VI. CALORIMETRIC MEASUREMENTS 201 



bomb at constant vohimpi (AE). The oxidation in animals, how- 

 ever, takes place at constant pressure. To obtain the heat of com- 

 bustion at constant pressure, or enthalpy, AH, one has to subtract 

 from AE the heat equivalent of the work of expansion against the 

 atmosphere. For carbohydrates this work is zero since the volume 

 of carbon dioxide produced is equal to the volume of oxygen con- 

 sumed. Thus, in this case the enthalpy is equal to the heat of com- 

 bustion measured in the bomb. In the combustion of fats, a greater 

 volume of oxygen is consumed than of carbon dioxide produced. 

 Under constant pressure there is, thus, a decrease in volume, and the 

 heat equivalent of the work done by the atmosphere in compressing 

 the sj'stem has to be added to the heat of combustion as measured at 

 constant volume. The heat equivalent of this work, however, 

 amounts to only 0.14% of the heat of combustion. 



For proteins, the heat production in the calorimeter bomb is 

 considerably greater than the heat the animal can derive from protein 

 catabolism. A certain amount of the chemical energy of the protein 

 is lost as chemical energy in urine. This loss is calculated and the 

 resulting catabolizable energy equivalents are given in Table IV. 

 Catabolizable energy or metabolizable energy measures the physio- 

 logical fuel equivalents of nutrients. 



TABLE V 



Estimation of Heat Production from Fat and Protein Catabolism 



From the "respiratory quotient" (R.Q.) one may estimate the 

 proportion of fat and protein that a starving animal metabolizes pro- 

 vided its glycogen content does not change significantly. This is the 

 rule in "postabsorptive condition." From these results one may 

 further calculate the caloric equivalents of carbon dioxide and oxy- 

 gen for a given R.Q. This is done in Table V. 



If the nitrogen balance is measured, one may subtract from the 



