188 STORRS AGRICULTURAL EXPERIMENT STATION. 
other materials, which comprise what are commonly designated 
as metabolic products. ‘These consist largely of the residues of 
digestive juices poured into the alimentary canal and not reab- 
sorbed, but contain also more or less of true excretory products. 
To determine actual digestibility it would be necessary to sepa- 
rate these metabolic products from the actually undigested food, 
and deduct the ingredients of the latter from those of the food 
eaten. ‘Thus far, however, no satisfactory method for separating 
the undigested residues from the metabolic products of the 
feces has been devised, consequently the actual digestibility of 
the food is not easily determined. Obviously the result ob- 
tained as explained above, and designated digestibility, is some- 
what below the true digestibility. 
On the other hand, the metabolic products may be considered 
as representing the cost of digestion in terms of food ingredients, 
and the total feces, including both these and the actually undi- 
gested residue of the food, may be taken as representing the 
portion of the food that is not available to the body for purposes 
other than digestion—the building of tissue and the yielding of 
energy. Thedifference between the amount of each ingredient 
in the total food and that in the total feces may, therefore, be 
designated as the availability, rather than the digestibility, of the 
ingredient. In the present discussion the term availability is 
therefore used to indicate what is frequently designated di- 
gestibility. 
While the above method of computation shows the amount 
of the different nutrients that are available to the body, the 
corresponding computation for energy represents the quantity 
of energy in the food absorbed from the alimeritary canal, and 
not the amount which the body actually utilizes, because a 
portion of the absorbed material is not completely burned in the 
body. ‘The absorbed carbohydrates and fats are believed to be 
completely oxidized, but the protein is not. When protein is 
burned in the bomb calorimeter, the carbon is oxidized to carbon 
dioxid, and the hydrogen to water, the nitrogen being reduced 
to the free state; but when burned in the body, the oxidation 
is less complete, the nitrogen, together with some carbon, 
hydrogen and oxygen, being excreted in such compounds as 
urea, uric acid, etc., that are capable of further oxidation 


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