CAUSES OF INCREASED METABOLISM. 



31 



INTERMEDIARY METABOLISM. 



The chemical reactions taking place during the so-called intermediary metabo- 

 lism of the resorbed material before it is finally utilized for the vital processes 

 have also to be considered as possible sources of heat production, although our 

 present knowledge of them is meager. 



This possibility is of special interest in connection with the marked effect of 

 protein on the energy metabolism, since this can hardly be ascribed to digestive 

 work in the strict sense. In the normal digestion of protein fermentations play 

 a very small part, while, as just shown, the digestive cleavage of protein is 

 substantially isothermic. Neither can we imagine that the mechanical work 

 of digestion or the secretion of digestive juices can account for the large 

 expenditure of energy. Rubner 1 has reported experiments in which the protein 

 katabolism of the fasting animal was artificially increased by the administra- 

 tion of phlorhizin, and in which a similar increase in the heat production is 

 computed, although there could have been no digestive work in the strict sense. 

 Falta, Grote, and Stahlein 2 have found that the products of the tryptic diges- 

 tion of casein when fed to a dog produce nearly as great an increase in the 

 metabolism as does a corresponding amount of casein, while in the familiar 

 experiments of Zuntz and Mering 3 the intravenous injection of the crude prod- 

 ucts of the peptic digestion of blood fibrin had a like effect. 



The katabolism of protein seems to consist in outline, first, of a hydrolytic 

 cleavage into peptids and amino-acids and, second, in a deamidization of these 

 latter compounds, and it is the nonnitrogenous products resulting from this 

 deamidization which serve as a source of energy for the body, the nitrogen 

 being split off as ammonia and excreted as urea. It is to a liberation of 

 energy in the form of heat in these preliminary processes of preparing protein 

 to serve as fuel that Rubner and other authors ascribe its specific dynamic 

 effect. 



Our knowledge of the intermediary metabolism of protein is too meager to 

 render any quantitative estimate of the amount of energy lost in this way of 

 much value. The cleavage of protein, as noted, seems to be substantially iso- 

 thermic. The deamidization of the simpler amino-acids with a small number 

 of carbon atoms seems at first thought to involve considerable loss of energy. 

 For example, the potential energy of 1 gram of glycocol and of alanin and of 

 equivalent amounts of acetic and propionic acids are: 



Glycocol. 



Alanin. 



Energy of amino acid 



Energy of equivalent fatty acid 



Difference 



Percentage loss 



Calories. 

 3.129 

 2.791 



Calories. 

 4.356 

 4. 129 



10.! 



.338 



.227 

 5.2 



A similar comparison of alanin with the equivalent amount of lactic acid 

 shows an apparent loss of about 14 per cent. With the higher members of the 

 series, the loss computed in this way is relatively small. It must be remem- 

 bered, however, that the amino group is split off as ammonia, which also con- 



1 Gesetze des Energieverbrauchs bei der Ernahrung. 



2 Beitrage zur Chemischen Physiologie und Pathologie, vol. 9, p. 372. 



3 Archiv fur die gesammte Physiologie des Menschen und der Thiere (Pfliiger), vol. 32, 

 p. 199, 



