METABOLISM 155 



may not keep pace with the resorption and there occurs an excretion 

 of sugar in the urine the so-called "alimentary glycosuria." The 

 amount of sugar which can be resorbed without producing alimentary 

 glycosuria, i.e., the limit of tolerance for sugar varies with the 

 kind of sugar, being highest with dextrose (220). 



216. Carbohydrates formed in the body. - In their relation 

 to the carbohydrates of the feed, the muscles and liver act, as 

 has been seen, as a sort of storage reservoir or regulator of the 

 sugar supply to the blood. The total withdrawal of carbohy- 

 drates from the feed, however, by no means results in the 

 disappearance of these substances from the body. The car- 

 bohydrates appear to be essential to the normal course of metab- 

 olism and if they are absent from the feed, they are manufac- 

 tured in the body from other materials. A carnivorous animal, 

 e.g., fed exclusively on meat or fat, shows a normal percentage 

 of dextrose in its blood, while its liver and muscles contain a 

 normal amount of glycogen. It is true that in such an experi- 

 ment small quantities of glycogen are contained in the meat 

 consumed, but their amount is entirely insignificant as com- 

 pared with the quantities of dextrose which there is reason to 

 believe are produced and katabolized in the organism. This 

 dextrose must obviously have its origin either in the proteins or 

 the fats. Which of the two is the source or whether both can 

 be thus utilized will be considered later in connection with 

 the metabolism of those substances (235, 253). 



217. Formation of fat. The mutual transformations of 

 sugar and glycogen tend to keep the dextrose content of the 

 blood approximately constant, while holding a supply of readily 

 available carbohydrate material at hand to meet promptly any 

 sudden demand. The amount of carbohydrates which can be 

 disposed of in this way is, however, limited. For man it is 

 estimated at about 300 grams and for cattle at about 2 kilo- 

 grams (96) . It is evident, then, that if the feed contains a per- 

 manent excess of carbohydrates over the needs of the body the 

 capacity to store them up as glycogen will soon be exhausted. 

 A surplus of carbohydrates over the amount which can be dis- 

 posed of in this way is applied by the organism to the pro- 

 duction of fat, which may be stored up in very large amounts 

 in the cells of connective tissue through the body, but especially 

 in those immediately beneath the skin and about the abdominal 



