96 PHYSIOLOGY FOR DENTAL STUDENTS. 



behavior of diabetic patients, where the power to use carbohy- 

 drates is much depressed). The marked protein-sparing action 

 of carbohydrates is illustrated in another way, namely, by the 

 fact that we can greatly diminish the protein break-down during 

 starvation by giving carbohydrates. In this way we can indeed 

 reduce the daily nitrogen excretion to about one-third what it is 

 in complete starvation. 



In the case of man living on an average diet, although the 

 daily nitrogen excretion is about 15 grammes, it can be lowered 

 to about 6 grammes provided that in place of the protein that has 

 been removed from the diet enough carbohydrate is given to bring 

 the total calories up to the normal daily requirement. If an excess 

 of carbohydrate over these energy requirements be given, the 

 protein may be still further reduced and yet equilibrium main- 

 tained. To do this, however, it is not the amount of carbohy- 

 drate alone that determines the ease with which the irreducible 

 protein minimum can be reached ; the kind of protein itself makes 

 a very great difference. This has been very beautifully shown 

 by one investigator, who first of all, determined his nitrogen ex- 

 cretion while living on nothing but starch and sugar, and then 

 proceeded to see how little of differnt kinds of protein he had 

 to take in order to bring himself into nitrogenous equilibrium. 

 He found that he had to take the following amounts : 30 gr. meat 

 protein, 31 gr. milk protein, 34 gr. rice protein, 38 gr. potato 

 protein, 54 gr. bean protein, 76 gr. bread protein, and 102 gr. 

 Indian corn protein. The organism is evidently able to satisfy 

 its protein demands when it takes meat protein much more 

 readily than with vegetable proteins. 



To understand why proteins should vary so much in Uitir 

 nutritive value, we must examine their ultimate structure very 

 closely. When the protein molecule is disintegrated, as by diges- 

 tion, it yields a great number of nitrogen-containing acids, the 

 amino acids, as well as several bases and aromatic substance's. 

 The most important of these acids are glycin, alanin, serin, valin, 

 leucin, prolin, aspartic and glutamic acids, the bases being lysin, 

 histidin and arginin and the aromatic bodies, phenylalanin, tyro- 

 sin and tryptophan. These substances constitute the available 



