METABOLISM OF FAT. 



931 



composition normal to the particular species of animal, are laid down 

 directly, and that other portions, such as the excess of stearin which 

 occurs in mutton fat, become broken down completely, and either directly 

 oxidised, or the products of their decomposition again built up to form 

 the normal fat. It has indeed been conclusively proved that the fat of 

 the food may be to a certain extent laid down unaltered in the body- 

 fat. Dogs which have been starved for a considerable time, so that 

 practically the whole of the body-fat has become removed, will, if fed 

 upon an excess of mutton fat and sufficient proteid, lay down a body-fat 

 of a melting point and composition very similar to mutton-fat. This 

 shows that at least a portion of the fat introduced with the food has 

 been, for a time at any rate, laid down directly as body-fat. 1 



It has been further shown that dogs to which there has been 

 administered, along with their food, forms of fat which do not ordinarily 

 occur in the animal economy, will lay down a certain amount of this 

 along with their body-fat. This has been determined for spermaceti, 

 linseed oil, and rape oil. 2 That in pigs the fat of the body may also be 

 derived from the fat of the food, was shown in some of the experiments 

 by Lawes and Gilbert. 3 



Formation of fats from fatty acids. The question of the form 

 in which fats are absorbed has been already considered in a previous article 

 dealing with that subject, and it has there been shown that the fats of the food 

 are in large part not absorbed in the form of fat, but in that of fatty acid, 

 into which and glycerin they are broken up by the fat-splitting ferment of 

 the pancreatic juice ; and that they undergo a subsequent synthesis into fat 

 by combination with glycerin in the columnar epithelial cells of the small 

 intestine. 



That such synthesis is possible even in the absence of glycerin given with 

 the food, is shown by the experiments of I. Munk, who found that when a 

 dog was fed upon fatty acids in place of the fats of its ordinary food, just as 

 much fat was absorbed into the chyle and was laid down in the body as if 

 it had been fed with the complete fat. The columnar epithelial cells become 

 filled with fat globules, as after a meal containing actual fats ; and the synthesis 

 of fatty acid and glycerin to form fat must therefore have occurred in these 

 cells, which must themselves have produced, in some way which is not under- 

 stood, the glycerin necessary for the synthesis. 4 



Are fats formed from carbohydrate ? This is a question of great 

 practical importance, seeing that carbohydrate foods are the cheapest forms 

 of nutriment, and that the fattening of animals is an important branch of 

 agricultural industry. The experience of all rearers of animals for market 

 points to the fact that carbohydrates do produce fat. Sheep and oxen 

 fed purely upon grass, which contains hardly any fat and but little 

 proteid in proportion to the carbohydrate present, lay on a large amount 

 of fat, and the artificial foods which are used for fattening purposes 



1 Lebedeff, CentralU. f. d. med. Wissensch., Berlin, 1882, S. 129 ; Ztschr. f. physiol. 

 Chem., Strassburg, 1882, Bd. vi. S. 149 ; Arch. /. d. ges. Physiol., Bonn, 1883, Bd. xxxi. 

 S. 11; I. Munk, Arch. f. Physiol, Leipzig, 1883, S. 273 (Verhandl. d. physiol. 

 Gesellsch. zu Berlin) ; Virchow's Archiv, 1884, Bd. xcv. S. 407. 



2 Radziewski, Virchow's Archiv, 1868, Bd. xliii. S. 286 ; Lebedeff, loc. cit. ; I. Munk, 

 loc. cit. See also Minkowski, Arch. f. exper. Path. u. PharmakoL, Leipzig, 1886, Bd. xxi. 

 S. 373, and I. Munk and Rosenstein, Virchow's Archiv, 1891, Bd. cxxii. S. 230, for evidence 

 that foreign fats pass into the chyle. 



3 See note 2 on next page. 



4 For further details regarding these and similar experiments, see article on "Fat 

 Absorption," p. 443. 



