Eespiratory Quotient. 449 



larger on the second day of fasting than on the first day with but three excep- 

 tions, the increase being, in one instance, as much as 70 grams, and in another 

 about 61 grams. In practically all cases the oxygen intake diminishes after 

 the second day, reaching a constant, 466 grams, on the sixth and seventh days. 



Considering experiments with the same individual, S. A. B., it is to be noted 

 that even on the first day of fast, there are noticeable differences in the 

 amounts consumed, which range from 534 grams in experiment No. 75, to 589 

 grams in experiment No. 71. The excessive amount in the latter case may, 

 however, be explained, in part at least, by the fact that a 10-minute period 

 of hard muscular exercise on the bicycle ergometer was taken on this day. A 

 similar variation appears, however, between the amounts for the first day of 

 experiments Nos. 68 and 69, made with the subject A. L. L. 



Obviously, as was the case with the carbon dioxide elimination, differences 

 in the bodily activity and body-weight of the subjects may explain in large 

 part the variations in the amounts of oxygen consumed during the experiments, 

 and indeed during different experiments with the same subject. The rela- 

 tions between the oxygen consumption, carbon dioxide excretion, heat pro- 

 duction, and muscular activity are discussed in detail in a subsequent section. 

 The discussion of the relations between the body-weight and the oxygen intake 

 are also considered elsewhere. 



RESPIRATORY QUOTIENT. 

 The ratio between the volumes of carbon dioxide excreted and oxygen con- 

 sumed is of great value in indicating the nature of the material undergoing 

 katabolism in the body. The combustion of the carbohydrate is accompanied 

 by an absorption of oxygen which in the course of the oxidation results in the 

 production of an equal volume of carbon dioxide. On the other hand, when 

 fat is oxidized, the oxygen absorbed is used to oxidize not only the carbon of 

 the fat, but also the organic hydrogen. While the combustion of the carbon 

 results in the formation of an aeriform combustion product, carbon dioxide, 

 oxidation of the hydrogen results in the formation of water-vapor which is 

 not measured in the ordinary methods of studying the respiratory gases. 

 Consequently, the volume of carbon dioxide excreted is less than that of the 

 oxygen consumed. The ratio between the volumes of carbon dioxide and the 

 oxygen is called the respiratory quotient. It is commonly expressed by the 



CO 

 fraction ^-? . In the case of the carbohydrate, this respiratory quotient is 



equal to 1 since equal volumes of oxygen and carbon dioxide are involved in 

 the respiratory exchange. The combustion of fats, on the other hand, results 

 in a respiratory quotient which is always less than 1, and for the ordinary 

 animal fats, it has been computed to be about 0.711. 140 



140 For a more detailed discussion of the theoretical respiratory quotients to be 

 derived from the combustion of different kinds of fats and carbohydrates, see 

 U. S. Dept. Agr., Office of Expt. Sta. Bui. 136. 

 29 



