A high retinol dietary intake increases its apical absorption by the proximal small intestine of juvenile sunshine bass (Morone chrysops × M. saxatilis)

Abstract

The relationship between dietary intake and systemic availability of retinol is likely to be complex because although retinol is an essential nutrient, it is toxic at high levels. The present study determined whether rates of transapical retinol absorption are modulated so that availability is increased at low dietary levels, but decreased when dietary intake is excessive. Juvenile hybrid striped bass were fed for 6 wk diets with 568 (below), 1657 (approximating the requirement) and 40,244 (excessive) μg/kg dry diet of trans retinol. Proximal small intestine segments were used to measure rates of retinol absorption and tissue concentrations. Initial and final body mass did not differ among groups; deficiency and toxicity symptoms were not observed. Uptake of tracer retinol was inhibited by unlabeled retinol, indicating the presence of saturable, carrier-mediated absorption. Increasing dietary levels of retinol increased the rates of absorption measured at 0.05 mmol/L [8.04 ± 0.65; 15.2 ± 1.53; 25.1 ± 3.4 pmol/(mg · min) for below, approximating and exceeding the retinol requirement; P < 0.0001]; this resulted in higher tissue concentrations of all-trans retinol (0.21 ± 0.03, 0.49 ± 0.21 and 338 ± 89 pmol/g; P < 0.0001) and dehydro-retinol (0.11 ± 0.04, 0.91 ± 0.04, and 454 ± 109 pmol/g; P < 0.001). These findings suggest that the systemic availability of various dietary levels of retinol is modulated after transapical absorption.