J aoe 
- >? ae 
EXPERIMENTS ON DIGESTION OF FOOD BY MAN. 157 
and mucus, and the gastric, pancreatic and other digestive 
secretions, but they include other materials, as epithelial debris 
from the intestinal walls. Recent investigation indicates very 
clearly that in man the undigested residues make a smaller and 
the metabolic products a much larger proportion of the intes- 
tinal secretion than has formerly been supposed. Thus the 
digestibility of most animal and many vegetable foods is found 
to be very nearly complete.** Prausnitz suggests very appo- 
sitely that what we have to consider here is not so much the 
digestibility of the food—z. e., the proportion of nutrients 
digested from a given material—as the amount of digestive 
juices used to render it capable of absorption from the ali- 
meutary canal. In other words, with ordinary diet the meta- 
bolic products make so large a part of the intestinal excreta 
that the amount of the latter depends more upon the quantity 
of digestive secretions needed to digest the particular kind of 
food than upon the quantity of undigested residue. Excep- 
tions to this rule may be found where some of the coarser 
vegetable foods, as for instance, unbolted flours and meals, are 
used. 
To determine the exact quantities of the food and nutrients 
actually digested it would be necessary to find the amount of 
undigested food residue in the feces, to determine the amounts 
of the several nutrients therein, and to subtract the undigested 
residue of each nutrient from the total amount of the latter in 
the food eaten. Numerous attempts have been made to elabo- 
rate methods for quantitative separation of the undigested food 
residues and the metabolic products of the intestinal secretions. 
While much valuable work has been done in this line the 
methods are not as yet as perfect as might be desired, and 
most investigators continue to follow the ordinary plan of 
taking the difference between the food and feces as the measure 
of the digestibility of the food. 
he error here is, however, not so great as might be sup- 
posed. The metabolic products represent material which, for 
the most part, has been used for the purposes of the digestion. 
In other words, they are not available to the body for the 
- yielding of energy, nor can they be utilized for building tissue 

* See especially the late investigations by Prausnitz, Hammerl, Kermauer and 
Moeller, Ztschr. Biol. 35, pass7m. 
