552 JOHN R. MURLIN 



sorted. The total heat value of the urine arising from the incomplete 

 oxidation of proteins, its heat value represents that fraction of the po- 

 tential energy of the proteins absorbed which the body does not utilize. 

 Utilization thus is used in two senses. From the standpoint of absorp- 

 tion it is that part of the food which exceeds the amount excreted through 

 the bowel. From the standpoint of energy it is that part of the absorbed 

 food diminished by the potential energy of the bodies excreted in the 

 urine. Comparing the method of Rubner with that of Atwater, it is seen 

 that in the former calorimetric heat value equals heat of the specific food 

 ingested less the heat of the feces less heat value of the urine. According 

 to Atwater the calorimetric heat value equals the heat value of the utiliz- 

 able food less heat value of the urine. 



The method of Rubner is more direct and thermochemically is more 

 correct ; but it is impracticable in its application to man for it requires the 

 ingestion of a perfectly pure (salt free) foodstuff. The method of Atwater 

 is open to the objection that he assumes the same heat value for the pro- 

 teins of the feces as for the corresponding food protein. It has the ad- 

 vantage of simplicity, however, in that it employs a coefficient of utiliza- 

 tion and can be used for a mixed diet both in animals and man. 



Woods made 56 determinations of the heat value of the urine in At- 

 water's laboratory and found an average value per gram of N of 7.9 Cal. 

 If this 1 gram of ~N represents 6.25 gm. of protein destroyed, for each 

 gram of protein absorbed and burned there is a loss of (7.9 -f- 6.25 =) 

 1.25 Cal. 



The heat value of a food protein may then be found by the follow- 

 ing method. Protein of meat has (table above) a utilization of 97 per cent. 

 Its heat value is 5.65 Cal. The energy of the portion utilized is 5.65 X 

 0.97 = 5.48 Cal. per gram. But from this value must be deducted 

 the heat value of the urine, which according to Wood's determination 

 is 1.25 X 0.97 = 1.20 Cal. The physiological heat value of meat for the 

 human subject, therefore, is (5.48 1.20 Cal. =) 4.28 or in round num- 

 bers 4.25 Cal. 



The bomb heat value of cereal protein Atwater found to be 5.8 Cal. 

 per gram ; its utilization was 85 per cent ; therefore, its physiological heat 

 value would be (5.8 X 0.85) (1.25 X 0.85) = 3.87 Cal. per gram. 

 The mean physiological heat value for all animal proteins was given by 

 Atwater at 4.27 Cal. and that of all vegetable proteins at 3.74 Cal. or 

 4.05 Cal. per gram for food proteins generally.. It is now known, how- 

 ever, that the utilization of cereal protein such as that of bread is more 

 commonly 92 per cent rather than 85 per cent as found by Atwater. This 

 would change his figure for vegetable protein from 3.74 to 3.98 Cal. per 

 gram, and if the percentage of animal and vegetable proteins in the diet 

 be placed at 40 and 60 which more nearly accords with practice in most 



