Improving vitamin A nutrition in low-income countries. In vitro bioaccessibility of provitamin A carotenoids in biofortified orange-fleshed sweet potato

Abstract: Vitamin A deficiency is a major nutritional disorder in a large number of low-income countries that is caused by an inadequate intake of preformed vitamin A and provitamin A carotenoids in the diet. The objective of the present thesis was to evaluate orange-fleshed sweet potato (OFSP) as a biofortified food source of provitamin A carotenoids with a potential to be included in a diet for alleviating vitamin A deficiency. The effects of various thermal processing and drying methods on the retention and in vitro bioaccessibility of ?-carotene in OFSP were investigated using HPLC and an in vitro digestion model. The cell structure and ?-carotene morphology in fresh and thermally processed OFSP were visualized by brightfield and coherent anti-Stokes Raman scattering microscopy. The interactive effects of simultaneous uptake of ?-carotene from micelles and iron were studied using a Caco-2 cell model. Biofortified OFSP cultivars from Uganda contained high levels of all-trans-?-carotene ranging from 108 to 315 ?g/g dry weight. Traditional preparation methods resulted in moderate losses of ?-carotene with approximately 78% of the all-trans-?-carotene content retained, and the effects of boiling, steaming and deep-frying were similar. However, the degree of cell wall damage strongly influenced the in vitro bioaccessibility of ?-carotene, which was clearly indicated by the 31% to 50% transfer of all-trans-?-carotene from the OFSP matrix to the micellar phase in samples that were homogenized prior to boiling, considerably higher than in samples that were thermally treated prior to homogenization (11% to 22%). These findings correlated qualitatively very well with the differences observed in size and number of ?-carotene bodies in the in vitro digested samples. Furthermore, ?-carotene bodies were co-located with starch granules in the OFSP matrix. The Caco-2 cell model showed that the uptake of ?-carotene was reduced in the presence of physiological amounts of ferrous chloride, and this effect was concentration dependent. However, no evidence of an impact of ?-carotene on iron uptake was observed. The results in the present thesis indicate that a meal supplemented with OFSP together with a small amount of fat would provide enough ?-carotene to completely cover the daily vitamin A requirements for preschool-age children. In summary, the high content and in vitro bioaccessibility of ?-carotene from thermally processed OFSP suggests that this food crop can be considered a promising food-based approach to prevent vitamin A deficiency in a sustainable manner.