150 METABOLISM 



equilibrium holds, an animal uses up all the protein that is 

 supplied. During starvation its use of protein becomes ex- 

 tremely economical. A liberal supply of protein given to a 

 starving animal leads at once to an almost entire destruction 

 of the protein. For a time some nitrogen is stored up in the 

 body, but the demand soon becomes equal to the supply. This 

 leads to a generalization: Destruction of protein is mainly 

 determined by the supply in the food. This fact, thus expressed, 

 has its raison d'etre in exogenous metabolism. A large part 

 of the nitrogen of the food passes by way of the liver, as a short 

 cut, to urea and is excreted. This process, while not depriving 

 protein food of its energy value to any great extent, does lead 

 to the preparation of a pabulum for tissue cells, freed from 

 superfluous nitrogen and containing, very likely, those amino- 

 acids in proper proportions suitable for the construction of 

 living substance. The relative small and constant amount 

 of endogenous metabolism indicates that the actual proto- 

 plasmic substance is comparatively stable, and that only a 

 small amount of the decomposition products of protein is 

 necessary to supply the waste. Experimentally, this has been 

 shown to be true whether the animal is at rest or not, and there 

 has, as a consequence, been established a second generaliza- 

 tion, which may be stated thus: Within normal limits nitro- 

 genous metabolism is nearly independent of muscular work. 

 For example, muscular work has little or no effect on the 

 excretion of urea. 



The energy for muscular work has its source mainly in non- 

 nitrogenous material. This was first made clear by two 

 experimenters, who, in climbing a mountain on a non-nitro- 

 genous diet, proved that the heat liberated was twice as great 

 as could possibly have come from broken-down protein, esti- 

 mated from the urea excretion during, and for some time after, 

 the climb. On the assumption that the urea excreted was 

 an adequate indication of the amount of protein broken down, 

 it was evident that a large fraction of the energy must have 

 come from the non-nitrogenous diet. This is corroborated 

 by the very great increase in carbon dioxide elimination which 

 takes place during muscular work. Excessive work in man 

 does cause an increase in nitrogen elimination which comes 

 on rather slowly and lingers for a day or two after cessation 



