Chap. XVI] METABOLISM 309 



the body is derived from fats, as excess carbohydrates are also 

 stored as fat. 



Function of fats. — The main function of fat is to serve as fuel 

 and yield heat and energy. The fat that is stored in the form of 

 adipose tissue constitutes an important reserve fund to be drawn 

 upon in time of need. In diseased conditions or when the supply 

 of food is insufficient, the body oxidizes first the glycogen stored in 

 the liver and then the fats stored in adipose tissue, and the proteins 

 of the tissues themselves. If the supply of fat is large, it follow^s 

 that the protein tissues will be protected. (For other functions, 

 see page 39.) 



The cause of obesity. — This condition is usually caused by 

 eating more carbohydrate and fat than the body needs. The 

 excess is stored as glycogen and adipose tissue. The needs of dif- 

 ferent individuals vary, depending on their mode of life, and on 

 their capacity to oxidize food materials, so that a diet which will 

 give an excess to one individual may in the body of another be en- 

 tirely consumed. A sedentary life and absence of worry lessen the 

 oxidation of food products and increase the tendency to take on 

 flesh, while a very active muscular life has the opposite effect. 



METABOLISM OF CARBOHYDRATES 



During the process of digestion all the carbohydrates are 

 changed to glucose, absorbed into the blood, and carried to the 

 liver. The liver cells take this glucose from the blood, and by put- 

 ting together a number of molecules and withdrawing water, the 

 soluble glucose is changed to insoluble glycogen, which is stored 

 in the liver cells and the muscles. In thus storing up glycogen 

 and doling it out as needed, the liver helps to maintain the normal 

 quantity of glucose — 0.1 to 0.15 per cent — in the blood. An 

 increased amount of glucose in the blood, resulting from the in- 

 gestion of a large amount of sugar, may provide more glucose than 

 the liver can take care of, and the excess is eliminated in the urine. 

 A permanent increase in the amount of sugar in the blood is irri- 

 tating to the tissues and acts as a poison, so that the liver by main- 

 taining the normal quantity protects the tissues from such irrita- 

 tion. 



Factors controlling the metabolism of carbohydrates. — The 

 metabolism of carbohvdrates is under the control of the nervous 



