



o 
DIGHOLIBILITY, AVAILABILITY. AND FUEL VALUE. post 
materials used for the food of man, however, generally contain 
but very little of the undigestible material the cost of handling 
which is so large. The removal of the hulls of grain in the 
milling, and the various processes in cooking help to reduce 
this cost to its lowest terms. 
There is, moreover, a portion of the energy of the available 
food which is not made available in the body at all, namely, 
that of the incompletely oxidized material which is excreted in 
the urine. This comes mostly from the protein and occurs in 
the forms of urea and kindred compounds. It is generally 
assumed that the available fats and carbohydrates of the food 
are, under normal conditions, completely oxidized in the body. 
Accordingly, their available energy is their total heat of com- - 
bustion. The available energy of protein, on the other hand, 
is taken as the difference between the total heat of combustion 
and the heat of combustion of the water-free substance of the 
corresponding urine. ‘This subject is discussed in the follow- 
ing article, pages 99 and 100. 
This view makes the total available energy of the food the 
total energy (heat of combustion) less that of the correspond- 
ing unoxidized residues of the feces and urine. It is also the 
total energy of the available nutrients less that of the corre- 
sponding unoxidized material of the urine. We might deduct 
from the total! available energy the amount required for the 
mechanical work of digestion and other operations by which 
the food is prepared for use, and call the difference the net 
available energy, but it does not seem necessary here to insist 
either upon this distinction or upon the corresponding one 
which would separate the nutrients oxidized for this purpose 
from the rest of the available nutrients. 
FUEL VALUE. 
The term fuel value is here used to designate the value of 
the food for its service as fuel. Exactly how this service is 
rendered is not yet known. Without discussing the question 
as to how much of the potential energy of the food is trans- 
formed directly into heat in the body, or what proportion is 
first used for internal muscular work and afterwards trans- 
formed into heat, we may for our present convenience assume 
that the chief uses of the nutrients of food as fuel are to:— 
1. Yield energy as heat. 
2. Vield energy for muscular work. ’ 
