State-of-the-art on absorption mechanisms and factors affecting the bioavailability of preformed vitamin A

Vitamin A deficiency remains a major health issue in many countries. Two types of vitamin A are naturally present in our diet, provitamin A carotenoids from plant sources and preformed vitamin A, mainly retinyl esters and retinol, found in animal products, fortified foods and supplements. The objective was to review current knowledge and gaps in understanding the mechanism and factors influencing preformed vitamin A bioavailability. The results show that the fate of retinyl esters in the intestine is well described, but that of retinol deserves further investigation. Preformed vitamin A present in liver, oil, sugar, and milk is well absorbed (70 to 99%), but data for other important matrices like eggs and fortified flours is lacking. The impact of the type and quantity of dietary lipids is unclear, especially whether low levels in a meal would be sufficient for optimal vitamin A absorption. Further research is needed to evaluate the effect of plant proteins and dietary fibers for which results are conflicting. The impact of host related factors like age is limited, and largely unknown for gender and genetic/epigenetic factors. It is well established that inflammation and other metabolic disorders affect vitamin A status, yet further study could help to distinguish absorption issues from other metabolic processes. An interesting avenue is the possible role of microbiota and probiotics on vitamin A absorption, as it could represent a complementary approach to food fortification. Further clinical research should prioritize the use of tracers to better quantify absorption under various nutritional conditions.