METABOLISM 185 



stances out of which the body can manufacture the compounds 

 which it requires. 



264. Functions of the proteins. The proteins furnish at 

 once the most familiar and the most striking example of this 

 dual function of the feed. 



Since the proteins may be katabolized in the body with the 

 formation of products (carbon dioxid, water, urea, etc.) con- 

 taining either no available energy or but a small fraction of 

 that found in the original proteins, it is clear that the latter 

 serve as carriers of energy. In fact, it has been shown to be 

 possible to maintain a carnivorous animal in normal activity 

 for an indefinite time on a diet containing substantially nothing 

 but protein as a source of energy. 



But proteins serve also as building material. Aside from 

 water, the working machinery of the body is composed largely of 

 proteins, while very many at least of the special substances 

 already mentioned are nitrogenous and probably derived from 

 the proteins. These protein tissues and other substances must 

 be built up in the growing animal and maintained in the mature 

 one, and for this purpose only proteins or their cleavage prod- 

 ucts can be utilized, and their presence in the feed is indis- 

 pensable. 



A point which sometimes causes perplexity is that the same 

 portion of protein may not only serve as structural material 

 but also yield energy for the vital processes, so that in esti- 

 mating the energy supplied by a feeding stuff that of its protein 

 as well as that of its other ingredients is included. The difficulty 

 disappears, however, when it is remembered that any given 

 portion of protein does not perform both these functions at the 

 same time. If a gram of protein in the feed of a mature animal 

 is used for structural purposes it practically takes the place of 

 an equal amount of tissue protein, while the latter is katabolized 

 and yields substantially the same amount of energy as would 

 have been available from the gram of feed protein had that 

 been katabolized instead. The latter, with its store of energy, 

 has been temporarily set aside from the katabolic process but at 

 some later time may itself be replaced by another gram of feed 

 protein and katabolized in its turn, liberating the corresponding 

 amount of energy. The repairing of a wooden building may 

 serve as an illustration. The old wood taken out to make way 



