THE BALANCE OF NUTRITION 233 



olizable energy means simply energy capable of transformation 

 in the body. It is the maximum quantity which the feed can 

 contribute to the energy changes in the organism. That it 

 does not necessarily measure nutritive value is indeed suffi- 

 ciently apparent from the method used for its determination. 

 As the example already given shows, this does not require any 

 measurement of the gain or loss by the animal, but, like a 

 digestion experiment, concerns itself simply with the feed and 

 the excreta. 



323. Synonyms for metabolizable energy. Two other 

 terms are frequently employed with substantially the same 

 significance as metabolizable energy, viz., fuel value and avail- 

 able energy. 



Fuel value. The metabolizable energy of the feed is evi- 

 dently capable of conversion into heat in the body. Since a 

 considerable portion and sometimes all of it is actually thus 

 converted, and since its amount is usually expressed as a matter 

 of convenience in heat units, the term fuel value (or physiological 

 heat value) has come into use as synonymous with metaboliz- 

 able energy. 



The term has the advantage of brevity, but has also certain 

 disadvantages. In conjunction with the unit of measurement 

 employed, it has a tendency to suggest that the purpose of 

 the feed is to supply heat energy and that it is of value 

 in proportion as it can do this, which is far from being the 

 case. Moreover, there appears to be some danger of confusion 

 due to the fact that the same term is used in a different sense 

 in relation to fuels. The " fuel value " of a coal, for example, 

 means the total amount of heat which it liberates when burned, 

 and corresponds, therefore, to the gross energy of a feeding stuff, 

 i.e., to its value if used as fuel under a boiler or in a heating 

 plant. The fuel value of a feeding stuff, on the other hand, in 

 the sense of its metabolizable energy, is the amount of heat 

 which it can furnish when oxidized as it is in the body, i.e., more 

 or less incompletely. 



Available energy. A much more unfortunate usage is the 

 employment of the term available energy, equivalent to the 

 German " Physiologischer Nutzwert," in the sense here assigned 

 to metabolizable energy. This usage dates back to Rubner's 

 investigations of the replacement values of nutrients in 1882- 



