UTILIZATION OF FOOD 421 



in the amount of heat evolved is due to processes essential to the 

 digestion of the food, since at 33 no heat is required to maintain 

 the body temperature of the animal. The heat evolved during 

 starvation being put at 100, the heat evolved during the same 

 length of time when food containing an equal amount of available 

 energy (100) was fed was found by experiments of Rubner 1 to 

 be as follows : 



Heat evolved 

 in starvation 100 



Heat when 100 calories flesh was fed 130.9 



Heat when 100 calories fat was fed 112.7 



Heat when 100 calories gluten was fed 1 28.0 



Heat when 100 calories cane sugar was fed 105.8 



That is to say, the consumption of the food caused an in- 

 creased production of heat, which was probably due to digestion 

 of the food. Since 100 calories of available energy was fed, the 

 increased quantity of heat evolved represents heat evolved by the 

 digestion; that is, the percentage of thermal energy contained in 

 the available energy. This ranged from 5.8 per cent, with the 

 sugar to 30.9 per cent, with the flesh. 



The second method is applicable to herbivorous animals, but 

 gives only indications. The animal is fed on a ration, and the 

 heat evolved from the body is measured. The feed to be 

 studied is added to the ration and the heat is again determined. 

 The increase in heat is due to the digestion of the added feed. 



The following figures of Armsby 2 afford an illustration : 



An increase of 2,264 calories in the digested food caused an 

 increase of 1,139 calories in the heat given off by the animal. 

 That is, about 34 per cent, of the energy of the food was con- 

 sumed in digestive processes. 



1 Quoted by Kellner, Die Ernahrung d. Niitztiere, p. 105. 



2 Proc. Soc. Prom. Agr. Sci., 1902, p. 100. 



