524 VI. OCCURRENCE OF LIPIDS IN THE \NIMAL 



pending upon the amount of fat in the diet, and the rate of growth; and 

 third stage animals (ruminants such as the cow) : depot fat unaffected by 

 linseed oil and consisting essentially of synthetic fat. Hilditch, 7 in com- 

 menting on the Shorland article, advances the fact that the composition of 

 pig fat, from an animal fed on whale oil, may resemble the latter fat to a 

 great extent. 8 Moreover, reference is made to the fact that linolenic acid 

 appears in the fat of grass-fed horses; this apparently originates from the 

 grass glycerides. 910 Although these examples are entirely correct, they 

 do not invalidate the Shorland hypothesis, since neither the pig nor the 

 horse can be classed as a ruminant. 



3. Sources of Fat in the Animal Body 



(1) Dietary Fat as a Source of Lipids 



The primary substances from which one might expect the tissue lipids 

 to originate are the fats which occur in the diet. This assumption is of 

 only extremely limited application, in the case of the higher animals, and a 

 correspondence between the ingested fat and that deposited in the fat de- 

 pots can ordinarily not be demonstrated. However, when large amounts of 

 unusual fats are given to animals, evidence of the deposition of the un- 

 changed food fat in the fat depots of the animal may sometimes be demon- 

 strated. As has already been stated, a much closer correspondence exists 

 between the dietary fat and the body fat in the lower animals than in 

 those higher in the evolutionary scale. 



Under ordinary conditions in which a mixed diet is employed, animals 

 lay down a fat characteristic of the species. The nature of this fat is 

 influenced by sex hormones as well as by other hormones, and probably 

 also by environmental conditions. These usually assist in establishing 

 the fatty acid composition which is characteristic of the particular species. 



In some cases, dietary fat plays no role in the composition of deposit fat. 

 Shorland 4 has pointed out that the composition of beef and mutton tallow 

 is substantially unaffected by the nature of the dietary fat. It is proposed 

 to designate this type of fat as heterolipids, to indicate its lack of resemblance 

 to dietary fat. On the other hand, most animal fats are subject to a 

 modification in composition as a result of the dietary fat. These fats are 

 referred to as homolipids, which term connotes that they readily incorporate 



7 T. P. Hilditch, note to a letter of F. B. Shorland, Nature, 170, 925-926 (1952). 



8 G. A. Garton, T. P. Hilditch, and M. L. Meara, Biochem. J., 50, 517-524 (1952). 



9 E. G. Brooker and F. B. Shorland, Biochem. J., 46, 80-85 (1950). 

 10 S. S. Gupta and T. P. Hilditch, Biochem. J., 48, 137-146 (1951). 



