RESUMEN
BACKGROUND: As currently applied, the paired retinol isotope dilution (RID) test, which is used to assess the impact of a vitamin A intervention on vitamin A total body stores (TBS), requires two doses of stable isotope-labeled vitamin A. OBJECTIVES: Objectives were to evaluate use of a single isotope dose (4 µmol) to assess TBS by RID before and after intervention in theoretical children with low/moderate TBS. METHODS: We selected six theoretical children with assigned values for TBS ranging from 82-281 µmol. Using Simulation, Analysis and Modeling software, we simulated the variable [plasma retinol specific activity (SAp)] and coefficients (Fa and S) used in the RID equation TBS (µmol) = FaS x 1/SAp in both the unsupplemented steady state at d 14 postdosing and during the subsequent 4 mo without or with vitamin A supplementation [2.8 µmol retinol/d (801 µg retinol activity equivalents/d)]. RESULTS: Fraction of dose in plasma on d 150 versus d 14 was similar in the unsupplemented and supplemented conditions [geometric mean, 32% (range, 20-48%) and 30% (20-48%), respectively] and simulated values for FaS were similar under the two conditions. After 2 and 4 mo of daily vitamin A supplementation with 2.8 µmol/d, TBS was 78% and 128% higher, respectively, than without supplementation. CONCLUSIONS: Results indicate that the paired RID method can successfully be done using a single 4 µmol dose of stable isotope. Furthermore, since values for the RID coefficient FaS were similar in the unsupplemented- and vitamin A supplemented conditions, these results in theoretical children indicate that FaS determined by population ("super-subject") modeling of steady state vitamin A kinetic data could be used to predict TBS by RID following a vitamin A intervention in individuals from the same or a similar group.
