346 



LECTURE XV. 



and at another time greater with a fat diet. On an average it was less 

 with the carbohydrate diet. Apparently sugar is a better sparer of body 

 fat than fat itself. The difference is, however, very slight, and we may 

 conclude from the experiments that isodynamic quantities of fat and 

 carbohydrates are equivalent in this respect. 



The question that next arises is with regard to the way in which carbo- 

 hydrates and fats behave when they are fed simultaneously. It would 

 seem possible that they would be decomposed equally, and the liberated 

 energy utilized, sometimes for one purpose and sometimes for another. 

 It is also conceivable that one substance may be used up more rapidly 

 than the other. This is a very hard problem to decide. We may determine 

 the amount of nitrogen and carbon eliminated in the urine and fseces, and 

 deduct from the total amount of carbon that corresponding to the protein 

 (as indicated by the nitrogen value). The carbohydrates and fats would 

 be represented by the rest of the carbon. It would be possible to decide 

 which nutrient gave rise to the greater part of the carbon dioxide formed 

 if we knew the amount of oxygen which was consumed at the same time. 

 If carbohydrates were exclusively oxidized, the ratio of the volume of oxygen 

 taken up, to that of carbon dioxide produced, would be equal to 1. The 

 ratio of CO 2 : Oo is called the " respiratory quotient." It is only 0.71 in 

 the case of the fats. Experiments which have been made in this direction 



