POTENTIAL ENERGY OF FOOD. 947 



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 

 nutritional metabolism. On the other hand, many of the chemical 

 processes occurring in the body are especially valuable on account 

 of the heat liberated. These reactions, for the most part, at 

 least, are oxidations; 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 necessary and that the carbon and 

 the hydrogen contained in the substances acted upon appear 

 eventually in the form of oxidation products 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-pro- 

 ducing 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 cell 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 oxidation or heat-producing changes and those 

 due to hydrolytic cleavages, synthesis, etc., which are attended by 

 a small liberation of heat, or, indeed, may be accompanied by an ab- 

 sorption of energy (synthesis). The great supply of heat energy 

 needed by the body to maintain its temperature comes from the 

 oxidation processes. The heat produced in and given off from the 

 body is estimated in terms of calories. The small calorie (c) or 

 gram-calorie is the quantity of heat necessary to raise one gram 

 of water one degree centigrade in temperature, while the large 

 calorie (C) or kilogram-calorie is the quantity of heat necessary 



* See Herzog, "Zeit. f. physiol. chem.," 37, 383, 1903, and Tangl, "Pfliiger's 

 Archiv," 115, 1, 1906. 



