PHYSIOLOGY OF ADIPOSE TISSUE 611 



cate that mobilization and replenishment of body fat proceeds continu- 

 ously, irrespective of the nutritional status of the animal. When an excess 

 of calories is ingested, the total fat stores are increased. It is well known 

 that, under the opposite condition, the total fat reserves become depleted. 

 However, even in this latter situation, new fat replaces some of the old fat 

 present in the fat depots. 



Schoenheimer 118 noted an apparent constancy in the fat stores in normal 

 adult animals. This is not to be traced to an inertia of the fat depots, but 

 rather to the fact that a very delicate balance exists between the processes 

 leading to the deposition of depot fat and the processes tending to cause a 

 mobilization and utilization of adipose tissue. A decrease in the magnitude 

 of the fat depots occurs not only in states of fasting or undernutrition in 

 which deposition of new fat is retarded, but in such abnormal conditions as 

 thiamine deficiency 425 and diabetes. 426 The extent of fat deposition may 

 also decrease as a result of an increased mobilization of depot fat such as 

 may occur in a variety of conditions described in Section c below. 



b. Fat Synthesis in Adipose Tissue. It has already been demonstrated 

 that fat is normally synthesized continuously from carbohydrate and pro- 

 tein in the animal organism (see pages 538-552). In addition to the earlier 

 demonstrations based upon balance experiments or upon changes in R.Q., 

 the subsequent demonstration of the incorporation of deuterium in the tis- 

 sue fat of animals receiving a carbohydrate diet clearly proves this synthe- 

 sis. 41 It has been generally believed that the newly synthesized fat, con- 

 sisting of Cie and Ci 8 acids, 427 is manufactured in the liver. 



However, several sets of experimental data are difficult to correlate with 

 the assumption that the liver is the sole site of fat synthesis. Although the 

 half-life of fatty acids in the liver is 2.6 to 2.8 days, the rate of synthesis of 

 fat from carbohydrate would require a complete turnover of fats in the liver 

 four times daily. 42s Secondly, Tepperman and associates 119 have postu- 

 lated that the conversion of carbohydrates to fats may occur not only in the 

 liver but also in the extrahepatic tissues. This conclusion is based upon a 

 high R.Q. following the administration of glucose, not only in control rats 

 which had been trained to consume their food rapidly, but also in rats in 

 which the liver was functionally absent. 



Since it was shown that glycogen can be deposited in adipose tissue, 

 Tuerkischer and Wertheimer 120 suggested that the adipose tissue is also a 



425 G. E. Boxer and De W. Stetten, Jr., J. Biol. Chem., 158, 607-616 (1944). 



426 De W. Stetten, Jr., and G. E. Boxer, J. Biol. Chem., 156, 271-278 (1944). 



427 H. E. Longenecker, Biol. Symposia, 5, 99-115 (1941). 



428 De W. Stetten, Jr., and G. F. Grail, J. Biol. Chem., U8, 509-515 (1943). 



