PROTEIN METABOLISM 



863 



left after deamination and their subsequent fate in the body. Since 

 they contain only the elements carbon, hydrogen, oxygen, one would 

 expect to find that they could replace either fat or carbohydrate. So 

 far as concerns the production of energy this is true. Moreover, as 

 we shall see in dealing with the metabolism of carbohydrates, we have 

 definite evidence that this non-nitrogenous moiety of the protein 

 molecule may be converted into sugar or glycogen. No experimenter 

 has yet succeeded in bringing forward indisputable evidence that fat 

 may be formed from this part of the protein molecule ; at any rate, 

 no fat which can be stored in the body and give rise to the production 

 of adipose tissue. On the other hand, the proteins, more than either 

 of the other two food-stuffs, cause a direct augmentation of the respira- 

 tory exchanges of the body. This is shown in the following Table by 

 Rubner, in which isodynamic quantities of proteins, fats, and carbo- 

 hydrates were administered during a period of starvation : 



It will be seen that the metabolism, i.e. the caloric output, of the 

 body on administration of protein increased 11-9 per cent., whereas 

 with fat the increase amounted only to 1-2 per cent., and with carbo- 

 hydrate to 4-7 per cent. In another similar experiment the animal 

 received 57-4 calories protein, 54-2 calories fat, and 57 calories carbo- 

 hydrate respectively per kilo body weight. During hunger the total 

 metabolism per kilo body weight amounted to 37'5 calories ; with meat 

 to 46 calories ; with fat, to 39'4 calories ; with carbohydrates, to 39'4 

 calories. Compared with the metabolism during starvation the rise per 

 cent, with protein was 24-3, and with fat and carbohydrates 5-1. This 

 surplus output of energy resulting from the administration of protein 

 cannot be ascribed to increased work thrown on the digestive organs. 

 There is no evidence that this is greater in the case of proteins than 

 it would be with carbohydrates or fats ; and even if the capacity of 

 these organs be strained to their utmost by administration of large 

 quantities of bones, the increase in the carbon dioxide output which 



