense, 

 and we will here confine our attention to those substances 

 more commonly regarded as such and classed as proteins, 

 fats, and carbohydrates, respectively (180). These sub- 

 stances, as they occur in the bodies of animals and plants used 

 for food, cannot be assimilated by the human body until 

 they have undergone certain chemical changes which render 

 them soluble and otherwise fit them for being absorbed. 

 Such changes are involved in the processes of digestion and 

 seem in large measure to be due to the breaking up of large 

 molecules into smaller ones by means of certain ferments 

 known as enzymes. In digestion, the enzymes act some- 

 what as catalyzers do in other chemical reactions and 

 cause the digestive processes to continue without being used 

 up themselves. 



215. Value of Foods. The carbohydrates, such as starch 

 and sugar, and the fats, are the chief sources of food for the 

 body. Energy may be derived from the proteins, it is true, 

 but this is not an economical use of such foods. They are 

 more useful in the growth and repair of the body. In order 

 to derive enough energy from proteins, an unusual amount 



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