No enhancing effect of vitamin A on iron absorption in humans

Background: Vitamin A and beta-carotene were recently reported to enhance iron absorption by counteracting the inhibitory effect of phytic acid in cereal-based meals and of polyphenol-containing beverages on nonheme-iron absorption in humans. Objective: Our objective was to further evaluate the influence of vitamin A on iron absorption. Design: Iron absorption from corn bread with or without added vitamin A (retinyl palmitate) was determined in 5 studies in young adult human subjects by using either a stable-isotope method (2 studies) or a radioisotope technique (2 studies). Iron absorption was measured by erythrocyte incorporation of the isotopic labels and by whole-body retention of Fe-59. Corn bread was served with water (studies 1 and 3) or coffee (studies 2, 4, and 5). The studies differed in the amounts and chemical forms of added tracer and fortification iron. The possibility of methodologic artifacts in earlier investigations was evaluated. Results: No effect of vitamin A on iron absorption from the test meals was identified in the individual studies by using paired Student's t test. A slightly negative effect of vitamin A on iron absorption was found with the use of analysis of variance. Conclusions: The previously reported findings of a positive effect of vitamin A on nonheme-iron absorption in humans was not confirmed. Incomplete isotopic equilibration of the tracer with native iron in the meal or with fortification iron cannot explain the previous findings. However, the present study does not exclude the possibility that suboptimal vitamin A status influences the effect of dietary vitamin A on iron absorption.