18<j Net Enerfjii Values aixl Stcircli Values 



energy or to 45-7 per cent, of the fuel value, while b}' the writers" method 

 of computation the net energj^ would be still less, viz., 32-3 therms. 



Maintenance and Production Values. 



Perhaps the most prolific source of confusion in the discussion of 

 starch values and energy values has been the fact that Kellner, in his 

 earlier publications, assumed that the fuel values, or metabolizable 

 energy, of feeding stuffs as determined by him represented their values 

 for maintenance. This was in accord with the notion then current and 

 still more or less widely held according to which the chief energetic 

 function of the maintenance ration is to supply heat to support the 

 normal body temperature. 



Were such the case, it is evident tb.at a feeding stuii would have two 

 distinct energy values, or starch values. Of these, the larger, equal to 

 its fuel value, or metabolizable energy, would express its value for 

 maintenance while the smaller, or net energy value, would represent 

 its value for production. Such a dualistic system would introduce most 

 perplexing complications into all comparisons of rations, but in fact it 

 lacks an)' adequate experimental foundation. No such sharp contrast 

 has been shown to exist between maintenance and production and the 

 prevention of loss of tissue in the former and the deposition of new 

 tissue in the latter appear to be substantially similar in their energetic 

 aspects. 



The writers were the first to show^ in 1902 that with cattle con- 

 suming submaintenance rations only about 60 per cent, of the metaboliz- 

 able energy of timothy hay actually contributed to the maintenance 

 of the animal, i.e. to the prevention of the loss of body substance, while 

 the remaining 40 per cent, simplj' increased his heat production, which 

 was already sufficient to maintain his body temperature. In other words, 

 it was shown that, qualitatively at least, the relations were the same as 

 in Kellner's experiments on fattening. 



Repeated subsequent experiments have abundantly confirmed this 

 resiUt^. Together with the almost simultaneous investigations of 

 Rubner and the later ones of Lusk and others on the so-called specific 

 dynamic action of foods they have rendered it evident that, under most 

 conditions, heat production is not an end but an incident of metabolism. 

 The body usually does not metabolize because it must produce heat 



' U.S. Dept. of Agr., Bur. Aiiim. Indus., Bull. No. 51. 

 - Compare The. Nutrition of Form Animals, pp. 271-272. 



