| β-carotene (βC) is valued in the human diet as a source of vitamin A (VA). Yet, the efficacy of plant-based diets to provide VA has been duly questioned as a result of reported poor absorption of βC from vegetables consumed by marginally VA-deficient populations and also by healthy individuals. This thesis examined the impact of dietary composition and βC isomeric structure on bioconversion of βC to VA utilizing the gerbil model.; The first study demonstrated that βC bioconversion is independently affected by the level of fat and type of fiber in the diet. Consumption of both low-fat (10% energy) and to a greater extent high-fat diets (30% energy) containing βC enhanced liver VA stores. While consumption of oat β-glucan had no adverse effect on liver VA stores, consumption of citrus pectin resulted in lower liver VA storage and higher liver βC accumulation suggesting less conversion of βC to VA.; The remaining two studies compared the relative VA activity of all-trans (at) βC, 9-cis (9c) βC, and 13-cis (13c) βC. While at βC was preferentially absorbed, transported, and accumulated in tissues compared to the cis-isomers 6 hours following a single dose in oil, liver VA stores were not different among groups. The extensive cis-to-trans isomerization, observed in the gastrointestinal tract after ingestion of 13cβC and 9cβC, may be responsible for a portion of the VA activity of cis isomers of βC. Following a 7-day period of dosing each βC isomer, the VA activity of 9cβC and 13cβC, determined by changes in liver VA storage, was 39% and 62% that of atβC, respectively.; In conclusion, βC bioconversion to VA was independently and adversely affected by consumption of low dietary fat and citrus pectin. 9-Cis βC was less and 13cβC was more than the currently assigned retinol equivalency of 50% of atβC. Thus, the actual contribution of βC to the VA requirement may be strongly influenced by the level of fat, the type of fiber, and the proportion of cis isomers of βC consumed in the diet. |
