548 METABOLISM 



4. Conversion of fat Into carbohydrate: 



2C 3 H 5 (C 18 H, 3 O 2 ) 3 + 64O, = 16C e H r ,O 6 + 18CO, + 8H 2 O 

 (Olein.) 



5. Conversion of carbohydrate into a mixed fat : 



13C 6 H J2 O 6 = C 55 H 104 O G + 23CO 2 + 26H,O. 

 ( Oloostearopalmitin. ) 



Taking carbohydrates first, the general formula may be written CH 2 0, 

 from which it is plain that, to oxidize the molecule, oxygen will be re- 

 quired to combine with the carbon alone, according to the equation, 

 CH 2 + 2 = C0 2 + H 2 0. In other words, the volume of carbon dioxide pro- 

 duced by the combustion will be exactly equal to the volume of oxygen 

 used in this process, in obedience to the well-known gas law that equi- 

 molecular quantities of different gases occupy the same volume. The 

 respiratory quotient is therefore unity (Equation 1). With fats and pro- 

 teins, however, the general formula must be written CH 2 -f-0, indicating 

 therefore that for its complete oxidation the molecule must be supplied 

 with oxygen in sufficient amount to combine not only with all of the car- 

 bon, but also with some of the hydrogen, forming water ; so that the vol- 

 ume of CO, produced will be less than the volume of oxygen retained, 

 and the respiratory quotient will be less than unity. As a matter of fact, 

 as the above equations show (2 and 3), the respiratory quotient for fats 

 and proteins lies somewhere between 0.7 and 0.8, being usually nearer 

 0.7 in the case of fats, and nearer to 0.8 in the case of proteins. 



That the conditions hypothecated in the equations exist in the animal 

 body during the combustion of the foodstuffs can easily be shown by ob- 

 serving the respiratory quotient of animals on different diets. An her- 

 bivorous animal, such as a rabbit, when it is well fed gives invariably a 

 respiratory quotient of about 1, whereas a strictly carnivorous animal, 

 such as the cat, gives a respiratory quotient of about 0.7. Even more 

 striking perhaps is the comparison of the respiratory quotients in an 

 herbivorous animal while it is well fed and after it has been starved for a 

 day or two. In the latter case the respiratory quotient will fall to a low 

 level because, by starvation, the animal has been compelled to change its 

 combustion material from the carbohydrate of its food to the protein and 

 fat of its own tissues. 



As already explained (page 545), it is from the respiratory quotient 

 that we are enabled to tell what proportions of fat and carbohydrate, 

 respectively, are undergoing metabolism. A useful table showing the 

 percentage of calories produced by each of these foodstuffs, after allow- 

 ing for protein, is given by Graham Lusk (see page 565). 



