LIPID STORAGE UNDER ABNORMAL CONDITIONS 629 



Although there is general agreement that the onset of obesity is pri- 

 marily associated with the hyperphagia, 379 ' 519 ' 522 ' 530 ' 534 Brobeck et al. 319 

 cited experimental evidence to indicate that other factors may be involved. 

 When a comparison in weight gain was made between normal rats and 

 operated animals, using the pair-feeding technic, it was found that the 

 same rate of gain obtained in the two groups in 75% of the cases. How- 

 ever, in 25% of the tests, the rats subjected to the hypothalamic injury 

 gained somewhat better than did their pair-fed controls. 



A second factor, other than hyperphagia, which may be partly respon- 

 sible for the obesity resulting from hypothalamic injury, is a decrease in the 

 spontaneous locomotor activity. Although a brief period of extreme hy- 

 peractivity occurs after the animal has recovered from the anesthetic 

 following the brain operation, this is followed by a period of inactivity, 

 which persists for several months. This inactivity has been observed not 

 only in the rat, 379,520,530 but also in the cat. 522 On the one hand, Hethering- 

 ton and Ranson 530 considered that the inactivity was the primary source of 

 the excess energy stored as fat by their animals, while the food intake was 

 thought to be a factor of minor importance. However, on the other hand, 

 most other workers 379 ' 520522 are of the opinion that the hyperphagia is the 

 principal cause of obesity, particularly when ad libitum feeding is employed, 

 while the inactivity is a contributory cause of obesity, important only under 

 certain conditions. It has been pointed out by Brooks 520 that inactivity 

 itself does not always lead to obesity. Moreover, although the extent of 

 activity may be decreased in rats with injuries of the hypothalamus, the 

 total energy expenditure for the decreased activity may exceed that of the 

 controls, inasmuch as the animal must exert more force to move the fat- 

 laden members of its body during the movements required for drinking, 

 eating, respiration, and for like activities. 



A third condition which may influence fat deposition in animals suffering 

 from hypothalamic injury is the reduction of the total heat production. 

 In this situation, the food would be more efficiently utilized and the pro- 

 portion of energy available for storage as fat would be increased. How- 

 ever, this cannot account for the obesity of the rats receiving the ad libitum 

 diet, because an actual increase of heat production obtains in the obese ani- 

 mals, which may become twice as great as it was before the operation. 379 - 534,535 

 Brobeck 380 is of the opinion that the energy surplus is mainly the result 

 of the hyperphagia. In spite of the marked increase in total heat produc- 

 tion in the obese rats following hypothalamic lesions, there is some evidence 



634 C. Mc C. Brooks and D. N. Marine, Federation Proc, 5, 12 (1946). 



535 J. M. Bruhn and A. D. Keller, Am. J. Physiol, 133, 229P-230P (1941). 



