NUTRITION 351 



much the same weight, and that could never be the case if storing 

 up occurred daily. What occurs is this, that of the total 

 amount of nitrogen received daily in the food, a similar amount 

 is recovered daily from the urine. Whatever portion of the 

 protein of the food is converted into tissue protein, an equivalent 

 amount of the latter is displaced, so that the total excretion of 

 nitrogen equals the total receipts. A body in this condition is 

 in nitrogenous equilibrium. It does not follow from this that it 

 is not increasing in weight ; it may be, owing to excess of non- 

 nitrogenous food ; on the contrary, it may be losing weight, due 

 to a deficiency of non-nitrogenous material. The matter is of 

 no moment at present, the main point being that in the urine an 

 equivalent amount of nitrogen is excreted to that received in the 

 food, and such an animal is in nitrogenous equilibrium. It is ob- 

 vious that nitrogenous equilibrium cannot exist in young growing 

 animals, nor in those in impoverished condition. In both these 

 cases nitrogen is being stored up, as the tissue requirements are 

 considerable, so that under these circumstances any attempt to 

 establish nitrogenous equilibrium experimentally would fail. An 

 adult animal in good condition gives off daily by the excreta as 

 much nitrogen as it receives in the diet. If the ingoing nitrogen 

 is increased, the outcoming nitrogen is increased ; if the ingoing 

 nitrogen is decreased, the outcoming nitrogen is decreased, and 

 this is true within physiological limits. 



It is evident, therefore, that in the same animal there may be 

 different levels of nitrogenous equilibrium, and these are depen- 

 dent entirely on the income. The more the body receives, the 

 more it spends ; the less it receives, the less it gets rid of, the 

 predominant feature being that the system is determined to 

 make its expenditure of nitrogen equal its income. This being 

 the case, how does the body behave when it receives no nitrogen ? 

 The improvident expenditure of nitrogen previously existing is 

 now rectified. None is being received by the mouth, and the 

 store in the tissues is therefore most economically handled. On 

 the second day of starvation, by which time the nitrogen of the 

 last meal is eliminated, the output of nitrogen falls suddenly, 

 and after that it remains either at a constant minimum or sinks 

 gradually until death approaches, when, after a temporary rise 

 in the amount excreted, it rapidly falls. It must not be imagined 

 that in this experiment the animal is living exclusively on its 

 own protein. Its body fat is also being utilised, and so long as 

 this lasts a steady daily loss of nitrogen occurs ; but when the 

 fat has disappeared, there is an increase in nitrogen excretion, 

 as shown by the temporary rise above noted. The fact is that 

 so long as the fat lasts, the waste of protein is kept down to the 

 lowest possible limit, the fat being sacrificed to spare the protein. 



