22 



the experiment each subject was given a heavy dose of lampblack in cap- 

 sules. This blackened the feces to such an extent that it was possible to 

 make a fairly accurate separation of that part derived from the food eaten 

 during the experiment. The lag of the lampblack on the wall of the 

 intestine did, in some instances, make the dividing point somewhat indis- 

 tinct, but in most cases it was quite clearly marked. The feces thus col- 

 lected were carefully dried, weighed, and analyzed, and the amount of 

 each constituent excreted calculated. Knowing the weight of each con- 

 stituent eaten and excreted, and assuming that what is not excreted is 

 digested and absorbed, we calculated the percentage digestibility of each 

 constituent. 



Strictly speaking, the results thus obtained do not represent actual 

 or true digestibility, because the feces contain, in addition to the portions 

 of the food not digested, some other materials, such as digestive juices 

 and excretory products. On the other hand, these waste materials, or 

 metabolic products, may be considered as representing the cost of diges- 

 tion in terms of food ingredients. Consequently, while the figures arrived 

 at may be a little below the true digestibility of the foods, they do repre- 

 sent the amount of food available to the body — for what is lost in the 

 metabolic products must be replaced from the food. 



The calculation of the amount of energy available to the body is a 

 little more complicated because all the food digested and retained in the 

 system is not fully oxidized. In computing the total fuel value of the 

 food, we figured on the perfectly correct assumption that all the nutrients, 

 excepting ash, may be completely burned. In the body, however, the 

 protein digested is only partially oxidized, as a portion is excreted in the 

 urine as urea, uric acid, etc., compounds capable of further oxidation. 

 Consequently, in computing the amount of energy available to the body, 

 account must be taken of the fuel value of these incompletely oxidized 

 residual products of protein excreted in the urine. This may be done by 

 collecting all the urine for the experimental feeding period and determin- 

 ing the heat of combustion of the organic matter in it. But, in the 

 absence of any means of marking the urine for a given period, similar to 

 that followed in the case of the feces, the only other alternative is to collect 

 the urine throughout the experimental period and determine its fuel value. 

 This may or may not be equal to that which would be formed by the 

 unoxidized nitrogen compounds from the food under investigation. A 

 much simpler method, and the only one open to us, is to calculate the 

 available energy. It has been found in a large number of experiments 

 conducted in Europe and on this continent that the average heat of com- 

 bustion of the organic matter of the urine corresponding to one gram of 

 digested protein amounts to 1.25 calories, t It is generally believed that 

 the energy of the urine calculated by this factor is reasonably accurate. 

 Consequently, the figures representing the per cent, of available energy 



tStorrs Experiment Station Report, 1899, p. 100. 



