486 



SCIENCE. 



[N. S. Vol. XVII I. No. 459. 



eaut, especially when taken in connection 

 with the very marked result of the experi- 

 ment on hay. 



Quite recently Rubner has published in 

 book form * an elaborate discussion of this 

 question, including the results of later ex- 

 periments on dogs, in which amounts of 

 fats, carbohydrates or proteids considerably 

 in excess of those i-equired for the simple 

 ■maintenance of the body were fed. Under 

 these conditions he finds that all these nu- 

 trients, but especially the proteids, may 

 cause a marked increase in the heat pro- 

 duction of the animal. In other words, he 

 shows that what appears to be true of rumi- 

 nants below the maintenance requirement is 

 equally true of carnivo.ra when the amount 

 of food consumed is relatively large. 



It would appear, then, that the law of 

 isodynamic replacement as it has been com- 

 monly taught must be modified. That law, 

 as stated above, is that the 'fuel values,' 

 or 'physiological heat values,' of the sev- 

 eral nutrients represent their relative worth 

 to the animal body except for the peculiar 

 constructive function of the proteids. In 

 other words, the food is regarded as the 

 fuel of the vital furnace. The fundamental 

 error of this view lies in the fact that it 

 more or less consciously assumes that the 

 production of heat in the body is an end in 

 itself. The truth appears to be that it is, 

 in a physiological sense, an incident. The 

 energy of the food is needed for the per- 

 formance of the vital processes. During 

 these processes it imdergoes various trans- 

 formations, but finally the larger part, or in 

 the resting animal all, is degraded into heat, 

 which incidentally serves to maintain the 

 temperature of the body, and, as it would 

 seem, is amply sufficient for this purpose 

 under a wide range of conditions. 



Such being the case, the value of a food 



*'Die Gesetze des Energieverbrauchs bei der 

 EiTiahrunff.' 



is not measured by the amount of heat 

 which it can liberate in the body, but by 

 the extent to which its energy is available 

 for the vital processes. When, as in the 

 case of the hay, 37 per cent, of the fuel 

 value is consumed in separating the valu- 

 able from the worthless portions and in 

 transporting the former to the point of 

 use, the final net advantage to the animal 

 is represented by the remaining 63 per 

 cent. If the gross receipts of a business 

 are one hundred dollars per day and the 

 running expenses thirty-seven dollars, it is 

 plain that the net receipts are only sixty- 

 three dollars, no matter how necessary the 

 expenses may be. 



Of course this has a limit. As the tem- 

 perature to which an animal is exposed 

 falls, and the consequent draft on the body 

 for heat increases, a point will be reached 

 at which the production of heat is just 

 equal to the demand. Below this point, 

 cold seems actually to stimulate in some 

 Avay the heat production of the animal. 

 Under these circumstances Rubner appears 

 to have demonstrated that the heat pro- 

 duced by the processes of digestion and 

 assimilation may be of use indirectly by 

 obviating the necessity of burning more tis- 

 sue to supply the necessary heat, and that 

 consequently at relatively low temperatures 

 the fuel value of the food may be the 

 measure of its worth, that is, that iso- 

 dynamic replacement may occur. Above 

 a certain temperature and a certain amount 

 of food, however, varying with the kind of 

 animal and with the nature of the food, 

 the law ceases to hold and the specific dif- 

 ferences in the availability of nutrients or 

 foods reveal themselves. 



But while it thiis appears that the law 

 of isodynamic replacement is of but limited 

 application, this should not blind us to the 

 vast importance of Rubner 's earlier work. 

 It established a new point of view and 



