The dried samples of foods and feces were analyzed for total nitrogen by the Kjeldahl process. Here again the nitrogen content of samples a and b supplied a check upon each other.

Heat Of Combustion

To obtain an energy balance, it was necessary that we should find the actual calories in the intake of food. Of the output it was possible for us to determine only the calories in the feces. Since, however, there are reasonably constant standard factors for computing the calories from the percentage of nitrogen in urine, we resorted to this method rather than attempt to dry down the 1,000 or more samples of urine obtained in the research and determine the heats of combustion with the bomb calorimeter. The daily output of energy in urine was found by multiplying the total number of grams of nitrogen by the factor 8.0. The energy as thus calculated was almost invariably somewhat under 100 calories, so that the error due to the method cannot at best be an appreciable portion of the total energy under consideration for the day.

The heats of combustion of the feces and dried foods were determined with a bomb calorimeter of the Kroker type in an adiabatic calorimeter. This calorimeter was developed in the Nutrition Laboratory and promotes rapid operation.1 The technique was finally so adjusted that after the various dried pellets of feces and food had been prepared and weighed and placed in nickel capsules, Miss M. A. Corson and her assistant were able to determine and compute four heats of combustion per hour. This made it possible to complete this extensive series of determinations within a reasonable time.

Since the total nitrogen and calories were obtained, it was deemed unnecessary to make an exact apportionment of the energy of the intake between protein on the one hand and fat and carbohydrate on the other. It is perfectly possible, knowing the total caloric value and nitrogen of the intake, to compute the calories due to protein. The remainder will be due to fat and carbohydrate. These were all mixed diets, with no special dietary adjustments other than decrease in the portions served. It hardly seemed advantageous to determine the fat in the food intake; indeed, the time requirement for such determination for all of the samples would alone preclude this additional work. A few special fat extractions were made which will be mentioned in the text from time to time, but there was nothing to indicate that exact information regarding the relative proportion of calories from fat and carbohydrate would have a special significance in the discussion of the results.

1 Benedict and Higgina, Journ. Am. Chem. Soc, 1910, 32, p. 461.