FATTY METAMORPHOSIS 401 



Physiological Formation of Fat 



Concerning the normal formation of fat we may summarize the evidence as 

 follows : 



(1) A large proportion of the fat of the body comes from the fat taken in the 

 food, as also does the fat of the milk. This can be shown, as Rosenfeld particu- 

 larly demonstrated, by starving an animal until it is as free from fat as possible, 

 then feeding with a large amount of some fat that is of a type different from that 

 normally found in the animal; the new fat that it then laid up in the fat depots of 

 the animal will partake of the characters of the fat given in the food. In case the 

 animal is lactating, the milk-fat will also resemble the fat of the food. As a 

 matter of fact, the body fat is not of constant composition, even in the same 

 individual; it varies greatly with age, having much less olein in infancy than in 

 later years, varying somewhat in composition in the different fat depots in the 

 same body, and apparently being more or less modified by diet. 



(2) Fat may also be formed from carbohydrates. According to Rosenfeld, this 

 fat differs from the fat formed on mixed diet in having less olein in proportion to 

 the palmitin and stearin, and it is deposited particularly in the subcutaneous and 

 mesenteric tissues rather than in the liver. Man does not seem to form fat readily 

 from carbohydrates, but rather burns them to protect his proteins; on the other 

 hand, swine and geese readily form fat from carbohydrates. As the fatty acid 

 radicals of ordinary fat (C18H36O2, Ci6H.-;202, C1SH34O2), are much larger than the 

 carbotiydrate radicals, a process of synthesis must be involved in the formation of 

 fat from carbohydrates.^ 



(3; Proteins are a possible source of fat, but it has not been established that 

 they are either a common or an important source of fat in either physiological or 

 pathological conditions, or, indeed, that they really ever do form fat. Upon this 

 statement rests our present tendency to refute the long-cherished conception of 

 fatty degeneration as a true degeneration of cell proteins into fat, as suggested by 

 Virchow. This view was supported by the earlier work of ^"oit and his school, who 

 believed that they had demonstrated that animals could form fat from protein 

 food, and their work was for a long time accepted as correct. Later Pfliiger and 

 his pupils pointed out what seem to have been essential errors in these investigations, 

 and, after much discussion and experimentation, the majority of physiologists now 

 support the view advanced in the sentence opening this paragraph. Since proteins 

 contain carbohydrate groups, and since fats can be formed from carbohydrates, 

 the possibility of the formation of fats from the proteins in this indirect way 

 cannot be denied. It is also possible that the nitrogen-containing groups may be 

 split out of the amino-acids of the protein molecule, and that the non-nitrogenous 

 residues can then be built up into fatty acid molecules as large as the molecules of 

 stearic, palmitic, and oleic acids; but we have no proof that either of these processes 

 occurs in the normal cell or in the cell that is undergoing degeneration. Atkinson 

 and Lusk' have obtained evidence of some fat formation from meat fed to a dog, 

 but this was only slight and obtained with difficulty. 



Pathological Fat Accumulation 



For a long time fatty degeneration was looked upon as one of the 

 chief evidences that fat was formed directly from protein, for the 

 cell protoplasm seemed, morphologically, to be changed directly into 

 fat in this process. Additional support was also claimed from the 

 supposed increase in fat in the ripening of cheese;^ from the forma- 

 tion of abundant fat by maggots living in fat-poor blood or fibrin; 

 and by the apparent conversion of proteins into fatty acids and soaps 



2 This, Magnus-Levy suggests, may be accomplished through lactic acid which 

 is formed from sugar, and then, after reduction to an aldehyde, several of these 

 molecules are combined into the higher fatty acid. See Leathes, loc. cit., p. 82. 



3 Proc. Natl. Acad. Sci., 1919 (5), 2-16. 



* Even the increase of fat in ripening cheese is doubtful (Nierenstein, Proc. 

 Royal Soc, B., 1911 (83), 301; Kondo, Biochem. Zeit., 1914 (59), 113). 



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