CHANGES IN THE INTESTINAL WALL 437 



group is probably to be ascribed to the fact that these animals grew nor- 

 mally, as compared with those in the other two groups, and used a consider- 

 ably large portion of the vitamin A for the growth process, so that less 

 remained for storage. 



Guggenheim^® reported that the rate of absorption, not only of carotene 

 but also of vitamin A, decreased following the prolonged administration of 

 atabrine. For an extended discussion of the absorption of vitamin A, 

 the reader is referred to The Lipids, Vol. II, pages 310 to 320. 



3. Changes in the Carotenoids and Vitamin A in the Intestinal Wall 



(1) The Transformation of ^-Carotene and of Other Provitamins A into 



Vitamin A 



a. /3-Carotene. Until recently, on the basis of the classic experiments of 

 Moore, ^^ it had been generally accepted that the liver is the main site, if 

 indeed not the only one, where carotene can be changed to vitamin A in 

 the animal body. However, a number of recent observations have led to 

 a revision of this concept. In the first place, Sexton and co-workers*^ 

 reported experimental data which led them to postulate that, in rats, the 

 intestinal wall is a probable site for the transformation of carotene to 

 vitamin A. A number of facts support this hypothesis. Thus, no appre- 

 ciable quantity of carotene appears in the hvers of rats, irrespective of the 

 amount of this provitamin in the diet. Moreover, when /3-carotene was 

 administered in oil solution parenterally by the intrasplenic route, the pro- 

 vitamin was deposited in the liver as such. Apparently, it could not be 

 changed to \'itamin A under these conditions, since it could not counteract 

 the symptoms of avitaminosis, and the rats receiving the vitamin A-defi- 

 cient diet died, displaying typical symptoms of vitamin A-deficiency, in 

 spite of the fact that their livers were found to contain sufficient /3-carotene 

 to supply the necessary vitamin A requirement for a year, had the liver 

 been able to transform the /3-carotene into vitamin A. No intrinsic impair- 

 ment of the carotene utilization occurred in these animals; the avitaminosis 

 symptoms promptly cleared up when the provitamin A was given orally. 

 Lease et aU^^ recorded a similar absence of utihzation of carotene when given 

 parenterally. A third experimental observation which argues for the con- 

 version of carotene to vitamin A in the gut wall is the fact that vitamin A 

 accumulates in the intestinal wall after carotene feeding, before any 

 increase can be noted in the liver. Higher levels of vitamin A were found 



151 J. G. Lease, E. J. Lease, H. Steenbock, and C- A. Baumann, /. Lah. Clin. Med., 

 27, 502-510 (1941-1942). 



