Katabolism of Protein. 453 



As commonly used, the respiratory quotient excludes completely any con- 

 sideration of the total nitrogen excretion, on the assumption that for the short 

 period of the respiration experiment the katabolism of protein is constant. 

 In the experiments made in this laboratory the protein broken down is as 

 accurately and regularly measured as is the carbon dioxide production, and 

 hence the total carbon dioxide excretion can be resolved into certain portions 

 representing the katabolism of the various ingredients of the body. 



The determination in metabolism experiments of the complete carbon dioxide 

 and nitrogenous output alone has served heretofore as the basis for the compu- 

 tation of the proportions of protein and fat katabolized. In this computation 

 it has been necessary to assume that the store of glycogen in the body remained 

 constant during the experiment. Such an assumption may not be grossly erro- 

 neous during food experiments in which a maintenance ration is employed, 

 though for periods of inanition where there may be very considerable drafts 

 upon the store of glycogen, this method of computation, admittedly but approx- 

 imate for experiments with food, may be entirely inadequate. 



It was early recognized in this laboratory that a fundamental study of 

 metabolism during inanition is necessary for a proper understanding of many 

 physiological processes. But until the means were at hand for determining 

 directly the oxygen consumption, it was apparent that such studies were of 

 but little value. With the added data regarding the total amount of oxygen 

 absorbed per day, a much more accurate apportionment of the total katabolism 

 among protein, fat, and carbohydrate is possible. The apportionment is based 

 upon the chemical determinations showing the losses of material to the body 

 in terms of chemical elements, and for this reason especial care has been taken 

 in making the elementary analyses. The losses to the body of nitrogen, carbon, 

 hydrogen of organic matter, oxygen, water, and ash have all been determined 

 as accurately as present methods of analysis will permit. The method of 

 obtaining the gains and losses of chemical elements has been discussed in detail 

 in connection with experiment No. 59 (see p. 36). From these gains or losses 

 of elements it is possible by means of the method of simultaneous equations to 

 compute the proportions of protein, fat, and carbohydrate katabolized. The 

 detailed treatment of this subject is likewise given in connection with the 

 discussion of the same experiment. 



It is only necessary here, therefore, to compare the total amounts of protein, 

 fat, and carbohydrate katabolized on different days of the different fasting 

 experiments. 



KATABOLISM OF PROTEIN. 



Since the protein katabolized is computed from the nitrogen excretion by 

 multiplying the amount of nitrogen by the factor 6.0, the discussion which 

 has been accorded the total excretion of nitrogen during fasting applies in 

 general in this connection. For while, as is well known, the nitrogen excreted 



