METABOLISM OF PROTEINS 



457 



It is, therefore, evident that the diet must consist of several compounds, 

 not of one alone. 



Many valuable observations have been made with a view of ascertaining 

 the effect upon the metabolism of a variation in the amount and nature of 

 food. These are of great assistance in the consideration of dietetics. 



METABOLISM OF PROTEINS. 



Nitrogenous Equilibrium. Experiments have been made, to a con- 

 siderable extent upon dogs, which demonstrate the necessity for protein 

 food. After a preliminary period without food, during which the output of 

 nitrogen as shown by the urea has diminished to a comparatively constant 

 amount, an animal is fed with a diet of lean meat which would suffice to pro- 

 duce the amount of urea, and so of flesh, which it has been losing during its 

 starvation period. The effect of this, however, is at once to send up the 

 amount of urea excreted to a point above that which had been lost previous 

 to the commencement of the flesh diet. Thus the output of nitrogen still 

 exceeds its income, and the weight of the animal continues slowly to dimin- 

 ish. It is only after a considerable increase of the flesh given in the food that 

 a point is reached where the income and the expenditure of nitrogen are equal, 

 and at which the animal is not using up or storing the nitrogen in its own 

 tissue, and is no longer losing flesh. This condition in which the nitrogen 

 of the egesta equals the nitrogen of the ingesta is known as nitrogenous 

 equilibrium. 



EXPERIMENT IN NITROGENOUS EQUILIBRIUM. 



In the dog, according to Waller, nitrogenous equilibrium does not occur 

 until the amount of flesh of the food is over three times as great as would be 

 necessary to supply the nitrogen of the urine during a period of starvation. 

 Thus a dog excretes during a starvation period 0.50 gram of urea per kilo 



