334 FOOD INGESTION AND ENERGY TRANSFORMATIONS. 



disposition of the total calories ingested or to the amount available or 

 useful to the body. 



Writers and experimenters in animal physiology, particularly in 

 animal nutrition, have considered the energy of intake under various 

 heads, and attempted its apportionment in some measure to the several 

 processes of digestion and absorption. It has long been assumed that 

 an increment in the heat production which is not directly available for 

 muscular work is of little, if any, value to the animal economy. Writers 

 have therefore been inclined to consider more especially that portion 

 of the food intake which participates in the heat produced inside the 

 body by muscular and glandular activity in distinction from the food 

 taking part in the production of heat in fermentative activities. Such 

 attempts to separate the various subdivisions of the energy consump- 

 tion produce great confusion. Perhaps no one has given this phase 

 of the matter more comprehensive treatment than Armsby in his 

 admirable treatise. 1 He considers as metabolizable energy that frac- 

 tion of the energy of the food which can enter into the metabolism of 

 energy in the body, without differentiating as to the use made by the 

 body of the energy thus metabolized. As the food of man contains but 

 little unoxidizable material, like cellulose or fiber, the human diet may 

 be considered as practically all digestible with the exception of the nitro- 

 genous portion of the protein molecule which is excreted unoxidized in 

 the form of urea. This material is still capable of being converted 

 into heat, for each gram of urea has an energy value of 2.528 calories. 2 

 In computing the caloric value of the food intake, therefore, due allow- 

 ance must be made for the unoxidizable material in the protein. 



A consideration of the heat production of the human body deals 

 chiefly with the disposition of the energy liberated after the food is 

 absorbed. For convenience, we may consider that the ingestion of a 

 definite amount of food produces an increase in the metabolism which 

 may be chargeable to the food itself. If this is expressed in terms of 

 calories, the total caloric value of the intake of food may properly be 

 compared with that of the excess heat production. In this publication 

 we have used for this purpose not the heat of combustion of the diet, 

 but the so-called "fuel value," i. e., the heat of combustion less the 

 unoxidized portion of the protein. 



In calculating the fuel values for the diets used in this research, two 

 methods were employed. If the heat of combustion had not been 

 determined, the energy derived from the protein, fat, and carbohy- 

 drate, respectively, was computed by means of the standard factors of 

 Rubner, 3 the factor 4.1 being used for multiplying the grams of both 

 the protein and the carbohydrate, and the factor 9.3 for multiplying the 



'Armsby, The principles of animal nutrition, 2d cd., 1900. 

 2 Emery and Benedict, Am. Journ. Physiol., 1911, 28, p. 301. 

 3 Rubner, Zeitschr. f. Biol., 1885, 21, p. 377. 



