METABOLISM 



279 



These results explain why a smaller amount of some 

 proteins than of others is sufficient to repair the wear and tear. 

 The more nearh^ the amino-acid make-up of the protein 

 in the food approaches that of the tissues, and chiefly of the 

 muscular tissue, of the animal consuming it, the smaller is 

 the quantity required. 



This has been shown by feeding a man on a carbohydrate 

 and fat diet, yielding the energy required and finding the 

 amount of nitrogen excreted, i.e. the amount of protein 

 oxidised, and then adding different proteins. It was 

 found that, while a small quantity of some is capable of 

 making good the loss of nitrogen, nuich larger quantities of 

 others are required. 



In wheat the defective gliadin forms half the protein 

 content. 



It has been found that in the dog the loss of nitrogen is 

 covered by the smallest protein intake when dog's flesh is 

 given — a physiological justification of cannibalism. 



On account of the different values of different proteins, 

 Lusk proposes to classify them into three groups according 

 to their amino-acid build-up and their resultant avail- 

 ability for growth and repair : — 



1st Class, e.g. Caseinogen of milk. 

 2nd Class, e.g. Gluten of wheat flour. 

 3rd Class, e.g. Gelatin. 



Salts. — It has been seen (p. 218) that salts as ions play 

 an essential part in regulating the osmotic pressure of the 

 body fluids. 



