610 VI. OCCURRENCE OF LIPIDS IN THE ANIMAL 



no hardening agent is present in the diet. For a further discussion of the 

 effect of diet on the nature of the fat deposited, see page 524. 



In spite of these variations in depot fat which can be ascribed to diet, 

 adipose tissue itself is able to bring about a certain amount of modification 

 of the fat supplied to it before new adipose tissue is laid down. While 

 Schoenheimer and Rittenberg 40 demonstrated that the body could desatu- 

 rate deuteriostearic acid to deuterium-containing unsaturated acids, the 

 site of the transformation was not determined. However, a number of 

 workers 419-422 proved that desaturation can be carried out in adipose tissue. 

 This assumption also affords a convenient explanation for the finding of 

 Henriques and Hansen 18 that the fats deposited in deeper layers of the 

 body are more saturated, as determined by their iodine number, than are 

 those laid down in the more superficial layers of storage fat. Wertheimer 

 and Shapiro 401 suggest that this relationship between tissue temperature 

 and the saturation of the deposited fat may result from the effect of tem- 

 perature on the equilibrium in the interchange between saturated and un- 

 saturated acids. Hilditch 116 proposed, as an alternative explanation, that 

 the several fat depots have the ability to select the correct proportion of 

 saturated and unsaturated fatty acids from the fatty acids furnished them. 

 The desaturation of the fatty acids would occur in other tissues, such as 

 the liver. Although this latter hypothesis has no definite experimental 

 support, its acceptance also predicates that the adipose tissue is not a pas- 

 sive storage depot, but a site in which active work can be done to make the 

 proper selection of fats for such deposition. 



a. The Mobile Nature of Fat in Fat Depots. The earlier and widely ac- 

 cepted explanation of the addition or removal of the fat from adipose tissue 

 was that these changes occurred only in the presence of a caloric excess or a 

 caloric deficit, respectively. However, the results of Schoenheimer and 

 Rittenberg 423 with labeled fats demonstrated that ingested fats become in- 

 corporated in the body fats even though no change in the total depot fats 

 obtains. After deuterio-fats were fed over a period of four days on a diet 

 which supported a caloric equilibrium, as much as 50% of the ingested fats 

 appeared in the storage depots. Moreover, Bernhard and Steinhauser 424 

 demonstrated a similar interchange of fat in fasted mice. These data indi- 



419 G. Quagliariello, Rend, accad. nazl. Lincei, [6], 16, Classe set. fis. mat. nat., 552-554 

 (1932). 



420 S. Yosii, /. Biochem. {Japan), 26, 397-424 (1937). 



421 B. Shapiro and E. Wertheimer, Biochem. J., 37, 102-104 (1943). 



422 J. Champougny and E. Le Breton, Compt. rend. soc. biol, 1J+1, 43-45, 45-48 (1947). 



423 R. Schoenheimer and D. Rittenberg, J. Biol. Chem., 114, lxxxvii (1936). 



424 K. Bernhard and H. Steinhauser, Helv. Chim. Acta, 27, 207-210 (1944). 



