The agreement between the results obtained by direct and indirect calorimetry in the calorimeter experiments was, in many instances, extremely unsatisfactory, so much so that for a long time we were disposed to question the value of our calorimeter measurements, particularly those with the Boston calorimeters. Subsequent experimentation has shown, however, that direct and indirect calorimetry may not necessarily agree under the abnormal conditions previously outlined which obtain when excessive amounts of carbohydrates are ingested.

To secure a satisfactory agreement between direct and indirect calorimetry is a problem that has received a great deal of attention ever since the earliest days of direct measurements of the heat output of man. The attempt was made in all of our experiments to determine the heat output directly with as high a degree of accuracy as possible. The respiration calorimeter at Middletown was designed primarily for 24-hour periods. On this basis the agreement between direct and indirect calorimetry has almost invariably proved satisfactory, especially after the apparatus was modified to permit the direct measurement of the oxygen consumption. Previous to the beginning of this research on the influence of food upon the metabolism, no attempt was made to compare direct and indirect calorimetry in periods shorter than 24 hours. When such an attempt was made, it was found that at least with the Middletown calorimeter, which had an air content of approximately 5,000 liters, great difficulty was experienced in the measurement of the residual air and particularly of the residual oxygen, and the possibility of experimental error was thus increased as the periods were decreased in length. Direct measurements of the heat production are also complicated by the difficulty in obtaining accurate measurements of the rectal temperature. Furthermore, the ingestion of large masses of food at a temperature above or below that of the body increases the difficulty, as the length of time required to bring the ingested food and the stomach wall to the temperature of the body is a matter of considerable speculation. Still, the general coincidence of the results obtained with both direct and indirect calorimetry lends credence to any deduction drawn from either. It should be said, further, that the researches conducted under the skillful guidance of Dr. E. F. Du Bois, at the Russell Sage Institute of Pathology in New York, have definitely demonstrated the fact that accurate comparisons of the direct and indirect calorimetry can be secured, even in periods as short as one hour.

Such values for the heat production as were obtained in this research by the indirect method were not computed with the idea of establishing a comparison between the direct and indirect heat values, but simply to obtain a general picture of the course of the metabolism after the ingestion of food. If both the direct and the indirect calorimetry show an increment in the metabolism, there is every reason to believe that such an increment actually took place. While the results obtained with the two methods by no means always agree closely, they yet supply a rough confirmation of each other. As a rule, the tabulated values for the heat production in the calorimeter experiments are those obtained by direct measurement. In one case both sets of figures are given for illustration (see table 101, page 179). Unless otherwise stated, the values for the heat measurements are for the heat actually produced - that is, the measured heat elimination corrected, in accordance with the usage of this laboratory,1 for changes in body-weight and body-temperature.

1Benedict and Joslin, Carnegie Inst. Wash. Pub. No. 136, 1910, p. 20.

Since it is the custom of many writers to compute the non-protein respiratory quotient and determine the non-protein metabolism in experiments of this kind, values for the nitrogen excretion in the urine have been given whenever obtainable, but with no idea of indicating the influence of the ingested food. Although basal values for nitrogen have been included in the tables whenever available, no effort was made to obtain such data for our experiments. In this we find ourselves at variance with Gigon, who assumed that the basal value for nitrogen was constant. It should be emphasized however, that this research was not planned to study the influence upon the protein katabolism of the ingestion of the various foods studied. The non-protein respiratory quotient is not of special significance in this research and it is deemed unwise to expand the data by including it, especially as it may be computed from the values for the nitrogen excretion as follows:

From the computations of Zuntz it is assumed, for the period in which the non-protein quotient is desired, that for each gram of nitrogen determined in the urine 5.91 liters of oxygen are absorbed and 4.75 liters of carbon dioxide are produced. The values obtained by multiplying these amounts of oxygen and carbon dioxide by the grams of nitrogen are considered to represent the carbon dioxide produced and oxygen consumed in the disintegration of the protein. Since the total oxygen consumption and carbon-dioxide production are determined, the subtraction of the amounts resulting from the katabolism of protein gives the liters of oxygen absorbed and carbon dioxide produced in the katabolism of fat and carbohydrate; the quotient from the division of the amounts so obtained, CO/O2, will thus be the non-protein respiratory quotient.

If it is further desired to compute the heat produced by the katabolism of body material, the grams of nitrogen in the urine multiplied by 26.51 calories1 will give the heat production resulting from the oxidation of protein. By employing the calorific value of oxygen found in the table of Zuntz2 for the non-protein quotient obtained in the above calculation, the heat that should result from the katabolism of the fat and carbohydrate is obtained. The sum of these computed values for protein and for fat and carbohydrate constitutes the heat produced (computed) for the period under observation.

In discussing the results of the experiments with carbohydrates, the experiments made with the Middletown and Boston calorimeters will first be considered and subsequently those made with the respiration apparatus in Boston. Except in one instance, the experiments in Middletown were carried out in 2-hour periods; in the Boston experiments the periods were only an hour in length, and the basal metabolism was usually determined on the same day.

1Loewy, Oppenheimer's Handbuch der Biochemie, 1911, 4 (1), p. 279. 2Zuntz and Schumburg, Physiologie des Marsches, 1901, p. 361.