946 NUTRITION AND HEAT REGULATION. 



(97 per cent.), which had practically disappeared from the body. 

 It is very significant that the central nervous system and the heart^ 

 organs which we may suppose were in continual activity, suffered 

 practically no loss of weight: they had lived at the expense of the 

 other tissues. We must suppose that in a starving animal the fat 

 and the protein materials, particularly in the voluntary muscles, 

 pass into solution in the blood, probably as a result of intra- 

 cellular digestion (autolysis), and are then used to nourish the 

 tissues generally and to supply the heat necessary to maintain the 

 body temperature. Examination of the excreta in starving ani- 

 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 



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

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

 1902; Benedict, "A Study of Prolonged Fasting," Carnegie Institution, No. 

 203, 1915. 



