ALIMENTATION 117 



of the amino acids can act as a source of heat or energy. Some 

 of the non-nitrogenous product of deaminization may be con- 

 verted to sugar and glycogen and then into fat, though most of 

 the body fat comes from fat taken as such in the food and from 

 carbohydrate. 



Absorption and Metabolism of Carbohydrates. — Since carbo- 

 hydrate food is taken mainly in the form of starch, the principal 

 end product of carbohydrate digestion is dextrose. This passes 

 through the walls of the villi into the blood capillaries, whence it 

 is carried by the blood of the portal vein to the liver. In the 

 liver a certain amount of sugar is removed and converted into 

 glycogen, an insoluble starchlike substance, which is stored in the 

 liver cells, leaving about 0.1 per cent of sugar in the blood. As 

 sugar in the form of glucose is used up in the general circulation 

 and tissues, glycogen is converted back into dextrose by the 

 enzyme glycogenase, thus maintaining a fairly constant sugar 

 content in the blood. Sugar may be absorbed in a form other 

 than glucose, in which case it is converted into glycogen by the 

 liver and released as glucose in the blood. 



The islet cells of the pancreas produce a hormone, insulin, 

 which plays an important part in sugar metabolism. Sugar is 

 used in the body (1) to provide a source of energy for cells and 

 for muscular work; (2) by oxidation to supply heat; and (3) 

 excessive amounts may be converted into fat and stored as such. 

 Energy is obtained from sugar by oxidation which may occur in 

 the circulation or in the tissues, but in order for oxidation to take 

 place, insulin must be present. If insulin is kept out of the 

 general circulation in an experimental animal by removing the 

 entire pancreas, the percentage of sugar rises, indicating that 

 the normal amount of sugar is not being used. If the pancreatic 

 ducts are ligated, all of the cells of the pancreas except the islet 

 undergo atrophy. In such an animal the amount of the sugar 

 in the blood remains normal. Thus the rise of sugar in the blood 

 in the first case must have been due to the absence of something 

 produced by the islet cells. 



In the disease, diabetes mellitus, practically all the carbohydrate 

 taken as food may be lost in the urine in the form of sugar and 

 even if no carbohydrate is eaten, sugar, derived from stored-up 

 glycogen, continues to be eliminated in the urine. That this 

 condition is due to lack of insulin is indicated by the fact (1) 



