Compartmental Modeling of Vitamin A Stable Isotope Data from Milk or Plasma Provides Comparable Predictions of Vitamin A Stores in Theoretical Lactating Women.

BACKGROUND

Previous compartmental models describing and quantifying whole-body vitamin A (VA) metabolism have been developed from plasma retinol kinetic data after human subjects ingest stable isotope-labeled VA. For humans, models based on data obtained from other sampling sites (e.g., excreta or milk) have not been proposed.

OBJECTIVES

Our objective was to determine whether comparable model predictions of VA total body stores (TBS) in theoretical lactating women were obtained using tracer data from only retinol in plasma or VA in milk.

METHODS

We used Simulation, Analysis and Modeling software to simulate values for TBS and the coefficients used in the retinol isotope dilution (RID) equation TBS = FaS/SAp (Fa, fraction of dose in stores; S, retinol specific activity (SA) in plasma/SA in stores; SAp, specific activity in plasma). We compared individual subject predictions of TBS and FaS based on modeling only plasma or only milk tracer data to previous results ("assigned values") for 12 theoretical lactating women when modeling was done based on tracer data for chylomicron retinyl esters, plasma retinol, and milk VA.

RESULTS

For subjects with a wide range of TBS, model-predicted TBS based on only plasma data were comparable with assigned values (range: 94%-106%). Using only milk data, predictions ranged from 72% to 178%, but when VA intake was included in modeling, predictions were improved (97%-102%). Similar results were obtained for simulated FaS.

CONCLUSIONS

If confirmed in free-living lactating women, results indicate that, similar to models based on serial plasma sampling, a model for whole-body VA kinetics, including predictions of TBS and FaS, can be identified based on tracer data for VA in milk when VA intake is included as a modeling constraint. Milk data have not been previously used for compartmental modeling of VA in humans.