Pre-absorptive metabolism of carotenoids

Vitamin A deficiency (VAD) is still a major public health issue in developing countries. Biofortification of staple crops with pro-VA carotenoids is proposed as a complementary and sustainable approach to alleviate VAD. However, it is unknown if the additional pro-VA carotenoids accumulated in the staple crops will be bioavailable. First, I focused on screening the relative bioaccessibility of beta-carotene (BC) from ten cultivars of cassava using the coupled in vitro digestion/Caco-2 cell model. All-trans-BC, 9- cis-BC, and 13-cis-BC were the most abundant carotenoids in boiled cassava and recoveries after digestion exceeded 70%. Efficiency of micellarization of total BC was 30% for various cultivars with no significant difference among isomers and was linearly proportional to concentration in cooked cassava. Accumulation of all-trans-BC by Caco-2 cells incubated with the diluted micelle fraction for 4 h was proportional to the quantity present in micelles. In addition to boiling, cassava is also traditionally prepared by fermentation and roasting. These different methods of preparation have the potential to affect both the retention and bioaccessibility of BC. So I next focused on the effects of traditional preparation of cassava on retention and bioaccessibility of BC in boiled cassava, gari (fermentation and roasting), and fufu (fermentation and boiling the paste). Apparent retention of BC was approximately 90% for boiled cassava and fufu. In contrast, roasting fermented cassava at 195°C for 20 min to prepare gari decreased BC content by 90%. Retention was increased to 63% when temperature was decreased to 165°C and roasting was limited to 10 min. These differences in retention and bioaccessibility of BC from cassava products prepared according to traditional processing methods suggest that gari and fufu may provide less retinol activity equivalence (RAE) than isocaloric intake of boiled cassava.;It is noteworthy that cassava has only BC as the primary carotenoid in the tuber. However, maize, another staple crop targeted for biofortification also has beta-cryptoxanthin (B-CX) and lutein (LUT) and zeaxanthin (ZEA). Studies have demonstrated interaction between carotenoids, therefore primary aim of my next study was to examine if LUT and ZEA attenuate the bioaccessibility of pro-VA carotenoids at amounts and ratios present in maize. BC incorporation into micelles during chemical preparation and during in vitro digestion of carotenoid-enriched oil was slightly increased when the concentration of LUT was 6-fold or greater than BC. Likewise, the efficiency of BC micellarization was slightly increased during simulated small intestinal digestion of white maize porridge supplemented with oil containing 9-fold molar excess of LUT to BC. Thus, bioaccessibility of pro-VA carotenoids in maize is likely to be minimally affected by the relative levels of xanthophylls present in maize.;I examined whether citral induces retinoic acid (RA) deficiency in the TC-7 clone of Caco-2 cells. I assessed RA status by monitoring the expression of several genes, and particularly beta-carotene 15, 15' monooxygenase 1 (BCO1), regulated by this ligand for retinoid acid receptor/retinoid X receptor. Differentiated monolayers of TC-7 cells first were exposed to various concentrations of citral for 16h to define non-cytotoxic doses. Lower concentrations (0.1-10 muM) of citral were not toxic for Caco-2 cells. The relative expression of BCO1 dose dependently increased in cultures treated overnight with 0.1-3 muM citral; increased time of exposure did not further elevate BCO1 mRNA. Citral mediated changes were blocked by simultaneous presence of 10 muM retinoic acid in medium. BCO1 activity in cell lysates and intact cells likewise significantly increased after pre-treatment with 1.0 muM citral for 16h. However, there was no significant difference in the amount in cellular accumulation and transepithelial transport of BC by control and citral treated cells. Relative expression of several other genes involved in retinoid and carotenoid metabolism showed that exposure to citral increased scavenger recepeptor type B class I and retinaldehyde dehydrogenase 1 mRNA and decreased cellular retinoic acid binding protein 1 and cytochrome p450 26A1 compared to control. Collectively, the results suggest that citral induced a moderate deficiency of RA in Caco-2 cells. Thus, citral represents an effective tool for the future investigation of the impact of VA and therefore RA deficiency on various activities of enterocytes. (Abstract shortened by UMI.)...