432 IX. CAROTENOIDS AND VITAMINS A 



The stimulating effect of thyroxine and the depressing effect of thiouracil 

 on the carotene utiHzation of lactating cows and goats is even more strik- 

 ingly evident in the data on digestibility than in those concerned with 

 absorption. Thus, Chanda et al.^'^ report the following average digesti- 

 bility figures for j^-carotene: Cows, control group, 57.4%, thyroxine- 

 treated group, 71.5%, and thiouracil-treated group, 49.4%; Goats, control 

 group, 61.5%, thyroxine-treated group, 74.0%, and thiouracil-treated 

 group, 50.9%. 



Comparatively few studies have been made on the digestibility of carot- 

 enoids other than /3-carotene. Johnson and Baumann'^ did report experi- 

 ments on the digestibilities of some of the more common carotenoids in 

 rats when they were fed in cottonseed oil in amounts of 2 to 78 fxg. daily. 

 The digestibilities were reported as 64% for cryptoxanthin, 60% for (3- 

 carotene, and 46% for a-carotene. At these low intakes, the percentage of 

 excretion did not vary with the dose, nor was any depressing effect exerted 

 by lutein on digestibility when the latter carotenol was fed simultaneously 

 with the several provitamins A. These values are fairly close to those 

 obtained earlier by Fraps and Meinke,^^ which were 57 and 64% for (3- 

 carotene and for a-carotene, respectivelj'', when dissolved in oil. "Neo-/3- 

 carotene," which is a stereoisomer] c form of /S-carotene, w^as found to have 

 about the same coefficient of digestibility as the natural all-trans-/3-caro- 

 tene.^^ The digestibihty of the carotenoids is discussed on pages 300 to 

 SO^, Vol II, oi The Lipids. 



(2) Vitamins A 



a. Normal Absorption of Vitamins A. Under normal conditions, vitamin 

 A is much more readily absorbed from the gastrointestinal tract than are its 

 provitamins. This is because of its smaller molecular size, and probably 

 also because it possesses a primary alcohol group which can readily form 

 an ester with fatty acids. Moreover, in contradistinction to carotene, 

 vitamin A is well utilized when given by the parenteral route. ^^ Sobel and 

 co-workers^° reported that, when an aqueous dispersion of vitamin A was 

 given to cows by the intravenous route, the increase in vitamin A in the 

 milk was approximately fifteen times that noted when the same amount of 

 the vitamin was administered orally, in oil. 



Hydrolysis of vitamin A esters to the free alcohol is beheved to be a neces- 



" R. M. Johnson and C. A. Baumann, Arch. Biochem., 19, 493-501 (1948). 



88 G. S. Fraps and W. W. Meinke, Arch. Biochem., 6, 323-327 (1945). 



89 E. L. Sexton, J. W. Mehl, and H. J. Deuel, Jr., /. Nutrition, 31, 299-320 (1946). 



90 A. E. Sobel, A. Rosenberg, and E. Engel, /. Nutrition, 48, 183-192 (1952). 



