THE UTILIZATION OF ENERGY. 445 
as will appear later, we are not yet in a position to make any such 
definite statements regarding its amount as in the case of muscular 
work, and although the writer’s few results on timothy hay cited 
on p. 424 afford no indication of such a loss, the utilization of the 
metabolizable energy for the production of gain seeming to have 
been practically equal to its net availability. It is obvious, how- 
ever, that the conversion of the resorbed nutrients of the food into 
the ingredients of tissue involves profound chemical changes, and 
we can hardly suppose that these take place without some evolution 
of heat. As a good illustration we may take the case of a carbo- 
hydrate. As resorbed into the blood it appears to be in the form 
of a sugar, and it would seem that this sugar can serve, without any 
very extensive chemical changes, to sustain the metabolism incident 
to the internal work of the body; that is, that it is oxidized more 
or less directly in the various tissues to supply energy for their 
physiological work. When, however, a surplus of a carbohydrate 
is to be utilized for the storage of energy in the form of fat, the case 
is different. The formation of fat from a carbohydrate is chemi- 
cally a process of reduction, and the oxygen which is removed 
from the carbohydrate must unite with the carbon and hydrogen 
either of other molecules of the carbohydrate or of other in- 
gredients of food or tissue, in either case giving rise to an evolu- 
tion of heat. If we suppose the transformation to take place 
according to the equation given in Chapter II (p. 24), the re- 
sulting fat would contain about 87 per cent. of the energy of 
the dextrose. Whether this percentage expresses the actual facts 
of the case or not, it is very improbable that this or any similar 
synthetic process takes place in the body without the evolution 
of some heat. 
Provisionally, then, we seem justified in assuming that only a 
part of the net available energy supplied to the organism above 
the maintenance requirement can be utilized to increase the store 
of potential energy in the body, and we may speak in this case, as 
in that of muscular work, of the coefficient of utilization. Repro- 
ducing here the essential parts of the graphic representation 
on p. 410, we may now complete it so as to represent in a general 
and qualitative way the relations indicated above, assuming pro- 
visionally that the effects are linear functions of the food. As 
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