Incorporation of all-trans- or 9-cis-beta-carotene into mixed micelles in vitro.

Various studies suggest that all-trans- and 9-cis-beta-carotene are absorbed in the intestine to different extents. The present study was undertaken to evaluate the degree of in vitro incorporation of the two isomers into intestinal mixed micelles, which is an essential early step in the absorption process. The micelles were designed to simulate those present during fat digestion in the lumen of the human small intestine with respect to bile salts, lipids, pH and temperature. Solutions of all-trans- and 9-cis-beta-carotene at various ratios were added to the lipid mixture. A direct correlation was seen between the 9-cis-beta-carotene level in the mixture and the degree of total beta-carotene incorporation into micelles. An increased level of all-trans-beta-carotene in the system led to a decrease in the percentage of beta-carotene incorporated into the micelles. In contrast, when carotene mixtures enriched with the 9-cis isomer were used, an increase in the level of total carotene in the solution was accompanied by a constant or even enhanced carotene incorporation. The results indicate that the differences in the absorption of beta-carotene isomers might be the result of their different ability to be incorporated into the lipid micelles of the intestinal lumen. In addition, the results point toward the possibility that ingestion of 9-cis-beta-carotene by humans may increase carotene availability.