706 PHYSIOLOGY OF RESPIRATION. 



throws light upon the nature of the material that is undergoing 

 oxidation in the body, as will be apparent from the following 

 considerations: Under ordinary conditions of rest and upon a 

 mixed diet the R. Q. varies between 0.65 and 0.95 (Loewy) or 

 between 0.75 and 0.89 (Magnus Levy). If, however, the material 

 oxidized in the body is entirely carbohydrate, the R. Q. should be 

 equal to unity: -q-' = 1. All the oxygen used in the combustion 

 might be considered as uniting with the C to form CO2, since 

 enough O is present in the sugar to account for that used in oxidiz- 

 ing the H to H2O. Or, as expressed in a reaction. 



Dextrose. 



CeHiA + 6O2 = 6CO2 + 6H2O. R. Q. = I = 1. 



The number of molecules of CO2 formed in the oxidation is equal 

 to the number of molecules of Oj used. If fats alone are oxidized 

 in the body the R. Q. should be low (0.7), since these substances 

 are poor in oxygen compared with the amount of C and H present 

 in the molecule. The combustion of palmitin may be represented 

 as follows : 



Palmitin, C3H,(C,eH3,0,)3 = a^H^Oe. 

 2(C5,H^06) + I45O2 = IO2CO2 + 98H2O. 

 R- Q- = iff = 0.703. 



In estimating the respiratory quotient for proteins one must 

 bear in mind the fact that these substances vary somewhat in 

 composition and, moreover, that they are not completely 

 oxidized in the body. Calculations based upon the amount of 

 unoxidized carbon and hydrogen escaping in the urine and 

 feces give the average figure of 0.801 for the R. Q. of proteins. 

 It is evident from these statements that an increase in the 

 proportion of carbohydrate food will cause the R. Q. to approach 

 unity, while an increase in protein and especially in fat will 

 lower its value. In this way we can understand the actual 

 variation observed in the average respiratory quotient of different 

 classes of animals, as shown in the following brief table (Loewy) : 



Horse, herbivorous — R.Q. =0.960 

 Sheep, " " =0.900 



Man, omnivorous " =0.800 

 Dog, carnivorous " =0.750. 



In starvation, when the body is living only on its own protein and 

 fat, the R. Q. is lower than under a normal diet, falling from about 

 0.8 to 0.7, and a similar drop is observed in diabetes, since in this con- 

 dition also the body is burning only protein and fats. By a deter- 

 mination of the respiratory quotient before and after varying certain 

 conditions one may ascertain whether the given condition causes 

 a change in the character of the body metabolism. For example, 

 Higgins finds that alcohol given to an individual may cause a fall 



