

AVAILABILITY AND FUEL VALUE OF FOOD MATERIALS. 99 
nitrogen is excreted in the form of urea, uric acid, and allied 
compounds. The carbon and hydrogen, however, appear to be 
oxidized completely to carbon dioxide and water with the excep- 
tion of the small amounts in combination with the nitrogen in 
the urine. The urine also contains a small amount of non- 
nitrogenous organic material, part of which may not improbably 
be derived from the digested fats and carbohydrates of the 
food. It is customary, however, to consider the heat of com- 
bustion of the urine as representing that portion of the energy 
of the protein which is broken down which the body cannot 
utilize. In other words, we assume that the total potential 
energy of the available fats and carbohydrates is itself available 
for use in the body while only a portion, although the major 
portion, of the energy in the available protein is thus available. 
This brings out the distinction between the availability of 
the material and of the energy belonging to it. In speaking of 
the available material we refer to the portion which is digested 
and absorbed less the corresponding metabolic products which 
are eliminated with the undigested residue, and are regarded as 
required for the process of digestion. ‘The available energy of 
the fats and carbohydrates is the total energy of their available 
‘material. In the case of protein, however, the available mate- 
rial is not fully oxidized, and the available energy is the total 
less that of the material which escapes oxidation. ‘The last 
statement requires qualification, because it does not distinguish 
between the proteids and non-proteids of the protein. To be 
strictly accurate we should have to consider the proteids them- 
selves, determine their amounts, availability and heats of com- 
bustion, and also learn by some means how much of their 
energy is transformed in the body by oxidation and how much 
is left in the residues excreted in the urine. ‘The different 
classes of non-proteids could be treated in like manner. Our 
present knowledge, however, does not permit such determina- 
tions, nor are the experimental methods by which they should 
‘be made as yet sufficiently elaborated. 
The best we are able to do at present is to determine as 
nearly as practicable the proportions of protein which are 
actually available and the heat of combustion of the material 
in the urine which is assumed to come from the incomplete 
oxidation of the protein compounds. 
