CHANGES IN THE INTESTINAL WALL 447 



the other hand, amino acids, which are probably not phosphorylated dur- 

 ing absorption, are unaffected by the thyroid hormone. On the basis of 

 this reasoning, it is difficult to see how the thyroid could enhance the 

 absorption of a hydrocarbon like carotene which cannot possibly be phos- 

 phorylated. Goodwin^" concludes that, although the bulk of evidence 

 suggests that the thyroid functions, in carotene metabolism, by increasing 

 the absorption of the provitamin, one cannot completely rule out the 

 possibility that this effect is to be ascribed to the influence of the hormone on 

 the carotenase activity. DrilP^* reviewed the subject of the thyroid in 

 relation to carotene metabolism up to 1943. He concluded that "all the 

 evidence to date (1943) indicates that, in the absence of the thyroid gland, 

 carotene is not metabolized to vitamin A." 



{})) The Effect of H ypervitaminosis A . In addition to hyperthyroidism, 

 hypervitaminosis A has been found to have a profound effect on the me- 

 tabolism of carotene. In this case there is no evidence to challenge the hy- 

 pothesis that the carotene — »- vitamin A reaction proceeds at an augmented 

 rate. Thus, Deuel and co-workers^^^-^^^ reported that, when massive doses 

 of vitamin A (70,000 to 1 ,690,000 Mg- per day) were given to cows, the plasma 

 carotene and also that present in the milk gradually decreased to mini- 

 mum values, coincident with a marked increase of vitamin A in the milk, 

 in spite of the fact that the cows were maintained on a uniform diet. The 

 depressing action of large doses of vitamin A on the carotene content of 

 the blood of calves and milk of cows was confirmed by Jensen et al.,^^^ by 

 Fountaine and Bolin,^^'' by Blaxter, Kon, and Thompson, ^^^ and by Eaton 



A similar physiologic response to excessive doses of vitamin A was 

 observed in the case of chickens by the Deuel group.^""-^"^ Thus, when the 

 vitamin A supplement present in the food was increased from to 60,000 



1" V. A. Drill, Phijsiol. Revs., 23, 355-379 (1943). 



13* H. J. Deuel, Jr., N. Halliday, L. Hallman, C. Johnston, and A. J. Miller, /. 

 Nutrition, 22, 303-313 (1941). 



135 H. J. Deuel, Jr., L. F. Hallman, C. Johnston, and F. Mattson, /. Nutrition, 23, 567- 

 579 (1942). 



"6 C. Jensen, P. D. Boyer, P. H. Phillips, I. W. Rupel, and N. S. Lundquist, /. 

 Dairtj Sci., 25, 931-937 (1942). 



1" F. C. Fountaine and D. W. Bolin, ./. Dairy Sci., 27, 155-158 (1944). 



"s K. L. Bla.\ter, S. K. Kon, and S. Y. Thompson, ./. Dairy Research, Lj, 225-230 

 (1946); Chem. Abst., 41, 3845 (1947). 



139 H. D. Eaton, A. A. Spielman, J. K. Loosli, J. W. Thomas, C. L. Norton, and K. L. 

 Turk, J. Dairy Sci., 30, 795-802 (1947). 



200 H. J. Deuel, Jr., M. C. Hrubetz, F. H. Mattson, M. G. Morehouse, and A. Richard- 

 son, J. Nutrition, 26, 673-685 (1943). 



20' F. H. INIattson and H. J. Deuel, Jr., ./. Nutrition, 25, 103-112 (1943). 



