FOODS AND THEIR USES 255 



the patient never suspects that the liver is responsible 

 for their production. 



The fate of the different classes of food in the economy 

 of the body may be summarized as follows: 



Proteins. 



produce energy; 



build up living tissue; 



may give rise to fat in the body. 



Fats.. I P roduce ener gy; 



I are converted into bodily fat. 



Carbohydrates. . 



produce energy; 



are converted into fats; 



may be stored up as glycogen. 



All foods produce energy for the performance of muscular 

 work and the production of bodily heat. Carbohydrates 

 and fats are chiefly energy producers and, although they 

 may be stored for a time, they may be oxidized later as 

 occasion demands, and hence used up for the production 

 of energy in the end. 



Besides the three classes of foods that have been de- 

 scribed there are several other substances that are essential 

 for the maintenance of life. Conspicuous among these 

 is water, as it forms about %o of the material of the blood 

 and about 59 per cent, of the substance of the body. 

 Since water is constantly being given off from the body 

 through the secretory activities of the kidneys, by the skin 

 in the form of perspiration or sweat, and by the lungs in 

 the breath, it must be supplied in considerable quantity. 

 While more or less water is contained in all our foods, it is 

 necessary to drink water or some beverage such as tea or 

 coffee, etc., consisting mainly of water, in order to supply 

 our bodily needs. A man may do without food for several 

 days and in exceptional cases for some weeks, but he will 



