DIGESTION, ETC. OF CAROTENOIDS IN THE G.I. TRACT 283 



One may likewise use a vitamin A bioassay to determine to what extent 

 the material yields vitamin A. However, these latter procedures are of 

 little value in determining absorption, in view of the fact that such small 

 quantities of the compound are involved. 



The various technics outlined above which concern the determination 

 of vitamin A as the proof of absorption of the carotenoid are a measure of 

 at least two factors; these are the rate of absorption of the carotenoid 

 and the speed of transformation of absorbed carotenoid to vitamin A. 

 If the synthesis of vitamin A were to occur in the lumen of the gut rather 

 than in the intestinal wall, then these last procedures would not be of 

 any value in proving the absorption of the provitamins A. However, 

 the best evidence indicates that the conversion of carotenoids to vitamin 

 A is a function of the wall of the intestine and not of its lumen. Vitamin 

 A determinations are of no value in assessing the absorption of carotenoids 

 which are not provitamins A. 



Still another method which has been used for the estimation of caro- 

 tenoid absorption is to determine the excretion of these substances in the 

 feces. Knowing the amount of the test substance fed, one can calculate 

 the quantity of material which has disappeared during its passage through 

 the gastrointestinal tract, either by absorption or by decomposition. 

 This procedure will be of no value if the carotenoid is synthesized in the 

 intestine. Although such a phenomenon is probably extremely rare, 

 McGillivray 215 has presented evidence that an appreciable synthesis of 

 carotene may occur in the ileum and cecum of the sheep. If this also 

 occurs in cattle, it would explain the earlier report of Whitnah et al. 216 

 that the excretion of carotenoids in the feces exceeded the quantity taken 

 in the food. Finally, this last technic is not an index of rate of absorption 

 but rather of completeness of utilization or digestibility. 



Barrick and co-workers 217 reported that no absorption of carotene 

 occurs in the cecum or colon. The jejunum and upper part of the ileum 

 are the areas of most active absorption of carotenoids, although some ab- 

 sorption may also occur from the lower part of the ileum; McGillivray 215 

 reported carotene synthesis in this area, as well as in the cecum. 



(1) The Absorption of Carotenoids from the Intestine 



a. The Absorption of /3-Carotene. Because /3-carotene is the commonest 

 and most widely distributed of the provitamins A and, in fact, of the caro- 



215 W. A. McGillivray, Brit. J. Nutrition, 5, 223-228 (1951). 



216 C. H. Whitnah, W. J. Peterson, F. W. Atkeson, and H. W. Cave, /. Agr. Research, 

 58, 343-355 (1939). 



217 E. R. Barrick, F. N. Andrews, and J. F. Bullard, J. Animal Sci., 7, 539 (1948). 



