FAT METABOLISM 735 



saturation is probably a preliminary step to this incorporation of the fatty- 

 acid molecule into lecithin, for it is well known that lecithin contains highly 

 unsaturated fatty-acid radicles. In support of such a view it is interesting 

 to note that in alcohol-ether extracts from normal and pathological livers, 

 the lecithins, which are precipitated by acetone, have higher iodine values 

 (i.e., are more unsaturated) than the neutral fats extracted from the same 

 liver, which also have higher iodine values than the depot fat of the same 

 animal. Conversion of fat into lecithin also occurs extensively in the 

 blood itself (page 728). 



Desaturation of fatty acids occurs in other places besides the liver. The 

 relative activity of the different tissues in this regard has been studied by 

 feeding cats with fatty fish and then determining the iodine value of fat 

 from various places in the body. The absorbed fat was more obvious in the 

 liver than in the subcutaneous tissues, because it had not become diluted 

 with fat deposited, it may have been months, previously, which would be the 

 case in the fat of the fat depots; and it was found that, although the 

 iodine value of the subcutaneous fat was slightly raised, that of the 

 liver was much more so, indicating that the desaturation process had 

 been more active in this organ, but had also occurred to a certain extent 

 in the depots. 



Before leaving this subject of fat in the liver, it is important to re- 

 call the old observation of Rosenthal, that a more or less reciprocal 

 relationship exists between glycogen and fat in the liver. When much 

 glycogen is present there is little or no fat, and vice versa. It is impor- 

 tant to note that the exact locations of fat and carbohydrate in the he- 

 patic lobule are somewhat different. 



A practical clinical application of the above work is found in the fact 

 that fats will be more readily utilized by the body when they contain a 

 high percentage of unsaturated fatty acids. It is possibly for this 

 reason that Norwegian cod-liver oil is of such undoubted nutritive value. 

 It is much more so than Newfoundland cod-liver oil, because in the prep- 

 aration of this variety oxidation occurs, which makes it no longer unsat- 

 urated. Fish oils in general are more unsaturated than other animal 

 oils, and are for this reason more nutritious. The presence of vitamines 

 rather than their assimilative properties may, however, be the factor 

 which determines the nutritive value of different brands of cod-liver oil. 



The fat in the tissues differs very materially from that of the liver or 

 the depots. Only 60 per cent of this fat consists of fatty acid, which is 

 present very largely as part of the lecithin molecule, thus accounting for 

 the high iodine value. Some is probably also present as simple glyceride, 

 in a highly unsaturated and therefore very fragile condition. 



