POTENTIAL ENERGY OF FOOD. 845 



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 hun- 

 ger, 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 libera- 

 tion 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. 

 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 sub- 

 stance, take place constantly as a part of general nutritional metab- 

 olism. On the other hand, many of the chemical processes occur- 

 ring in the body are especially valuable on account of the heat 

 liberated. These reactions, for the most part, at least, are oxida- 

 tions ; they are effected under the influence of oxidizing enzymes or 

 by some other means of activating the oxygen. The various stages 

 in the process are not explained, but we know that oxygen is neces- 

 sary and that the carbon and the hydrogen contained in the sub- 

 stances acted upon appear eventually in the form of oxidation prod- 

 ucts namely, carbon dioxid and water Liebig designated the 

 fats and carbohydrates as respiratory foods on the hypothesis 

 that their fate in the body is to be oxidized and furnish heat. While 

 this view is, in the main, correct, it is evident now that a portion at 

 least of the protein molecule, after the splitting off of the nitrogen, 

 may also undergo oxidation and furnish heat. In Liebig' s sense, 

 therefore, the proteins play the part of respiratory or heat-producing 

 foods as well as acting as tissue formers. On the other hand, fats 

 and carbohydrate material may enter to some extent, together with 

 the protein, into the synthesis of living material, and thus play the 

 role of a plastic or tissue-forming as well as of a respiratory food. 

 We cannot divide the foodstuffs, therefore, strictly into two such 

 classes, but we may perhaps consider the chemical processes in 

 the body under the two heads mentioned above namely, the oxi- 

 dation or heat-producing changes and those due to hydrolytic 

 cleavages, synthesis, etc., which are attended by a small liberation 

 of heat energy, or, indeed, may cause a loss of heat (synthesis). 

 The great supply of heat energy needed by the body to maintain its 

 temperature comes from the oxidation processes. This classifica- 

 tion is employed by some physiologists, and is helpful in empha- 

 sizing the fact that many chemical changes occur in the body that 



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

 gern," 1890. See also Weber, " Ergebmsse der Physiologic," vol. i, part i, 1902. 



