FUEL VALUES OF DIGESTED NUTRIENTS, 123 
FUEL VALUES OF DIGESTED NUTRIENTS IN 
EXPERIMENTS IN SHEEP. 
BY W. O. ATWATER AND CHAS. D. WOODS. 
— — 49 ——— 
In connection with the digestion experiments with sheep de- 
scribed in the preceding paper (pp. 107-122), the heats of combus- 
tion of the feeding stuffs and feces were determined by use of 
the bomb calorimeter, with the purpose of getting light upon the 
potential energy or fuel value of the digested material.* 
The problem is, however, not as simple as these brief state- 
ments imply. Indeed it is extremely complex, but for the 
present purpose it will suffice to say that while most of the uncon- 
sumed material of the food leaves the body in the undigested 
food residue of the feces, a small portion escapes in urea and 
other substances of the urine. ‘These last substances come from 
the digested material of the food; they are digested material 
which has not been completely consumed, and they contain poten- 
tial energy. If, therefore, we are to find how much of the 
potential energy of the food has been set free in the body we 
must subtract from the total energy of the food, the sum of the 
amounts excreted in both feces and urine. The remainder is 
taken as the measure of the actual fuel value of the food. 
Food has two chief uses. The first is to build up the materials 
of the body and repair their wastes, the second is to yield energy 
in the form of heat to keep it warm and strength for its work. 
The nutritive ingredients or nutrients of the food make blood 
and muscle, bone and milk and other tissues and fluids of the 
body and replace them as they are used up. But only the por- 
tions which are actually digested serve these purposes. Hence 
to learn the nutritive value of food we endeavor to find how 
much of each of its nutrients is digested and thus made useful 
for building and repair. To do this we learn how much of 

* The bomb calorimeter is described in a succeeding article, page 135. Brief explanations of 
potential energy and fuel values are given on pages 43 and 44. 
