SOURCES OF FAT IN ANIMAL BODY 525 



dietary fatty acids into their structure. While most animal fats belong to 

 this latter group, some, such as cow's milk fat, are of an intermediate type; 

 the composition of this type of fat is not readily altered by the nature of 

 the dietary fat. However, by feeding certain oils, such as rapeseed oil, it 

 is possible to incorporate small proportions of erucic acid into the milk 

 fat. 



There are many instances in which the effect of diet on homolipid fats 

 has been followed. When an unusually large proportion of fat is given in 

 the diet, it will not all be metabolized, but will be laid down in the tissues, 

 largely in its original form. If this resembles the normal body fat of the 

 particular species involved, then the only result may be that the total 

 proportion of fat in the tissues is increased. On the other hand, if some of 

 the ingested fatty acids are foreign to the body, they may be absorbed and 

 deposited in the fat depots in sufficient amount to reveal the effect of the 

 food fat on the body fat. The higher the concentration of the fat in the 

 diet, the shorter the period which will be required for the storage fat to be 

 altered. 



The fats in the several tissues have different rates of turnover; this 

 explains why a substitution of a new fat may occur in one tissue before it 

 occurs in another tissue. Thus, the blood and liver lipids are the most 

 susceptible to alteration resulting from the fats consumed, since they are 

 primarily concerned with fat transport and fat oxidation. In contradistinc- 

 tion to the rapid interchange of food fats with those of the blood and liver, 

 the storage fats are much more slowly altered by dietary means. How- 

 ever, the nature of the fats in the fat depots may also be greatly changed 

 before any appreciable alteration obtains in brain lipids. 11,12 The slug- 

 gishness of the brain in altering its fat composition may be related to the 

 inability of this tissue to oxidize, fat or ketone bodies. For a more com- 

 plete exposition of the relationship of diet to fat deposition, the reader is 

 referred to the monograph of Leathes and Raper, 13 as well as to the more 

 recent reviews of Anderson and Williams, 14 and of Burr and Barnes. 15 



The classical experiments on the interplay of dietary and body fats are 

 those of Lebedeff. 16 This investigator fasted two dogs until each had lost 



11 R. G. Sinclair, J. Biol. Chem., 86, 579-586 (1930). 



12 K. P. McConnell and R. G. Sinclair, J. Biol. Chem., 118, 131-136 (1937). 



13 J. B. Leathes and H. S. Raper, The Fats, 2nd ed., Longmans, Green, London-New 

 York, 1925. 



14 W. E. Anderson and H. H. Williams, Physiol. Revs., 17, 335-372 (1937). 

 16 G. O. Burr and R. H. Barnes, Phijsiol. Revs., 23, 256-278 (1943). 



16 A. Lebedeff, Centr. Med. Wissensch., 20, 129-130 (1882); Arch. ges. Physiol. 

 (Pfliiger's), 81, 11-59 (1883). 



