CHAPTER XIII. 
THE UTILIZATION OF ENERGY. 
Accorpine to the conceptions discussed in the preceding chap- 
ter a certain portion of the metabolizable energy of the food is 
consumed in what has been called in a broad sense the work of 
digestion and assimilation, while the remainder constitutes net 
available energy and contributes to the maintenance of the store of 
potential energy in the body. If the food is sufficient to supply 
net available energy equal to that dispensed by the internal work 
of the body, the balance between income and expenditure of energy 
is just maintained. If we increase the food beyond this maintenance 
requirement we supply the body with an excess of net available 
energy. In general terms we can say that this excess may be 
disposed of in two ways: it may be utilized for the peformance of 
external work, or it may give rise to a storage of potential energy 
in the body in the form of new tissue,* particularly of fat tissue. 
It appears probable, however, that neither of these processes takes 
place without more or less loss of energy in the form of heat. 
This is certainly true of the performance of muscular work, as has 
already been mentioned (p. 189) and as will be shown in detail 
on subsequent pages. Out of the total potential energy of the 
material metabolized rather more than one third, in the most favor- 
able case, is actually recovered in the form of external work, the 
remainder taking the form of heat. In this case, then, we might 
speak of the coefficient of utilization of the energy as being about 
one third. 
In the utilization of surplus energy by storage of tissue it 
appears likely that there must be also a loss of energy, although, 
* From this point of view the production of milk is to be regarded as 
the formation of new tissue. 
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