NORMAL PROCESSES OF ENERGY METABOLISM 605 



meat. Feeding fat they observed no increase in the heat production un- 

 less the amount fed was far in excess of the body requirements. Feeding 

 carbohydrate in the form of starch, they found that 370 grams in the 

 food increased the metabolism 17 per cent over that of the starving animal. 

 More exact information concerning the influence of carbohydrate came 

 with the invention of methods by Zuntz and by Benedict by which the 

 oxygen absorption could be determined, since, without this knowledge, 

 it was impossible to distinguish the part taken by fat in the total heat 

 production from that taken by carbohydrates. Magnus-Levy, using the 

 Zuntz method with human subjects, came to the conclusion, substantially 

 in accord with those of Pettenkofer and Voit, namely, that moderate quan- 

 tities of fat do not increase the heat production (absorption of oxygen), 

 but that both carbohydrate and protein increase it considerably. Rubner, 

 using only the excretion of C0 2 as the measure of heat production, formu- 

 lated laws regarding the influence of different foods given to dogs, as fol- 

 lows : Since the different foodstuffs affect the heat production to a different 

 degree, we may speak of their "specific dynamic action." The proper basis 

 of comparison is the amount of heat produced by the fasting animal. Tak- 

 ing this quantity as the minimal requirement of the animal for energy 

 (in potential form), and feeding this quantity in the form of different 

 foodstuffs, the effect is for protein an increase of heat production of 30 

 per cent, for fat 11 per cent, for carbohydrate 5.8 per cent. In order to 

 keep the animal in an energy equilibrium, therefore, it is necessary to feed 

 him in protein 140 per cent of the requirement, hi fat 114 per cent, and in 

 carbohydrate 10G per cent. 



Lusk and his co-workers, using the small respiration calorimeter (de- 

 scribed on page 579),. have demonstrated that the increased heat pro- 

 duction in dogs after ingest ion of proteins is due to the ammo-acids into 

 which the protein is broken up by digestion. It is, however, not the mere 

 absorption of the ammo-acids themselves, nor their direct oxidation which 

 accelerates the metabolism, but the stimulating effect of the intermediate 

 oxyacids which are formed from them. Quantitatively the results of these 

 more modern researches confirm the conclusions of Rubner as to the speci- 

 fic effect of protein. These, however, relate to the dog. In man the dyna- 

 mic effect is ordinarily not so great. The dynamic effect of protein in 

 milk upon the metabolism of the infant will be discussed later (page 644). 

 It need only be added here that protein which becomes a part of the body 

 does not affect the heat production. 



The dynamic effect of fat, it turns out, is not so high as Rubner found 

 it, if reckoned for the entire day, but for individual periods up to six hours 

 after feeding, may increase the metabolism as much as 30 per cent (Murlin 

 and Lusk), as contrasted with protein (meat) which may raise it 100 per 

 cent. Bloor found that the fat in the blood also increases up to six hours 

 after feeding. 



