14 



MAINTENANCE RATIONS OF FARM ANIMALS. 



carbohydrates, and protein mutually replace each other as fuel ma- 

 terial as one or the other is most available fully substantiate the 

 assertion made on page 9 that the controlling factor in the katabo- 

 lism of the fasting body is the demand for energy. As there stated, 

 the body is essentially a converter of energy, and protein occupies a 

 peculiar position in nutrition simply so far as it is a part of the 

 necessary mechanism for this conversion. These facts can hardly 

 have failed to suggest that the demand for energy must be relatively 

 constant in the same individual, and that such is in fact the case has 

 been demonstrated by a large number of experiments. 



For example, in the experiment by Voit upon a dog, just cited, the energy of 

 the protein and fat katabolized on the three days, as computed from the data 

 for the urinary nitrogen and for the fat katabolism, was as shown in the fol- 

 lowing table, from which it appears that the total energy katabolized, especially 

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

 the different days. 



Constancy of katabolism of fasting dog — Voit. 



Day. 



Live 

 weight. 



Energy 



from 

 protein. 



Energy 

 from 

 fat. 



Total 

 energy. 



Total en- 

 ergy per 

 kilogram 



live 

 weight. 







Kilos. 



Calorics. 1 



Calories. 1 



Calories. 1 



Calories. 1 



Second day 





32. 87 



289.3 



816.9 



1,106.2 



33.66 



Fifth day.. 





31.67 



142.2 



978.5 



1,120.7 



35. 38 



Eighth day 





30.54 



117.2 



942.4 



1,059.6 



34.70 



1 Throughout this bulletin the word " calorie " signifies the large, or kilogram, calorie, 

 unless the contrary is specifically stated. 



The same constancy is illustrated by Rubner's experiments on a rabbit, a 

 dog, and a guinea pig, whose relative protein katabolism was tabulated on 

 page 13. The latter is repeated in the following table, together with the 

 heat production as measured directly or the carbon dioxid excreted, which may 

 be assumed to be an approximate measure of the energy katabolized. As the 

 table shows, notwithstanding very considerable variations in the relative 

 amount of protein katabolized, the total energy liberated in the body was rela- 

 tively very constant. 



