DIGESTION, ETC. OF CAROTENOIDS IN THE G.I. TRACT 309 



occurs, namely, that carotene is converted to vitamin A more efficiently 

 than in the animal which possesses a normal thyroid function. 



There are a number of reports in the literature on the effect of hypo- 

 thyroidism on carotene metabolism when the deficiency has been pro- 

 duced by the administration of thiouracil or thiourea. Thus, Johnson 

 and Baumann 394 reported that very little storage of vitamin A occurred in 

 the livers of rats treated with thiourea or thiouracil followed by the ad- 

 ministration of carotene; thyroxine increased the ability of these animals 

 to convert carotene to vitamin A. Kelley and Day 395 confirmed the 

 fact that vitamin A deposition in the liver after the feeding of carotene 

 was decreased in thiouracil-treated rats and increased when thyroid was 

 given. However, differences recorded by these workers in the several 

 conditions are not great. Canadell and Valdecasas 396 likewise reported 

 that carotene was unable to relieve the ocular symptoms of a vitamin A 

 deficiency in a thiouracil-treated animal. However, these symptoms 

 could be cleared up if small amounts of desiccated thyroid were admin- 

 istered with the carotene. 



Although all the above reports are in agreement in supporting the hy- 

 pothesis that the carotene -> vitamin A change is regulated by the thyroid 

 gland, several reports are in disagreement. Thus, Wiese and her col- 

 laborators 397 were unable to demonstrate any differences in the liver 

 storage of vitamin A after the administration of 348 fig. of /3-carotene, 

 irrespective of whether or not the rats had previously been treated with 

 thiouracil. In a later study by these same investigators, 398 it was found 

 that, although the extent of maximum growth, after vitamin A or caro- 

 tene feeding, is markedly depressed by hypothyroidism, the point of 50% 

 response to carotene or vitamin A was unaltered. These results are taken 

 to indicate that carotene and vitamin A are equally well utilized by hypo- 

 thyroid rats when fed at low levels. In these later tests, no differences 

 were observed in the comparative amounts of vitamin A and carotene 

 required to bring about growth or to relieve eye symptoms in vitamin A- 

 deficient hypothyroid rats. Morgan and Arnrich 399 also concluded that 

 the normally functioning thyroid gland is not essential for carotene utiliza- 

 tion in rats and dogs. These workers reported that young vitamin A- 



394 R. M. Johnson and C. A. Baumann, J. Biol. Chem., 171, 513-521 (1947). 



395 B. Kelley and H. G. Day, /. Biol. Chem., 175, 863-866 (1948). 



396 J. M. Canadell and F. G. Valdecasas, Experientia, 3, 35-36 (1947). 



397 C. E. Wiese, H. J. Deuel, Jr., and J. W. Mehl, Proc. Soc. Exptl. Biol. Med., 66, 

 213-214 (1947). 



398 C. E. Wiese, J. W. Mehl, and H. J. Deuel, Jr., /. Biol. Chem., 175, 21-28 (1948). 



399 A. F. Morgan and L. Arnrich, Federation Proc, 12, 424-425 (1953). 



