Establishing Energy Values 199 



classes of nutrients to which they belong. If, then, 

 we multiply the calculated quantities of digestible pro- 

 tein, carbohydrates and fats by their respective as- 

 sumed energy factors, we get a number which may be 

 taken as an expression of the available energy of the 

 ration under consideration. This method cannot be 

 regarded as entirely accurate, because the calorific value 

 for protein may not be the same as that for any single 

 albuminoid, and the heat units of the nitrogen-free 

 extract are likely to vary materially from those found 

 for the starches and sugars, while the ether extract is 

 very far from representing the pure fats. At the same 

 time, it is possible in this way to learn the energy 

 value of a ration closely enough, perhaps, for all prac- 

 tical purposes. 



Energy value of digested nutrients. — A second 

 method, which is probably a step in the direction 

 of greater accuracy, is to determine by the use of 

 a calorimeter the heat units of the ration and also 

 of the urine and feces. The differences between the 

 food heat units and those found for the excreta are 

 assumed to represent the energy value of that por- 

 tion of the ration appropriated by the animal. Pro- 

 vided the heat units obtained in calorimeter combus- 

 tion and physiological combustion are equivalent, this 

 method must be considered as furnishing a reliable 

 energy measurement. However probable this equivalence 

 may seem, it has not been fully demonstrated. We 

 still need more complete experimental proof that the 

 oxidation of the several food compounds in ordinary 

 combustion and in the animal produces identical re- 



