488 NUTRITION. 



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 it 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 grammes 

 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 dimin- 

 ishes. 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 examintion 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 pro- 

 teid, as we have already ( 401) suggested, explains at the same time why 

 the meat diet so largely and immediately increases the urea of the egesta. 



The characteristic effect of proteid food to increase the metabolism of the 

 body is shown 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 of 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 per cent, 

 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 420). Similar observations 

 carried out on sheep showed that in these animals the storing up of nitro- 

 genous material was even less, only about 4 per cent, of that given in the 

 food. 



Every quantity of proteid material taken into the alimentary canal thus 

 appears to affect proteid metabolism in two ways. On the one hand, it ex- 

 cites a rapid proteid metabolism giving rise to an immediate, and generally 

 large, increase of urea ; on the other hand, it serves to maintain the more 

 regular normal proteid metabolism continually taking place in the body, 

 and so contributes to the normal regular discharge of urea. It seems very 

 natural to suppose that the proteid which plays the first of these two parts is 

 not really built up into the tissues, does not become actual living substance, 

 but undergoes the changes which give rise to urea outside the actual living 

 substance in the blood or elsewhere ; and we have seen that under the influ- 

 ence of the pancreatic juice some of the proteid food may undergo the greater 

 part of such a change while it is as yet within the alimentary canal. Heiice 



