A relationship of unusual interest is that of the increase in the heat production following the ingestion of food to that of the fuel value of the food taken. While it may seem at first sight a gross misuse of engineering terms or terms of efficiency to apply them to the apportionment of the caloric value of the ingested food of man, one might consider from an engineering standpoint or from that of industrial efficiency that the ingestion of food containing a certain number of calories would result in a certain amount of excess heat. Excess heat production represents an expenditure, either necessitated by the ingestion of food or resulting from the ingestion of food, and hence may logically be attributed to and in a sense chargeable to it.

In considering the metabolism subsequent to the ingestion of food, one should bear in mind the following facts: A considerable portion of the diet, at least with ruminants, is distinctly indigestible, this portion consisting of woody fiber, cellulose, etc. Secondly, only part of the protein of the diet is oxidized inside the body. This is true of all animal life, the unoxidized portion of the protein molecule being with mammals excreted chiefly in the form of urea. Furthermore, and this applies more particularly to ruminants, fermentation processes take place in the large intestine and cause a considerable production of marsh gas and a liberation of heat as the result of bacterial action. Finally, the ingestion of food per se causes an increase in the heat production. It is clear, therefore, that a measure of the heat of combustion of the intake has but little significance in relation to the ultimate 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 consumption 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 fraction 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 nitrogenous 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 allowance 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 carbohydrate, 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 grams of fat in the diet. The sum of the calories found represented the total fuel value of the food.

1Armsby, The principles of animal nutrition, 2d ed., 1906. 2Emery and Benedict, Am. Journ. Physiol., 1911, 28, p. 301. 3Rubner, Zeitschr. f. Biol., 1885, 21, p. 377.

If, however, the heat of combustion of the diet had been determined, another method was followed. Since the heat of combustion of protein 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 portion 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, however, 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.