


DIGESTIBILITY, AVAILABILITY AND FUEL VALUE. 69 

DISCUSSION OF THE TERMS DIGESTIBILITY, 
AVAILABILITY AND FUEL VALUE. 
BY ew. O. AL WADE RY 

In order to make food available for use in the body it must 
_ be digested. The digestion is done at the expense of a certain 
amount of material which the food itself must supply. ‘This 
material is essentially that which is poured into the alimentary 
canal in the digestive juices. That which is not re-absorbed 
remains in the feces in the so-called metabolic products. These 
latter include also the fragments of intestinal ephithelium and 
minute quantities of other substances. In addition a small 
part of the food escapes digestion. The feces are, accordingly, 
made up of (1) metabolic products which are mainly the residues 
of digestive juices and (2) the undigested residues of the food. 
DIGESTIBILITY AND AVAILABILITY. OF NUTRIENTS. 
This brings out the distinction between the terms digesti- 
bility and availability as they are here used. 
Digestibility.—This term is here used to designate the quan- 
tity or proportion of material digested. It is measured by the 
difference between the total food and the undigested residue. 
The statement applies likewise to the several nutrients—pro- 
tein, fats, carbohydrates and mineral matter. To determine 
the amount of each which is digested the total amounts in the 
‘food and the corresponding amounts in the feces are deter- 
mined, and the latter are subtracted from the former. ‘The 
methods for distinguishing between the metabolic products and 
the undigested residue of the food have not been made suffi- 
ciently accurate to enable us to determine exactly the propor- 
tion actually digested.* ; 
Avatlability.—The term availability is here used to desig- 
‘nate the quantity or proportion of the food and of the several 
* nutrients which can be used for the building and repair of tissue 
and the yielding of energy. ‘The metabolic products, which 
come from the digested food, are not used for either building 

* See Report of this Station for 1897, p. 157. 
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