940 NUTRITION AND HEAT REGULATION. 



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

 gree. In round numbers an adult man not doing muscular work 

 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 car- 

 bohydrate, 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 process can be measured directly. So far as 

 the fats and carbohydrates 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 pro- 

 duced 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 combustion equivalent, and it measures the amount of potential 

 energy of these foodstuffs which is available for the production of 

 heat or for the supply of energy in other forms to the working cells. 

 With regard to the protein, 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 nitrog- 

 enous wastes are capable of further oxidation with liberation 

 of heat, so that, as far as they are eHminated, the body 

 loses a possible supply of heat energy, which must be subtracted 

 from the total heat energy that the protein gives upon oxida- 

 tion 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 protein when burnt outside of 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- 

 tein appears as urea and that 1 gm. of protein yields \ gm. of urea, 

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

 to 5778 — 841 (or ^ 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- 

 portion is given off in the feces. Rubner has calculated the avail- 

 able heat energy of proteins by direct experiments upon animals. 

 In these experiments the heat value of the protein fed was directly 

 determined by burning a sample in a calorimeter. Then after feed- 



