Use of Equations for Estimating Vitamin A Total Body Stores using Stable Isotope Methods

Stable isotope methods for estimating vitamin A total body stores (TBS) quantitatively are based on the principle of isotope dilution. Briefly, a single oral dose ²H- or ¹³C-labeled vitamin A is administered to an individual, and plasma concentrations of labeled and unlabeled retinol are measured by mass spectrometry at a pre-specified time after dosing, usually 14-21 days. TBS is calculated using the retinol isotope dilution (RID) equation or the mass balance equation. The RID and mass balance equations require information on retinol specific activity in plasma (SAp; i.e., tracer-to-tracee ratio), which is obtained from mass spectrometry measurements. Both equations require values for coefficients to account for absorption and storage of the oral dose of stable isotope-labeled retinol at the time of TBS estimation. Published values for the coefficients can be used in the equations. Alternatively, TBS can be determined by model-based compartmental analysis of plasma retinol kinetic data using WinSAAM Simulation, Analysis, and Modeling software. Briefly, a super-subject study design with model-based compartmental analysis of plasma retinol kinetic data can be used to determine group TBS and population-specific values for the composite coefficient (FaS), which is subsequently used in the RID equation to determine individual TBS. The super-subject design requires an estimate of mean dietary vitamin A intake for the group of participants, and blood sampling at ~11-16 time points over ~28-91 d, with 5-7 participants/time point, but each participant provides only 2-3 blood samples. Liver vitamin A concentration can be estimated from TBS, using an assumption for the proportion of TBS found in liver and an estimate of liver weight. Vitamin A status is assessed by comparing estimated liver vitamin A concentration with proposed cutoff values for categorizing status across the full continuum, from deficient to excess vitamin A stores.