METABOLISM, NUTRITION AND DIETETICS 455 



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 formed 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 proteids ; 

 and it is more likely that the stearin was formed from the 

 proteids of the food or tissues than directly from fat. And 

 if the stearin was produced from proteids, it is evident that 

 the olein and palmitin might have been formed from proteids 

 too, the portion of the latter 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 ' proteid-sparing ' action, and a 

 portion, though not the whole, of the fat laid on when an 

 animal is fed with fatty acids does arise from the carbonaceous 

 residue of the proteids that are saved from complete com- 

 bustion. It is possible also that the fat which is normally 

 excreted into the intestine (p. 372), and which is perhaps 

 derived from broken-down proteids, may be reabsorbed, and 

 take its place among the fat ' put on.' 



Formation of Fat from other Sources than the Fat of the 

 .Food. (i) From Proteids. Dry proteid contains on the 

 average 15 per cent, of nitrogen and 50 per cent, of carbon ; 

 and urea contains 46 per cent, of nitrogen and 20 per cent, 

 of carbon. Urea is therefore rather more than three times 

 as rich in nitrogen as the proteid from which it is derived, 

 but two and a half times poorer in carbon ; and less than 

 one-seventh of the carbon of proteid will be eliminated in 

 the urea, which carries off all the nitrogen. A carbonaceous 

 residue is left, which under certain circumstances may be 



