METABOLISM, NUTRITION, AND DIET. 429 



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

 not of one alone. 



Many valuable observations have been made with a view of ascertain- 

 ing 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. 



Effect of a Proteid Diet. Experiments have been made, to a consider- 

 able extent upon dogs, which demonstrate the effect of proteid food. 

 After a period without food, during which the output of nitrogen, as 

 shown by the urea, had diminished to a certain amount, the animal is 

 fed with a diet of lean meat which would suffice to produce the amount 

 of urea, and so of flesh, which it had 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 it has been previous to the 

 commencement of its flesh diet, so that again the output of nitrogen 

 would exceed its income, and the weight of the animal would continue 

 slowly to diminish. It is only after a considerable increase of the flesh 

 given that a point is reached where the income and expenditure are 

 equal, and at which the animal is not using up quickly or slowly the 

 nitrogen of his 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. In the dog, according to Waller, it 

 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 grms. of urea per kilo of body weight; in order to satisfy this it 

 would be necessary to administer 1.5 grms. per kilo of meat; this at 

 once increases urea excreted to about 0.75 grms. per kilo of body weight, 

 and nitrogenou^ equilibrium is not attained until over three times viz., 

 5 grms. 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 increase of the metabolism of the 

 tissues. 



It must not be thought however that during nitrogenous equilibrium 

 there is, of necessity, equilibrium of carbon. On the contrary, it is very 

 possible that the carbon, as supplied by the large amount of meat, is not 

 entirely eliminated, but may be partially retained in the body. If re- 

 tained in the body it is probably retained in the form of fat, although 

 possibly it might be retained partially as some carbohydrate, e.g., gly- 

 cogen ; but the amount of glycogen obtained from the body is too small 

 for the latter to be appreciable. The animal in nitrogenous equilib- 

 rium, therefore, may gain weight, although not in the form of flesh. 

 The converse may also be the case, the animal getting rid of more carbon 



