MAINTENANCE REQUIREMENTS OF MATTER 319 



material (229). The increased nitrogen excretion on a high 

 protein diet is simply the method by which the organism gets 

 rid of surplus nitrogen while retaining the larger share of the 

 energy of the protein for fuel purposes. It does not mean the 

 total destruction of the corresponding amount of protein, but 

 simply its transformation into compounds which can serve as 

 sources of energy. 



405. Fluctuations of body protein. Although in the mature 

 animal a surplus of feed protein is largely katabolized, so that 

 a continued increase of the protein tissue of the animal cannot 

 be brought about, as can that of the adipose tissue, simply by 

 a surplus in the feed, the protein content of such an animal is 

 not to be regarded as absolutely fixed, so that the protein supply 

 has no effect upon it. On the contrary, a considerable range of 

 variation is possible. 



Thus it is a familiar fact that a fasting animal may live and 

 continue to perform the essential bodily functions for some 

 time while losing daily a not inconsiderable amount of protein. 

 To cite a single striking example, Rubner observed in a fasting 

 rabbit up to the time of death, on the nineteenth day, a loss of 

 45.2 per cent of the computed nitrogen of the body. While 

 this is an extreme instance, nevertheless it is evident that there 

 must be a relatively large loss of body protein in those more mod- 

 erate cases in which the deprivation of protein is not continued so 

 long as to cause death. Furthermore, the losses occurring in 

 these latter cases may be made good by subsequent feeding 

 and the animal restored to its original state. Illustrations of 

 the same fact are familiar in the human subject in the emacia- 

 tion due to illness and the restoration of the body during con- 

 valescence. In brief, it is evident that the body of the mature 

 animal may fluctuate within somewhat wide limits as regards 

 its protein content without necessarily causing any serious or 

 permanent derangement of its functions. 



406. Storage of feed protein. Similar, although smaller, 

 fluctuations in the protein content of the body appear to be 

 caused by variations in the supply of feed protein, an increase 

 in the latter giving rise to more or less storage of nitrogenous 

 matter in the body, while a decrease has a contrary effect. 



In other words, as regards its stock of nitrogenous materials 

 the organism may exist and function at a higher or lower level 



