LOO cp 
EXPERIMENTS ON DIGESTION OF FOOD BY MAN. 155 
ordinarily followed in this laboratory.* For supper of the day 
previous to the commencement of each experiment charcoal 
was given, usually with bread and milk, in order to separate 
the feces of the food preceding the experiment from those of 
the experiment proper. At the end of each experiment an- | 
other separation was made, charcoal being given with milk 
and bread for either the supper of the last day of the experi- 
ment or the breakfast of the day following. 
Heats of combustion. Fuel values.—Specimens of the dried 
food, feces, and residue from the evaporation of the urine 
were burned in the bomb calorimeter.t The results are des- 
ignated as ‘‘heats of combustion, determined.’’ The differ- 
‘ences between the heats of combustion of food and feces is 
taken as the heat of combustion of the food which was actually 
utilized by the body for the building and repair of tissue and 
yielding of energy. ‘These quantities are assumed to represent 
the available ingredients of the food. ‘The term digestible is 
commonly applied to this material, but for reasons referred to 
beyond, the word available seems more appropriate. 
Assuming that the body neither gains nor loses protein, fats 
or carbohydrates, these available materials or their equivalent 
will be burned in the body. The carbohydrates and fats are 
assumed to be completely oxidized, and hence their fuel value 
is taken as equivalent to their heats of combustion. The oxi- 
dation of the protein compounds, however, is not complete. 
Their nitrogen, in combination with hydrogen, oxygen and 
other elements, is excreted by the kidneys in various com- 
pounds, of which the most important is urea. If the heat of 
combustion of this unoxidized residue is subtracted from the 
total heat of combustion of the protein, the remainder is 
assumed to represent the amount of energy of the protein 
which has been made available to the body and is taken as a 
measure of the fuel value of the protein. ‘Thus in digestion 
experiment No. 37, page 159, the total energy—z. e., the heat 
of combustion—of the food eaten was 21,384 calories. The 
heat of combustion of the corresponding feces was 1,052 calo- 
ries, leaving 20,332 calories as the measure of the heat of 

* See Report of this Station for 1891, p. 47. 
+ This apparatus and the method of its use are described in the Report of this 
Station for 1894, page 135, and a later article in the present Report. 
