GENERAL METABOLISM 



395 



excreting, will not at once stop the loss of weight. Suppose, 

 for instance, that to a fasting animal using daily 30 grras. of the 

 proteins and 160 grms. of the fats of his body, a diet containing 

 these amounts is given, the disintegration of proteins and of 

 fats will at once rise, say, to 50 grms. of protein and 280 of fat. 

 Thus the result will be that, instead of his losing 30 grms. of 



o o 



protein, he will lose only 20 grms. per diem, and instead of 160 

 grms. of fat, only 120 grms. But, if the diet is sufficient to 

 supply the energy required, in a few days the intake and output 

 will balance, and the individual is then said to be in metabolic 

 equilibrium, and he neither gains nor loses weight. The 

 following table and fig. 168 give an idea of how this adjust- 

 ment of the metabolism is reached : 



If the amount of food be further increased, a small proportion 

 of the proteins and a larger proportion of the fats are retained, 

 and weight is gained. As already indicated, the power of 

 storing proteins is generally small (see p. 386). 



Protein Diet. Proteins contain all the chemical elements 

 required for the building and repair of the tissues, and from 

 the complexity of their molecules they also supply latent energy. 

 It is therefore theoretically possible for an animal to sustain 

 life on proteins, and certain animals can be fed exclusively upon 

 them. Thus Pfliiger kept a dog for many months upon a purely 

 protein diet without injury to its health. But to supply the 

 necessary energy in proteins alone requires the consumption 

 of excessively large quantities, and herbivorous animals are 

 unable to digest and use such quantities. Further, when large 

 quantities are taken, the greater portion is broken up in the 

 intestinal wall and formed into urea by the liver and ex- 



