THE FASTING KATABOLISM 



257 



may mutually replace each other as fuel material according as 

 one or the other is most available, render it evident that the 

 controlling factor in the katabolism of the fasting body is the 

 demand for energy for the performance of the internal work and 

 can hardly have failed to suggest that this demand must be 

 relatively constant in the same individual under like conditions. 

 That such is in fact the case has been demonstrated by a large 

 number of experiments. While not mathematically invariable, 

 the fasting katabolism, expressed in terms of energy, tends 

 to approach a uniform value in proportion as the experimental 

 conditions are maintained constant. The fasting organism 

 requires approximately the same quantity of energy from day 

 to day for the performance of its necessary internal work, but 

 seems more or less indifferent as to whether this energy is 

 derived from the katabolism of fats, carbohydrates' or proteins. 



For example, in Voit's experiment cited in the previous section to 

 illustrate the interrelations of protein and fat katabolism (Table 33), 

 the computed energy of the protein and fat katabolized on each of 

 the three days was as shown in the following table, from which it 

 appears that the total energy katabolism, especially when computed 

 per kilogram of live weight, was approximately the same on the 

 different days. 



TABLE 34. ENERGY KATABOLISM OF FASTING DOG 



The same thing is true of Rubner's determinations of the fasting 

 katabolism of a rabbit, a dog and a guinea pig, whose results as re- 

 gards the protein katabolism have been already considered (337) and 

 likewise of Benedict's investigations upon fasting men (334). 



344. Energy expenditure in fasting a measure of main- 

 tenance requirement. In the fasting animal in a state of 



