764 



PHYSIOLOGY 



the origin of the urea, but the nature of the substances that are 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 pro- 

 duction of energy this is true. Moreover, as we shall see in dealing with the 

 metabolism of carbohydrates, we have definite evidence that part of this non- 

 nitrogenous moiety of the protein molecule may be converted into sugar or 

 glycogen. Thus, of the ammo-acids formed by the digestion of proteins, 

 glycine, alanine, aspartic acid and glutamic acid can be converted quanti- 

 tatively under appropriate circumstances into glucose. On the other hand, 

 leucine, phenylalanine and tyrosine yield no glucose, even in the diabetic 

 animal, but may in the liver undergo conversion into aceto-acetic acid, which 

 is a stage in the oxidative disintegration of fats. In spite of this latter fact 

 we have no 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 

 respiratory 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 carbohydrate to 4-7 per 

 cent. In another similar experiment the animal received 574 calories 

 protein, 54-2 calories fat, and 57 calories carbohydrate 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 ; 



