784 PHYSIOLOGY 



COMPOSITION OF FAT 



The fats occur generally in the form of triglycerides of various fatty 

 acids. In adipose tissue the acids are chiefly stearic, palmitic, and oleic, 

 the consistency of the fat depending on the relative amount present of 

 triolein, with its low melting-point. In certain animals the glycerides of 

 more unsaturated fatty acids occur. Thus lard contains about 10 per 

 cent, of fats belonging to the linoleic series. The fats of cows' milk, though 

 consisting chiefly of the three above mentioned, include also the esters of 

 butyric and caproic acids in fair amounts, and traces of the intermediate 

 acids, caprylic, capric, lauric, and myristic acids. 



The ' fat ' extracted from the tissues (e.g. heart muscle) includes a 

 considerable amount of ' phosphatides ' (lecithins, &c.). It also contains 

 a much larger proportion of unsaturated fatty acids, of the linoleic and even 

 lower series, so that its ' iodine value ' is generally found relatively high 

 (120 as compared with 40 to 60 in adipose tissue). 



FUNCTIONS OF FAT 



First and foremost must be mentioned the significance of fat as a reserve 

 food store. The power of the organism to store up reserve carbohydrate is 

 strictly limited. The liver of man can probably not accommodate more 

 than 150 grm. of glycogen, and assuming that the muscles of the body may 

 contain an equal amount, 300 grm. represents the extreme limit of storage 

 of carbohydrates in the body. On the other hand, in most animals there 

 is practically no limit to the amount of fat which can be laid down, and 

 over-feeding, whether with carbohydrates or fats, leads to the deposition 

 of fat. This fat does not enter into the normal metabolism of the body, 

 but is available for use whenever the needs of the body are increased above 

 its income. 



As to the part taken by fat, especially the hidden fat of the working 

 cells, in the chemical processes which determine the life of the cell, our 

 knowledge is still very scanty. Fats enter into the constitution of the 

 complex bodies lecithin and myelin, which form important constituents of 

 the limiting membrane of every living cell. As constituents of the mem- 

 brane itself, fatty substances therefore have a protective action, and also 

 regulate the passage of substances into the cell across the membranes. 



The presence of lecithin as an integral constituent of all protoplasm, 

 and of the first products of disintegration of protoplasm, suggests that 

 this substance may play a part in the normal transformations which occur 

 within the cell, and may represent, so to speak, the currency into which 

 fat is transformed in order to participate in the vital processes, and that 

 it is in this form that the energy of fat is utilised for the needs of the cell. 



ORIGIN OF FAT IN THE BODY 



Fat formation is the result of an excess of income over expenditure. 

 As soon as the latter exceeds the former the fat store is drawn upon, so 



