Does the Amount of Stable Isotope Dose Influence Retinol Kinetic Responses and Predictions of Vitamin A Total Body Stores by the Retinol Isotope Dilution Method in Theoretical Children and Adults?

To minimize both cost and perturbations to the vitamin A system, investigators limit the amount of stable isotope administered when estimating vitamin A total body stores (TBS) by retinol isotope dilution (RID). We hypothesized that reasonable increases in the mass of stable isotope administered to theoretical subjects would have only transient impacts on vitamin A kinetics and minimal effects on RID-predicted TBS. We adapted previously used theoretical subjects (3 children, 3 adults) with low, moderate, or high assigned TBS and applied compartmental analysis to solve a steady state model for tracer and tracee using assigned values for retinol kinetic parameters and plasma retinol. To follow retinol trafficking when increasing amounts of stable isotope were administered [1.39–7 (children) and 2.8–14 μmol retinol (adults)], we added assumptions to an established compartmental model so that plasma retinol homeostasis was maintained. Using model-simulated data, we plotted retinol kinetics versus time and applied the RID equation TBS = FaS/SAp [Fa, fraction of dose in stores; S, retinol specific activity (SA) in plasma/SA in stores; SAp, SA in plasma] to calculate vitamin A stores. The model predicted that increasing the stable isotope dose caused transient early increases in hepatocyte total retinol; increases in plasma tracer were accompanied by decreases in tracee to maintain plasma retinol homeostasis. Despite changes in kinetic responses, RID accurately predicted assigned TBS (98–105%) at all loads for all theoretical subjects from 1 to 28 d postdosing. Results indicate that, compared with doses of 1.4–3.5 μmol used in recent RID field studies, doubling the stable isotope dose should not affect the accuracy of TBS predictions, thus allowing for experiments of longer duration when including a super-subject design (Ford et al., J Nutr 2020;150:411–8) and/or studying retinol kinetics.