HEAT ENERGY OF FOODS. 815 



are of no importance from the standpoint of heat production, and 

 that the changes that do give rise mainly to heat form, as it were, 

 a special group, which is not connected with the building up or 

 breaking down of the living matter, but furnishes the energy by 

 means of which these latter changes and perhaps other functions, 

 such as muscular work, are made possible. Roughly speaking, an 

 adult man forms in his body and gives off to the surrounding air 

 about 2,400,000 calories of heat per day. By calorie or small 

 calorie (c) is meant the quantity of heat necessary to raise 1 

 gm. of water 1 in temperature. This great supply of heat is derived 

 from the physiological oxidation of the carbohydrate, fat, and 

 proteid material of the food. These same materials may be oxi- 

 dized 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 process can be measured directly. So far as the fats and carbo- 

 hydrates are concerned, the end-products of the oxidation in the 

 body are the same as in their combustion out of the body, and we 

 may believe, therefore, that the amount of heat produced is the 

 same in both cases. Consequently the heat value of a gram of fat 

 or carbohydrate burnt outside the body is spoken of as its combus- 

 tion equivalent, and it measures the amount of potential energy 

 of these foodstuffs with regard to their capacity for the production 

 of heat or of muscular work in the body. With regard to the pro- 

 teid, the case is somewhat different. Its end-products in the body 

 are carbon dioxid, water, and urea or some other of the nitrogenous 

 waste products. These nitrogenous wastes are capable of further 

 oxidation with liberation of heat, so that, as far as they are elimi- 

 nated, the body loses a possible supply of heat energy, which must 

 be subtracted from the total heat energy that the proteid gives upon 

 oxidation outside the body, in order to determine the available heat 

 energy yielded within the body. The figures obtained for the heat 

 equivalents of the foodstuffs by burning them outside the body in 

 some form of calorimeter are as follows : 1 gm. of fat yields an aver- 

 age of 9300 calories, or 9.3 large calories (C), 1 gm. of carbohydrate 

 yields an average of 4100 calories (4.1 C.). These figures may be 

 taken, therefore, to express the quantity of heat given to the body 

 by the oxidation within its tissues of these elements of our food. 

 A gram of proteid when burnt outside the body yields on the aver- 

 age 5778 calories. The heat value of the urea is estimated as 1 

 gm. = 2523 calories. If we assume that all the nitrogen of the pro- 

 teid appears as urea and that 1 gm. of proteid yields gm. of urea, 

 then the available heat energy of a gram of proteid should be equal 

 to 5778 841 (or J of 2523) = 4937 calories. Later workers, however, 

 have given reasons for believing that this last figure is too high. 

 All of the nitrogen is not eliminated as urea, and, moreover, all of 

 the nitrogenous waste is not excreted in the urine; a distinct pro- 



