NORMAL PROCESSES OF ENERGY METABOLISM 593 



between about 5.5 and 4.0 Cal. At a constant quotient the authors find 

 that it varies somewhat with the subject, and for the same subject it de- 

 creases with training (see page 588). 



The question may fairly be raised, Where does protein stand in the scale 

 of efficiency as a source of muscular work ? This question has been studied 

 in relation to the specific dynamic action of protein by Rubner(o) and more 

 recently by Anderson and Lusk. Both sets of observations show that there 

 is practically complete summation of the extra energy production due to 

 the specific dynamic action of meat and the energy production caused 

 by the muscular work. There is nothing specifically uneconomical in 

 doing work 011 a high protein diet except in the sense that the extra heat f of 

 dynamic action is added to the extra heat of muscular work and this throws 

 extra burdens on the organs charged with the dissipation of heat. With 

 cane sugar, as proved in Rubner's experiments or glucose as proved in 

 Lusk's, the specific dynamic effect of the food disappears, i. e., merges into, 

 the extra metabolism of muscular work. These facts make it clear 

 that the mechanism of energy release in muscular work is more nearly 

 akin to the mechanism by which carbohydrate raises the metabolism 

 (metabolism of plethora, see page 606) than it is to the mechanism of pro- 

 tein stimulation. The work of Fletcher and Hopkins and of A. V. Hill 

 on the details of muscular contraction make it appear that certain reac- 

 tions take place between definite substances which must be closely allied to 

 carbohydrates. It becomes more intelligible therefore why carbohydrate 

 should support muscular work more economically than fat 6 and why its 

 dynamic action, unlike that of protein, should not be superimposed upon 

 the metabolism of muscular work. 



III. The Energy Metabolism is Determined in Part by 

 the Environing Temperature 



1. How Heat is Lost from the Body. In general, there are four main 

 avenues of escape for the heat which is produced in the body of a warm- 

 blooded animal : ( 1 ) Warming the food and air which enter the body ; 

 (2) Vaporization of water and setting free of CO 2 in the lungs; (3) 

 Evaporation of water from the surface of the body; (4) Radiation and 

 conduction from the surface of the body. 



Tigerstedt(a) gives the following calculations made by Rubner for a 

 man producing 2,700 calories daily : 



"Krogh and Lindhard note that the standard metabolism (called basal metabolism 

 more commonly) is somewhat higher when the respiratory quotient is low than when 

 it lies in the median range. There is just a hint in this fact that the so-called waste of 

 energy when muscular work is supported By fat may be bound up with the specific 

 dynamic action of th&t foodstuff as it is in the case of protein. 



