CHAP, v.] NUTRITION. 795 



labour thrown on the economy by the very presence of the 

 food. This however is not the case as far as the nitrogen of the 

 meal is concerned ; the larger portion passes off as urea at once, 

 and only a comparatively small quantity is retained. If the diet 

 be continued, and we are supposing the meals given to be large 

 ones, the proportion of the nitrogen which is given off in the form 

 of urea goes on increasing until at last a condition is established 

 in which the nitrogen of the egesta exactly equals that of the 

 ingesta. This condition, which is spoken of as "nitrogenous 

 equilibrium " is attained in dogs with an exclusively meat diet 

 only when large quantities of food are given, and is not easily 

 maintained for any length of time. The exact quantity of meat 

 required to attain nitrogenous equilibrium varies with the previous 

 condition of the dog ; equilibrium is frequently attained when 

 1500 or 1800 grms. of meat are given daily. 



Thus the most striking effect of a purely nitrogenous diet is 

 largely to increase the nitrogenous metabolism of the body ; and 

 we shall see later on that it increases the metabolism not only of 

 the nitrogenous but also of the other constituents of the body. 



The establishment of nitrogenous equilibrium does not mean 

 that a body-equilibrium is established, that the body-weight 

 neither increases nor diminishes. On the contrary, when the 

 meal necessary to balance the nitrogen is a large one, the body 

 though it is neither gaining nor losing nitrogen may gain in total 

 weight ; and the increase is proved by calculation from the income 

 and output, and indeed by actual examination of the body, to be 

 due to the laying on of fat. The amount so stored up may be far 

 greater than can possibly be accounted for by any fat still 

 adhering to the meat given as food. We are therefore driven 

 to the conclusion that the proteid food is split into a urea moiety 

 and a fatty moiety, that the urea moiety is at once discharged, 

 and that such of the fatty moiety as is not made use of directly by 

 the body is stored up as adipose tissue. And this disruption of 

 the proteid, as we have already ( 487) suggested, explains at 

 the same time why the meat diet so largely and immediately 

 increases the urea of the egesta. 



This characteristic effect of proteid food to increase the 

 metabolism of the body is shewn on other animals besides the 

 dog, and not only by means of calculations of what is supposed to 

 take place in the body, but also by direct analysis. Thus the 

 analysis of the body of a pig, which had been fed on a known diet, 

 compared with the analysis with that of another pig of the same 

 litter, killed at the time when the first was put on the fixed diet, 

 gave as a result that of the dry nitrogenous material of the food 

 only about 7 p.c. was laid up as dry proteid material during the 

 fattening period, though the amount of proteid food was low. 

 This contrasts strongly with the amount of fat stored up during 

 the same period (see 506). Similar observations carried out on 



