DYNAMIC ACTION OF PROTEINS 121 



heat production induced by each foodstuff is different, 

 and is specific for each foodstuff. Rubner, therefore, 

 has named this effect "specific dynamic action." 



It has been shown under the correct conditions that if 

 the energy metabolism of a fasting dog be represented 

 as 100 calories, food must be given in the following 

 amounts to prevent body loss, 106 calories of sugar, 

 or 114 calories of fat, or 140 calories of protein. In 

 an experiment by Rubner it was found that a fasting 

 man metabolized 2042 calories. After the ingestion 

 of 2450 calories of sugar he metabolized 2087. When 

 given 2450 calories of meat 2566 calories were metab- 

 olism. Whatever the cause of the greater metabolism 

 of protein ingestion it is believed that there is produced 

 a liberation of energy which cannot be used by the 

 tissues in support of their activities but it is possible 

 that it may contribute to the maintenance of body 

 temperature. On a mixed diet this liberation of heat 

 unavailable for energy purposes is not of great sig- 

 nificance in the total metabolism since it increases the 

 metabolism of energy less than one tenth on a mainte- 

 nance diet above that when no food is eaten. 



Another explanation for the increased energy metab- 

 olism after the ingestion of foodstuffs has been put 

 forward by Zuntz, who ascribes this effect fo the 

 mechanical work of the intestinal canal (Darmarbeit) 

 performed during digestion and assimilation. Such a 

 theory would seem to fit in well with the greater 

 specific dynamic action of protein and its probably 

 greater difficulty of digestion in comparison with 



