DEPOSITION OF LIPIDS IN DIFFERENT SPECIES 523 



an altered metabolism related to the increasing complexity of the animal 

 organism. Each animal presumably lays down its own typical fat. For a 

 further discussion, see Volume I, pages 184-194. 



Shorland 3 has since proposed a new theory to account for species dif- 

 ferences in fat composition. This ascribes more importance to the in- 

 gested fat and less to the ability of the animal to lay down its own typical 

 fat. Thus, in the early stages of evolutionary development, an organism 

 such as the fish absorbs the dietary fat, and deposits it, with only minor 

 changes in its composition, such as hydrogenation, dehydrogenation, or, 

 in some cases, shortening of the carbon chains. At this stage of develop- 

 ment, the animals are unable to synthesize fat from proteins and carbo- 

 hydrates, with the result that the fat more or less resembles that ingested. 



In the case of amphibians, which represent an intermediate stage in 

 development, the animal can still deposit the dietary fat in an unchanged 

 form. However, at this stage, the animals have developed the capacity 

 to synthesize fats. These fats have, as their main components, palmitic, 

 stearic, and oleic acids. The resultant depot fat of any species in this 

 category will depend upon the relative proportion of dietary and synthetic 

 fat which the animals contain. 



The final stage is represented by ruminants, which have, to a large 

 extent, lost their ability to deposit any of the dietary fat 4 in their tissues; 

 most of the fatty acids of such fat are broken down by bacteria in the 

 rumen before assimilation by the animal. 3 Under such conditions the 

 depot fat largely mirrors the composition of the fat synthesized in the body 

 from carbohydrates and proteins. 



Proof that the fat in the lower forms is related to diet has been adduced 

 in several ways. For example, Green and Hilditch 5 showed that, when 

 carp are fed on grass, they deposit diene and triene fatty acids character- 

 istic of the grass lipids 3 ; these differ in composition from those of other 

 marine forms. Lovern 6 likewise demonstrated that the composition of the 

 original body fat of eels may be modified if fats of a different fatty acid 

 makeup are fed to this species in relatively large amounts. Shorland 3 

 has indicated that the following effects on the body fat of different species 

 would be noted on feeding linseed oil: -first stage animals (fishes): depot 

 fat resembling linseed oil; second stage animals (amphibians): depot fat 

 consisting of a mixture of synthetic fat and linseed oil, the proportion de- 



3 F. B. Shorland, Nature, 170, 924-925 (1952). 



4 F. B. Shorland, Nature, 165, 766 (1950). 



5 T. G. Green and T. P. Hilditch, J. Soc. Chem. Ind., 55, 4-8T (1936). 



6 J. A. Lovern, Biochem. J., 32, 1214-1224 (1938). 



