174 NUTRITIONAL PHYSIOLOGY 



often nicely balanced. Complete equilibrium demands 

 strict equality between the income and outgo of water, of 

 mineral matter, of nitrogen, and of carbon. The realiza- 

 tion of these conditions is not likely, though it is often 

 closely approached. Partial equilibrium, that is, equality 

 between intake and output for one class of compounds with 

 inequality for another, is more common. The most fre- 

 quent striking of a balance is between the nitrogen of the 

 food and that of the excreta. Nitrogenous equilibrium is 

 the rule rather than the exception. The tendency of the 

 body to establish this correspondence, in spite of wide 

 variations in the diet, was noted long ago. It was said 

 that the organism refused to store protein when supplied 

 with large amounts of this kind of food. This is true in the 

 narrow sense that the body does not add freely to the adult 

 measure of its living tissues when offered extra protein. 

 Yet, as we have seen, the return of all the nitrogen fed 

 does not of necessity mean that the body has retained no 

 part of the protein supplied to it. The chief reason why 

 there is such a marked disposition to make the output of 

 nitrogen equal the income is found in the fact that all the 

 amino-acids beyond the small quantity used for protein 

 synthesis are deaminized. So long as this is the case, 

 raising the nitrogen of the food must result merely in 

 adding to the urea excreted. The non-nitrogenous resi- 

 dues may find more or less permanent lodgment in the 

 tissues in the form of glycogen or fat. 



Nitrogen retention is to be expected during growth when 

 the protein syntheses are more extensive than the endogen- 

 ous decomposition. The recovery from illness or from a fast 

 is another instance when the body protein must definitely 

 increase. This is really only a special case of growth. 

 Change of climate or the pursuit of athletic training may 

 encourage some degree of protein storage. And without 

 seriously qualifying what has just been said it may be 

 stated that abundant nitrogenous food may have the same 

 effect, but the nitrogen retention secured by forced feeding is 

 always limited to a very small fraction of the protein given. 



