538 VI. OCCURRENCE OF LIPIDS IN THE ANIMAL 



of weeks during which the replacement diet continued. The replacement 

 of the "soft" fat by "hard" fat was accelerated if the animals were first 

 subjected to a fasting period before the new regimen was instituted. It 

 was reasoned that the unsaturated fatty acids were oxidized preferentially 

 by such a procedure, which left a more saturated fat in the tissues when the 

 corn diet was started. However, Longenecker 51 failed to demonstrate any 

 preferential utilization of the unsaturated fatty acids in rats having a normal 

 body fat, when these rats were fasted until they had lost 15 to 25% of their 

 body weight. 



{2) Carbohydrates as a Source of Body Fat 



It has already been shown that the nature of the storage fat laid down in 

 the rat or in the pig is profoundly influenced by the presence of carbo- 

 hydrate in the diet. It has long been recognized that foodstuffs other than 

 lipids can be a source of body fat. The widespread use of corn as a fatten- 

 ing agent for hogs is certainly not based upon its fat content, which is 

 minimal, but rather upon its starch content, which makes up over 70% of 

 the grain. For a discussion of the interconversion of foodstuffs, the reader 

 is referred to the excellent review of Rapport. 102 Deuel and Morehouse 103 

 have also discussed the interplay between carbohydrate and fat. 



a. Experimental Proof of the Conversion of Carbohydrate to Fat. The 

 first experimental evidence of the transformation of carbohydrate to fat 

 was obtained by Lawes and Gilbert, 104 in 1866, in England, and by Meissl 

 and Strohmer, 105 in 1883, in Germany. By the use of balance experiments, 

 these workers found that a considerable portion of the ingested carbon 

 could not be accounted for in the excreta (urine, feces, and expired carbon 

 dioxide), after large amounts of food had been given. After subtracting 

 the carbon, which could have been retained in the stored protein (as de- 

 termined from the nitrogen retained), a large balance of carbon was still 

 unaccounted for. Since this residual carbon exceeded the carbon present 

 in all stored carbohydrate (as determined by the analysis of the carcass), 

 the conclusion is inescapable that this carbon moiety must have been con- 

 verted to fat and stored as such. The conversion of carbohydrate to fat 



102 D. Rapport, Phijsiol. Revs., 10, 349-472 (1930). 



103 H. J. Deuel, Jr., and M. G. Morehouse, Advances in Carbohydrate Chem., 2, 119-160 

 (1946). 



104 J. B. Lawes and J. H. Gilbert, Phil. Mag. [4], 32, 439-451 (London, Edinburgh and 

 Dublin, 1866). 



106 E. Meissl and F. Strohmer, Sitzber. Akad. Wiss. Wien, Math.-naturw. Klasse, 88, 

 Abt. Ill, 205-218 (1883). 



