METABOLISM, NUTRITION AND DIETETICS 523 



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 (Hof mann, Pettenkof er 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 

 Lebedeff, 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. 413) 

 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 

 containing 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. 415), and which is 



