Eelations between Factors of Metabolism. 20'.) 



ratio between these two gases is of very considerable significance in interpreting 

 the nature of the materials katabolized during an experiment with man. 



From the chemical analysis of carbohydrate, it is seen that the hydrogen and 

 oxygen of the molecule exist in the proportions to form water and in the oxida- 

 tion of this material, the oxygen of the air is required only to oxidize the carbon, 

 although this is far from stating that the oxidation proceeds in such a sharply 

 defined direction that the carbon only is affected. The ultimate result is an 

 oxidation of carbohydrate amounting to the oxidation of the carbon in the 

 carbohydrate. It has been found that the volume of carbon dioxide resulting 

 from the oxidation of carbohydrate is exactly equal to the volume of the oxygen 

 absorbed. 



On the other hand, since there is a relatively small amount of oxygen in the 

 fat of food, there is not enough to oxidize the organic hydrogen in the molecule, 

 and when the fat is burned in the body, oxygen is required not only to oxidize 

 the carbon but also to oxidize a portion of the hydrogen. Here, again, it is not 

 permissible to assume that the oxidation travels in any definite line or that the 

 oxygen of the molecule has any selective function in combining with this or that 

 hydrogen or carbon atom, but assuming that all the oxygen of the molecule is 

 completely combined with the hydrogen, it can be seen from a computation of 

 the molecular weights that the oxygen is still insufficient to completely oxidize 

 the hydrogen of the molecule. Consequently, when oxygen is absorbed in the 

 combustion of fat, it forms not only carbon dioxide but also a certain amount 

 of water, and as a result, there is a larger volume of oxygen consumed than 

 carbon dioxide produced, so that the ratio between the volumes of these two 

 gases is very different when fat is oxidized. 



Although, as has been pointed out in a number of previous discussions, the 

 molecular rearrangements involved in the oxidation of carbohydrate or fat are 

 comparatively simple, when we consider the oxidation of the complex protein 

 molecule, the calculation and the problems of the oxidation are somewhat more 

 difficult to trace. The oxidation of protein is very much complicated by the 

 fact that the process of oxidation does not completely break down the protein 

 molecule into carbon dioxide, water, and nitrogen. Indeed, there is a cleavage 

 of the molecule in the body ; the nitrogen is in large part excreted in the form 

 of urea and allied compounds in the urine, and certain amounts of carbon, nitro- 

 gen, and hydrogen are also excreted unoxidized in the feces. 



The calculation of the oxygen required to oxidize the remaining portion of 

 the protein molecule has been made by a number of writers, but perhaps the 

 clearest is that given by Loewy 1 in which he assumes that 100 grams of fat- 

 free, dry substance of flesh contains 52.38 grams of carbon, 7.27 grams of hydro- 

 gen, 22.68 grams of oxygen, 16.65 grams of nitrogen and 1.02 grams of sulphur. 

 Computing the amounts of the elements found in the urine and feces as a result 



1 Loewy, Oppenheimer's Handbuch der Biochemie des Menschen und der Tiere. 

 Jena, 1908, 4, p. 156. 



