936 NUTRITION AND HEAT REGULATION. 



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. This possibility is emphasized by modern 

 theories regarding the intermediary metabolism of the carbohy- 

 drates (p. 897), according to which it would appear that the 

 lactic acid formed from the carbohydrates may be converted to 

 an amino-acid, after passing through a ketonic acid stage. 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 libera- 

 tion of heat, or, indeed, may be accompanied by an absorption 

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

 by the body to maintain its temperature comes from the oxid- 

 ation 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 

 to raise the temperature of one thousand grams of water one de- 

 gree. In round numbers an adult man produces in his body and 

 gives off to the surrounding air about 2,400,000 calories (2400 C.) 

 of heat per day. This great supply of heat is derived from the 

 physiological oxidation of the carbohydrate, fat, and protein 

 material of the food. These same materials may be oxidized 

 outside the body by burning them at a high temperature or under 

 a high pressure of oxygen, and the heat that they give off in the 



