GROWTH 381 



The rate of gain of energy as thus computed is notably 

 greater for young carnivora (dogs and cats) during the first two 

 or three weeks than that of pigs or calves. Aside from this, the 

 results on farm animals, although more or less irregular, present 

 in general the same picture as those on the rate of gain of pro- 

 tein, viz., a diminishing energy of growth with advancing age. 

 The few instances showing a wide divergence from the majority 

 may probably be assumed to be due to rapid fattening. 



2. THE UTILIZATION or FEED IN GROWTH 



The utilization of protein 



465. Relative values of proteins for growth. A considera- 

 tion of the utilization of protein in growth necessarily raises 

 the question of the relative values of different individual pro- 

 teins in this respect. 



As was pointed out in Chapter IX (398), it appears probable 

 that the protein requirement for maintenance is essentially 

 an amino acid requirement and that the relative values of 

 proteins for maintenance may prove to depend largely or wholly 

 on their ability to supply certain specific " building stones " 

 required for the performance of specific functions. In the 

 growing animal there is, in addition to this requirement for 

 functional purposes, a demand for amino acids out of which 

 new body proteins may be built up. In growth, therefore, the 

 amino acid requirements may differ from those for maintenance 

 not only in being quantitatively greater but in being qualita- 

 tively different. A striking illustration of this is afforded 

 by the investigations of Osborne and Mendel l on the relation 

 of lysin to growth. 



In common with other investigators they have found that trypto- 

 phan is indispensable for maintenance (399). Wheat gliadin con- 

 tains tryptophan but only a minute amount of lysin. While they 

 have repeatedly secured maintenance for long periods on rations con- 

 taining gliadin as the sole protein, they have been unable to secure 

 growth with such rations, but the simple addition of lysin enabled 

 growth to proceed at a normal rate. The body proteins contain lysin, 

 ox muscle, for example, yielding 7.6 per cent (50). Evidently this 

 1 Jour. Biol. Chem., 12 (1912), 473; 17 (1913), 325; 86 (1916), 293. 



