19 



charcoal taken with a meal of bread and milk, which gives feces of a 

 characteristic color and consistency. The digestibility of the nutrients 

 of the diet as a whole was taken as the difference between the amounts 

 in the food and those in the feces, no attempt being made to determine 

 the metabolic products of the feces/' 



In order to compute the digestibility of the nutrients of the bread 

 alone, it was assumed that 97 per cent of the protein, 95 per cent of 

 the fat, and 98 per cent of the carbohydrates of milk were digested. 

 The undigested nutrients of the milk as calculated by the use of these 

 factors subtracted from the nutrients of the total feces give the esti- 

 mated undigested nutrients from bread, which, subtracted from the 

 total nutrients of the bread, give the digestible nutrients in bread. 

 The latter, divided by the total nutrients in the bread, give the coeffi- 

 cients of digestibility of bread alone. 



The values used for the digestibility of the nutrients of milk have 

 been deduced from the results of a large number of digestion experi- 

 ments with milk. Even if, in the experiments here reported, the diges- 

 tibility of the milk nutrients varied from these assumed coefficients, 

 the figures for the digestibility of the nutrients of the different kinds 

 of bread are still strictly comparable because the same factors for 

 milk were used in all cases. 



As has been already explained,'' the energy of the estimated feces 

 from In-ead alone was computed by proportion from the energy of the 

 total feces. The ratio of the heat of combustion of the bread feces 

 as computed by factors to the actual energy was assumed to be the 

 same as the ratio of the computed energy of total feces to the heat of 

 combustion as determined. 



Although the energy of the urine was determined, in the calculation 

 of the availability of the energy of the total food and of the bread 

 alone, it was assumed, for the sake of uniformity with experiments 

 previously reported, that 1.25 calorics of energy would appear in the 

 urine for ever}^ gram of digestible protein in the total food or in the 

 bread alone. For the sake of making an approximate estimate of the 

 available energy in those experiments where the digestibility of the 

 bread fat could not be computed, it was assumed that 90 per cent of 



«It should be observed that the results thus obtained do not represent actual 

 digestibility. The true digestibility could be found by subtracting from the ingredi- 

 ents of the food the corresponding ingredients of the feces that come only from undi- 

 gested portions of the food. But no satisfactory method has been found for separating 

 these from the metabolic products in the feces, which consist largely of the residues 

 of the digestive juices that have not been reabsorbed. These latter represent the 

 cost of digestion as expressed in terms of food ingredients. What the results of these 

 experiments do represent, therefore, is the proportions of the food, or of the several 

 ingredients, that are available to the body for purposes other than digestion itself. 

 In accordance with common usage, however, the term digestibility, which indicates 

 the apparent digestibility, has been employed here; the term availability is some- 

 times used to express the same idea. 



&U. S. Dept. Agr., Office of Experiment Stations Bui. 101, p. 22. 



