440 A TEXTBOOK OF PHYSIOLOGY 



when fed with the same carbohydrate. The figures given in the table 

 (p.4S8) show that the fat deposited from the carbohydrate in the diet 

 differs considerably from that of the same animal when fed on other 

 fats, and probably represents the animal's own characteristic fat. 

 It is this fat which makes for a hard body and physical fitness. 



The Eatabolism of Fat. In regard to the katabolism of fat, evidence 

 is scant}'. It is quite probable that there exists two different forms 

 of katabolism (1) of the ingested fat taken; (2) of the fat formed 

 within the animal's own body from other todies. The fat of the food 

 may be regarded as a wanderer. It is formed, in the first place, 

 within some plant or animal from lower bodies, and, unless used up 

 again within the parent organism, it passes from plant to animal, 

 and from animal to animal, until burnt up and used in the processes 

 of metabolism. Such foreign fat is eventually burnt to carbon dioxide 

 and water. The fatty acids, if saturated, are probably first broken 

 down to the higher unsaturated fatty acids, and then to the lower 

 unsaturated fatty acids, such as 3-oxybutyric acid and aceto-acetic 

 acid, and finally to CO., and water. 



Fat 



Glycerine Saturated fatty acid 



i 

 Unsaturated fattv acids 



/\ 

 CO 2 H 2 



If, however, the fat formed within the organism for example, 

 from carbohydrate in the time preparatory to hibernation be kata- 

 bolized e.g., during hibernation there is good evidence to show 

 that such fat is broken down by way of carbohydrate again. 



Fat 



I 

 Carbohydrate 



/\ 



CO 2 H 2 



The best evidence in favour of this view has been obtained from 

 the observation of the respiratory quotient of hibernating animals. 

 Normally, fat gives a respirator}- quotient of 0-7 (see p. 319)- This 

 is the lowest of the figures given by the three classes of foodstuffs. 

 In hibernating animals, however, much lower figures have been ob- 

 tained e.g., 0-23 for the dormouse, 0-5 for the hedgehog. The 

 explanation given for these low figures is that the stored fat is being 

 converted into carbolrvdrate ; and since carbohydrate contains more 

 oxygen in its molecule, oxygen is used up in the process, and the 

 respiratory quotient of the animal correspondingly reduced. The 

 process may be summarized by the following formula : 



2C 2 H 5 (C 18 H 33 2 ) 3 +640 2 = 16C 6 H 12 6 + 18C0 2 + 8H 2 



Olein Dextrose 



. C0 2 _18 

 ~ = 



