THE METABOLISM OF FAT 



549 



has been laid up in the form of fat. Even with a diet of pure fat 

 and with such a diet digestion and absorption are carried on under 

 unfavourable conditions more carbon is retained than can have 

 come from the metabolism of the proteins of the body, as measured 

 by the nitrogen given off in the urine and faeces: the fat passes 

 rapidly from the blood into the organs, and especially into the liver 

 (Hofmann, Pettenkofer and Voit). It is thus certain that some of 

 the absorbed fat may be stored up as fat in the body. 



This is borne out by the careful experiments of Munk and Lebe- 

 deff, who found that, when dogs are fed with excess of foreign fat 

 (linseed oil, rape oil, mutton fat), a fat is laid down which is quite 

 different from dog's fat, and has the greatest resemblance to the fat 

 of the food. Thus, when rape oil, which contains a fatty acid, 

 erucic acid, not found in animal fat, was given, erucic acid could be 

 detected in the fat laid on. When the dogs were fed with mutton 

 fat, whose melting-point is much higher than that of dog's fat, the 

 fat laid on did not melt till it was heated to 40 C. or more. When 

 they were fed with linseed oil, the body-fat was found liquid even 

 at o C. We have already referred (p. 438) to the fact that neutral 

 fat can be built up in the wall of the intestine from fatty acids given 

 in the food. Munk has shown that fat formed in this way can also 

 be laid down as body-fat. But besides the fat and fatty acids of 

 the food, the fat of the body has other sources, and some of it is 

 produced by more complex processes. 



The fat of a dog consists of a mixture of palmitin, olein, and 

 stearin. When a starved dog was fed on lean meat and a fat con- 

 taining palmitin and olein, but no stearin, the fat put on contained 

 all three, and did not sensibly differ in its composition from the 

 normal fat of the dog (Subbotin). Stearin must, therefore, have 

 been formed in some way or other in the body. If it was produced 

 from the olein and palmitin of the food, the portion of these deposited 

 in the cells of the adipose tissue must have undergone changes before 

 reaching this comparatively fixed position. But there is conclusive 

 evidence that fat may be derived from other sources, certainly 

 from carbo-hydrates, and probably from proteins; and the stearin 

 may have been formed from the carbo-hydrates or proteins of the 

 food or tissues, and not directly from fat. And if the stearin was 

 produced from proteins or carbo-hydrates, it is evident that the 

 olein and palmitin might have been formed in this way too, the 

 portion of the carbo-hydrate or protein devoted to this purpose 

 being sheltered from oxidation by the combustion of the fats of the 

 food. It is well known that not only neutral fats, but also fatty 

 acids, exert such a ' protein-sparing ' action. It is possible also that 

 the fat which is normally excreted into the intestine (p. 438), and 

 which is perhaps derived from broken-down proteins, may be re- 

 absorbed, and take its place among the fat ' put on.' 



