MAMMALIAN CAROTENOIDS 



oils as a purge, have no effect on carotene absorption. Similarly, it is 

 important to note that alumina gel which may be administerecf over 

 protracted periods in the treatment of peptic ulcer, has no effect on 

 carotene absorption. ^ ^ ^ Absorption is improved by dispersing 

 agents. ^ ** ^ 



(b) Other Mammals 



When the absorption of carotenoids in animals other than humans is 

 considered, the same general situation is apparent except that in animals 

 which preferentially store carotenes these are probably preferentially 

 absorbed ; in the case of rats some xanthophylls are, however, absorbed 

 and then presumably suffer oxidative destruction. ^ ^ ^ Although, as 

 in humans, fat facilitates absorption, it is not essential in rats and ab- 

 sorption has been achieved on a diet containing only 0-04 per cent, of 

 fat. ^ ^ * Even under optimum conditions and using doses of only 1-2 

 fjig. per day, rats excrete 10-15 per cent, of the dose. ^^^-^^^ Cama 

 and Goodwin ^ ^ ' have recently shown that in the rat, the activity of 

 the thyroid gland conditions the efficiency of the absorption of carotene; 

 hypothyroidism reduces the absorption and vice versa. This has recently 

 been confirmed in cows, sheep ^^** and goats. ^^'^^ Fraps^^^ has 

 recorded somewhat indefinite results on the effect of bulk on the 

 absorption of carotene in rats and is has been stated that small rats 

 utilize p-carotene better than large ones. ^ ° ^ These and other con- 

 siderations raise considerable problems when the biological assay of 

 vitamin A preparations using crystalline [3-carotene as the International 

 Standard is considered. A discussion of this is outside the scope of this 

 chapter and readers are referred to a critical discussion of the problem 

 by Morton. 30 1 It has recently been agreed, internationally, that 

 crystalline vitamin A, and not (3-carotene, shall be used as the Inter- 

 national Standard for vitamin A assays although crystalline ^-carotene 

 is retained as the standard for assaying pro-vitamins A. ^^^^ 



Experiments on choledochocolostomized animals ^ ° ^ and on isolated 

 intestinal loops of normal animals ^ ° ^ indicate that bile is essential for 

 the absorption of carotene but not of vitamin A. This accounts for 

 the poor carotene absorption in jaundice. ^ ^ Dispersing agents increase 

 the rate of absorption of carotene in cows. ^ " ^ 



Even the presence of 0-08 per cent, of mineral oil in the diet of cows 

 (just sufficient to prevent dustiness in salt mixtures and lucerne leaf 

 meal) has a pronounced deleterious effect on carotene utilization. ^ » i 



(3-Carotene when homogenized in milk is well absorbed by cows 

 when a nipple feeder is used for administration ; it is less well 

 absorbed when given by stomach tube, ^^^a 



251 



