RESPIRATION 5 



introduced into the lungs must increase the rate of oxidation and 

 heat production. 10 This conclusion seemed to be confirmed when 

 he introduced his well-known method for the determination of 

 urea in urine and it was found that every increase in the amount 

 of nitrogenous food eaten was followed by a corresponding in- 

 crease in the amount of urea excreted, although during complete 

 starvation the excretion of urea was not diminished below a certain 

 minimum. He inferred that it is only the "vital force" which pro- 

 tects the body against indefinite oxidation, and that when more 

 food is introduced than is really required this protection is not 

 extended, so that the food material falls a prey to oxygen. In 

 assuming this influence of the "vital force" he was only applying 

 to the phenomena of physiological oxidation the ideas held by 

 the majority of contemporary physiologists. 



When, however, the phenomena of physiological oxidation 

 came to be studied more closely by Bidder and Schmidt, Voit, and 

 other physiologists, it was found that although the excretion of 

 urea might fall greatly during starvation there was very little 

 fall in the consumption of oxygen. It thus became evident that any 

 diminution in the consumption of protein was accompanied by 

 increase in consumption of the fat and of any carbohydrate 

 remaining in the body. Further investigation of the ratios in which 

 protein, carbohydrate, and fat replaced one another in the oxida- 

 tions occurring in the body resulted in the striking discovery by 

 Rubner that within wide limits of variation in their supply to the 

 body they replace one another in proportion to the energy which 

 they liberate in their oxidation within the body. 11 Thus I gram of 

 fat furnishes as much energy as 2% grams of protein or carbohy- 

 drate, and I gram of fat from the reserve in the body takes the 

 place of 2*4 grams of protein or carbohydrate when the supply 

 of the latter in the food is cut off. The idea that the rate of oxida- 

 tion in the living body is determined by the rate of food supply 

 is thus erroneous. On the contrary the oxidation is regulated with 

 marvelous accuracy in accordance with its energy value in satis- 

 faction of what are commonly called the "energy requirements" 

 of the body. Rubner's discovery is one of the main physiological 

 foundations of scientific dietetics. 



Just as the rate of physiological combustion, other things being 

 equal, is not determined in the higher organisms by the supply .of 

 food material, so it is not determined by the abundance of the 



10 Liebig, Letters on Chemistry, Third English Edition, p. 314, 1855. 

 "Rubner, Zeitschr. f. Biologie, XIX, p. 313, 1883. 



