20 



oneroT of the total feces as the calculated energ^y of the feces from food 

 other than bread is to ,/', the value desired. For example, in experi- 

 ment No. 469 the computed heat of combustion of the total feces was 

 745 calories, while the value obtained in the calorimeter was 787 calo- 

 ries. The computed heat of combustion of the feces from food other 

 than bread was 242 calories. The proportion was therefore as fol- 

 lows: 745:787=242:;r, or i/'=257 calories. The heat of combustion of 

 the feces from bread alone was found by suljtracting- this value from 

 the determined heat of combustion of the total feces (787 calories). 

 This result (530 calories) subtracted from the total heat of combustion 

 of bread (4,770 calories) <^ives the heat of combustion of the estimated 

 digestible nutrients in bread (4,240 calories), which divided by the 

 total heat of combustion of bread (4,770 calories) and nuiltiplied by 

 100 gives the proportion (88.9 per cent) of the total energy of the 

 bread estimated to be contained in the digestible nutrients. 



The value as thus determined, however, does not represent the 

 energ}' actually available to the body, since a portion of the energy of 

 the digested nutrients is lost in the incompletel}' oxidized material of 

 the urine. The proportion of the energy of the total food and of the 

 bread alone which was actually available to the body was computed 

 as in previous investigations, according to the assumption that the 

 amount of the energy lost in the organic matter of the urine was 1.25 

 calories per gram of digestible protein. Thus in experiment No. 469 

 the total amount of energy lost in the urine w^as found by multiplying 

 the digesti})le protein in total food (401.4 grams) by 1.25, which gave 

 502 caloi-ies. This product was sul)tracted from the energy of the 

 total food digested (9,756 calories), and the diti'erence (9,254 calories) 

 divided by the energy in the total food (10,543 calories) and multiplied 

 b}^ 100 gave the proportion of energy in the total food actually avail- 

 able to the body (87.8 per cent). The proportion of energy in bread 

 alone which was actually available (87 per cent) was obtained in like 

 maimer by dividing the difference between the estimated energy in 

 the bread digested (4,240 calories) and the energy lost in the urine 

 (70.9X1.25 = 89 calories) by the total energy in the bread (4,770 

 calories). 



As pointed out in former reports, it is well known that the digestion 

 coefficients obtained in the manner described are relative rather than 

 absolute. It is believed, however, that while the figures for a single 

 digestion experiment may be open to criticism, the results of a series 

 of experiments as reported in this bulletin are comparable because 

 whatever error is introduced in one experiment is common to all. 

 Since the determinations of dry matter, nitrogen, and energy are 

 known to be of greater accuracy than the other chemical data included, 

 particular attention is given to the results and conclusions based upon 

 these values. 



