POTENTIAL ENERGY OF FOOD. 935 



mals has shown that a greater quantity of protein is destroyed dur- 

 ing the first day or two than in the subsequent days. This fact 

 is explained on the supposition that the body is at first supplied 

 with a certain excess of protein material, circulating protein, de- 

 rived from its previous food, and that after this is metabolized 

 the animal lives entirely, so far as protein consumption is con- 

 cerned, upon its "tissue protein." If the animal remains quiet 

 during starvation, the amount of nitrogen excreted daily soon 

 reaches a nearly constant minimum, showing that a practically 

 constant amount of protein (together with fat) is consumed daily 

 to furnish body heat, and material for the energy needs and tissue 

 waste in the active organs, such as the heart. Shortly before 

 death from starvation the daily amount of protein consumed may 

 increase, as shown by the larger amount of nitrogen eliminated. 

 This fact is explained by assuming that the body fat is then ex- 

 hausted and the animal's metabolism is confined to the tissue 

 proteins alone. The general fact that the loss of protein is greatest 

 during the first one or two days of starvation has been confirmed 

 upon men in a number of interesting experiments made upon 

 professional fasters. For the numerous details as to loss of weight, 

 variations of temperature, etc., carefully recorded in these latter 

 experiments, reference must be made to original sources* It may 

 be added, in conclusion, that the fatter the body is, to begin with, 

 the longer will starvation be endured, and if water is consumed 

 freely the evil effects of starvation, as well as the disagreeable 

 sensations of hunger, are very much reduced. 



The Potential Energy of Food. The food material during 

 digestion and after absorption undergoes numerous chemical 

 changes in the body. .Some of these changes are not attended by 

 the liberation of heat to any marked extent. Such is the case, for 

 instance, with the hydrolytic cleavages of the molecule which 

 have been described especially in connection with the digestive 

 processes. As an example of this fact one may take the inversion 

 of the double sugars one molecule of maltose yields two molecules 

 of dextrose. The heat value of a gram molecule of maltose is 

 1350.7 calories. The heat value of the dextrose resulting from its 

 inversion is 1347.4 cal., so that the process of hydrolysis liberates 

 only 3.3 cal. or about 0.2 per cent, of the total available energy in 

 the maltose, f Similar hydrolytic cleavages occur doubtless within 

 the tissues, and other changes connected with muscular, nervous, 

 and glandular activity, and the building up and breaking down 

 of the living substance take place constantly as a part of general 



*"Virchow's Archiv," vol. 131, supplement, 1893; and Luciani, "Das 

 Hungern, " 1890. See also Weber, "Ergebnisse der Physiologic," vol. i., 

 part i., 1902. 



fSee Herzog, "Zeit. f. physiol. chem.," 37,383,1903, and Tangl, 

 "Pfliiger's Archiv," 115, 1, 1906. 



