POTENTIAL ENERGY OF FOOD. 937 



process can be measurerd 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 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, ihe 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 eliminated, 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 J gm. of urea, 

 then the available heat energy of a gram of protein 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- 

 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- 

 ing a known amount of the protein the urine and feces were col- 

 lected and their heat value was determined in the same way. The 

 difference between the total heat value of the protein fed and the 

 heat value lost in its excreted products in the feces and urine gave 

 the actual heat energy obtained from the protein by the animal 

 body. Results obtained by this method give an average value 

 for 1 gm. protein of 4100 calories (4.1 C.), or, since protein contains 

 an average of 16 per cent, of nitrogen, we may say that 1 gm. of ni- 

 trogen ingested as protein has a heat value of 4.1 X 6.25 = 25.6 C. 



