METABOLISM 173 



but may, as has already been indicated (217, 236), manu- 

 facture fats from other ingredients of its feed. 



This view, first propounded by Liebig in 1843, was contrary 

 to the opinion then prevailing and led to a lively controversy 

 which, however, was definitely resolved in favor of the newer be- 

 lief. Indeed, the feed fats, especially in case of herbivorous ani- 

 mals, are usually of subordinate importance as sources of body fat, 

 a large share of the latter being produced de now in the body. 

 This fact explains in part the general uniformity of composition 

 of the body fat of each species. The steer produces beef 

 fat and the sheep mutton fat on substantially identical rations 

 largely because the fat deposited in the body is derived only 

 in small part from the feed fat, most of it being produced by the 

 specific metabolic activities of the body cells. The seat of this 

 synthetic production of fat, however, as well as the manner 

 in which it is deposited in the reserve tissues, are still unknown. 



The sources of animal fat 



247. Experimental evidence. The sources of animal fat 

 have been already indicated. Aside from whatever feed fat 

 may be stored up in the adipose tissues, the body can produce 

 fat from the carbohydrates of the feed (217) and in all prob- 

 ability from the non-nitrogenous residue of the proteins (236). 

 In view of the historic interest attaching to the long controversy 

 over this question, however, as well as of its intrinsic importance, 

 an outline of the experimental evidence seems appropriate. 



That the feed fat is a source of body fat was never seriously 

 questioned. When the correctness of Liebig's contention that 

 the animal body can also manufacture fat had been demon- 

 strated, it was assumed that the source of this new-formed 

 fat was to be found in the carbohydrates of the feed and this 

 was for years the accepted view. Following Liebig's termi- 

 nology, the proteins were designated as. the " plastic materials," 

 serving to build up tissue, while the carbohydrates and fats were 

 " respiratory materials," serving as sources of heat and of fat. 



248. Fat from protein. Several earlier investigators ob- 

 served facts pointing to the formation of fat from protein in 

 the animal body, but Carl Voit l was the first to distinctly ad- 



1 Ztschr. Biol., 5 (1869), 79-169. 



