314 ESSENTIAL FATTY ACIDS 



studied in regard to their requirements for essential fatty acids;^'*-^^ it is 

 possible in Ephestia to differentiate effects of linoleic and arachidonic acids. 

 Several studies have dealt with the requirements of mammals for essential 

 fatty acids under various conditions. Rats are most commonly used, since 

 the deficiency causes well-defined signs and symptoms. In dogs, fat defi- 

 ciency is characterized by skin changes and decreased resistance to infection." 

 Although nutritive value of dietary fats depends on their content of linoleic, 

 linolenic, and arachidonic acids, the relative proportions are also important, 

 and forced exercise can elicit deficiency symptoms in rats fed a diet con- 

 taining horse fat, even though this fat supports growth better than does 

 linseed oil.^^ In rapidly growing pigs, less linoleic acid is incorporated into 

 back fat than during slower growth.^^ In rats, there is no sex difference in 

 the composition of stored fat, but males are more susceptible to essential 

 fatty acid deficiency.^'' Deficiency in essential fatty acids does not affect fat 

 absorption in rats.'*^ Effects of diet on distribution and composition of fat 

 in the rat have been intensively studied.^-' *^ Synthetic margarine, contain- 

 ing no unsaturated fatty acids or phosphatides, is well absorbed but pro- 

 duces diarrhea, renal damage, and yellow atrophy of the liver.^* Rats main- 

 tained for long periods on a high carbohydrate diet with deficiency in 

 essential fatty acids show a markedly increased absorptive capacity for 

 glucose, whereas mature rats fed a high fat diet for relatively short periods 

 show a decreased absorption of glucose.*^ Caffeine is reputed*^ to increase 

 the unsaturated fatty acid content of the liver when it is given orally, but 

 not subcutaneously. If a fat-free diet is given to rats during pregnancy, 

 cis,cis-9 , 12-linoleic acid may be found in the mother but not in the young.^^ 

 Offspring of rats fed a fat-poor diet develop Schachtelhalmschwanz, a rush- 

 like jointed tail.''* If deuterium-tagged fatty acids are fed to pregnant rats, 

 they are found also in the fetuses. ^^ It is possible to produce an essential 



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