ENERGY RELATIONSHIPS. 335 



grams of fat in the diet. The sum of the calories found represented the 

 total fuel value of the food. 



If, however, the heat of combustion of the diet had been determined, 

 another method was followed. Since the heat of combustion of pro- 

 tein is 5.5 calories per gram, the difference between 4.1 (the Rubner 

 factor used for calculating the energy derived from protein) and 5.5, 

 namely, 1.4, corresponds to the potential energy of the unoxidized por- 

 tion of the protein molecule. With carbohydrates and fat the fuel 

 value and the heat of combustion are essentially alike, although at 

 times investigators have made slight allowances for the so-called 

 "digestibility" of fat. Such correction of the values for fat is, how- 

 ever, a questionable procedure, and thus in calculating the fuel value 

 from the heat of combustion we need only make correction for the 

 unoxidized protein. The loss of energy from the unoxidized protein 

 was found by multiplying the protein in 1 gram of the food by 1 .4 (the 

 potential energy of the unoxidized portion of the protein molecule) ; 

 the resulting value deducted from the heat of combustion represented 

 the fuel value of the diet per gram. (See table 50, page 124.) The 

 fuel value of the total intake of food was then found by multiplying 

 the grams of food ingested by the fuel value per gram. 



If we compare the fuel value of the diet with the subsequent increase 

 in the heat production, we obtain a mathematical relationship which 

 may properly be designated as the "cost of digestion." This designa- 

 tion is in harmony with a convenient phraseology for similar relation- 

 ships which is finding increased usage in all economic and many 

 industrial processes and is beginning to be used by physiologists. 1 



For a true measure of the cost of digestion, it is necessary to have an 

 accurate measure of the total heat production. We may not therefore 

 content ourselves, as is too frequently done, with the simple measure- 

 ment of the maximum or peak effect of the food ingested, but it is abso- 

 lutely necessary to continue the measurements until the basal values 

 are again reached and the total increment in the heat which is charge- 

 able to the ingestion of the particular diet studied has been obtained for 

 the entire period of measurement. Unfortunately, in a considerable 

 number of our observations the experimental period was not continued 

 a sufficient length of time to insure the return of the metabolism to the 

 basal value and hence in the large majority of cases our measurement of 

 the cost of digestion is a low rather than a maximum value. This 

 should be taken into consideration in any estimate of our values for the 

 cost of digestion. 



The data regarding the cost of digestion in the studies made of the 

 various nutrients and diets have been collected and tabulated. 



'See MacDonald, Proc. Roy. Soc. (B), 1915-17, 89, p. 394. 



