Simplified Approaches for Estimating Vitamin A Stores and β-carotene Bioconversion in Humans
Georg Lietz *
Newcastle University, Newcastle upon Tyne, UK.
*Author to whom correspondence should be addressed.
Abstract
Objectives: Better methods are needed to assess vitamin A (VA) status and the efficiency of bioconversion of β-carotene (BC) to retinol (ROH).
Methods: Data on plasma ROH kinetics from 2 h to 14 d after an oral tracer dose of [13C10]BC and [13C10]retinyl acetate (RAc) to 33 healthy young adults were analyzed using model-based compartmental analysis (WinSAAM, the Windows version of the Simulation, Analysis and Modeling software).
Results: The 6-compartment model that fit data for all subjects predicted total body VA stores (TBS) of 146±89 μmol (mean ± SD). A simplified isotope dilution ("Olson") equation for TBS was derived, eliminating several factors and assumptions: TBS = F * (1 / SA), where F (fraction of dose [FD] absorbed and retained) was estimated as 0.61 from the kinetic data and SA (specific activity) is FD 13C10[ROH] in plasma 3 d after dosing / plasma ROH pool (μmol). TBS calculated using the equation was 149±85 μmol, essentially the same as the value predicted by the model. BC bioconversion, calculated as FD 13C5[ROH] (derived from BC) / 13C10[ROH] (derived from RAc) at 2 d, averaged 23±11% and was significantly correlated (R=0.942) with WinSAAM's estimate based on areas under the curves (26±12%).
Conclusions: Our results indicate that both TBS and BC conversion to ROH can be estimated based on blood samples taken 3 and 2 d, respectively, after dosing. With further refinement, one sample at 3 d may suffice.