408 



METABOLISM, NUTRITION, AND DIET 



METABOLISM OF PROTEIDS. 



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

 siderable extent upon dogs, which demonstrate the necessity for proteid 

 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 diminish. 

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

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

 and at which the animal is not using up quickly or slowly the nitrogen of its 

 own tissue, and is no longer losing flesh. This condition in which the nitro- 

 gen of the egesta equals the nitrogen of the ingesta is known as nitrog- 

 enous 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 urea during a period of starvation. 

 Thus a dog excretes during a starvation period 0.5 gram of urea per kilo 

 of body weight; in order to satisfy this waste it would be necessary to ad- 

 minister 1.5 grams per kilo of meat; this at once increases the urea excreted 

 to about 0.75 gram per kilo of body weight, and nitrogenous equilibrium 

 is not attained until over three times, viz., 3 grams per kilo of body weight, 

 of meat is given. Foster gives even a larger figure. The effect, therefore, 

 of proteid food is largely to increase the excretion of urea, which indicates 

 an increase in the metabolism of the tissues. 



Studies in nitrogenous equilibrium are based on the fact that when an 



