342 LECTURE XV. 



we remember that the animal was fasting, and that such an organism is 

 as economical as possible with its subsistence. On the other hand, it is 

 possible that all three nutrients, fat, carbohydrate, and albumin, are drawn 

 upon as sources of the energy required for the work of the glands, and that 

 the consumption, of albumin especially, was so small, that it did not 

 change the nitrogen content of the urine enough to be shown by our 

 present methods of analysis. 



We have already stated that the carbohydrates are also to be looked upon 

 as a source of heat. It is possible to cause glycogen to disappear by merely 

 chilling an animal. Albumin may also act in this way as a source of heat. 



At all events, the discovery that the fats act as direct sources of mus- 

 cular force, proves that the nutrients stand in intimate relation to one 

 another. They replace one another partly by being transformed and 

 partly by reason of their calorific values. This, however, does not by any 

 means include all of the relations existing between the individual nutrients. 

 As we have already seen, it is possible to keep a dog alive on meat alone. 

 In this case the organism must obtain all of its requirements from the 

 albumin (aside from the small amounts of fat and carbohydrate contained 

 in the meat). If the quantity of meat is insufficient, the animal must 

 draw upon its stores of fat and carbohydrate. It is a simple matter to 

 determine the amount of meat necessary to just satisfy the energy require- 

 ments. If we exceed to the slightest degree the quantity of lean meat 

 which is necessary to keep the metabolism in equilibrium, there will be 

 no accumulation of albumin, but, on the contrary, there is an increased 

 decomposition of albumin as a consequence. An accumulation of albumin, 

 i.e., an increased amount of albumin in the cell material, may indeed be 

 brought about by a liberal and long-continued feeding of albuminous 

 material. 1 It has been shown, however, that the animal organism con- 

 stantly seeks to maintain its albumin content, and consequently the func- 

 tional condition of its cells, at a constant level. As soon as the diet ceases 

 to contain the excess of protein, the accumulation of albumin in the cells 

 quickly disappears. The previous equilibrium in the economy of the 

 individual cells will be reestablished. 



If we make an animal go hungry, the elimination of nitrogen continues, 

 and this is also true when an abundance of fat or carbohydrate is fed to 

 the animal. The albumin, therefore, is not entirely replaceable, although 

 it is possible to reduce the destruction of albumin by means of fats. This 

 fat may come from the food or from the body itself. It is possible by 

 means of fat and albumin to .establish a new nitrogen balance; i.e., to 



1 More recent experiments make it seem doubtful whether the nitrogen retention 

 and deposition of albumin correspond to one another. Great caution should be used in 

 the estimation of the nitrogen balance in this direction. Cf. E. Abderhalden and 

 Bloch: Z. physiol. Chem. 53. 464 (1907). 



