100 



METABOLISM AND NUTRITION 



The proteid metabolism behaves in essentially the same way when the diet 

 contains a constant quantity of nonnitrogenous organic foodstuffs in addition 

 to proteid. This may be seen from the preceding table. 



Just as the body can destroy very different quantities of proteid., it can 

 also be placed in nitrogenous equilibrium with very different quantities of 

 this foodstuff (cf. first table on page 91). 



The following experiments from Voit will give some idea of the time 

 required to reach nitrogenous equilibrium: 



II 



The two experiments were carried out on the same animal. Previous to the 

 first the dog had received 17 g. of N (500 g. meat) daily. Equilibrium had not 

 been established with this quantity, for on the last day of this period he lost 

 1.6 g. of N" from the body. The supply of N (in meat) was then increased to 

 51 g. ; immediately the N-metabolism rose, but perfect equilibrium was not 

 reached until the seventh day, and during this time 26.4 g. N were retained 

 in the body. 



What became of this nitrogen? It might have been retained as dead pro- 

 teid, as living protoplasm, or in the form of decomposition products. The last 

 possibility has been rendered very improbable by various experiments of Voit. 

 We shall return later to the question of whether the nitrogen is stored as proteid 

 or as protoplasm. 



The second experiment is just the reverse of the first. Here the animal had 

 previously received 51 g. N (1,500 g. meat) and had been in nitrogenous equi- 

 librium. The N-supply was then cut down to 34 g. The result was that on the 

 very first day the N-excretion was less than before, and during the following 

 days it sank lower and lower until on the fifth day it reached approximately the 

 level of the amount supplied. During these five days the animal lost 14.8 g. 

 N from his body. 



Nitrogen excretion runs a similar course during the first few days of starva- 

 tion. If the same animal be made to fast, in the one case after feeding a plenti- 

 ful supply of meat, and in the other after feeding a scant supply, the excretion 

 of N in the urine during the first few days behaves very differently : the greater 



