The Use of Datasets for Theoretical Subjects to Validate Vitamin A-Related Methods and Experimental Designs

Use of Population-Based Compartmental Modeling and Retinol Isotope Dilution to Study Vitamin A Kinetics and Total Body Stores among Ghanaian Women of Reproductive Age

Background

Limited data are available on vitamin A kinetics and total body stores (TBS) in women. Such information can be obtained using compartmental modeling and retinol isotope dilution (RID).

Objectives

Objectives were to apply population-based ("super-subject") modeling to determine retinol kinetics in nonpregnant Ghanaian women of reproductive age and to use RID to predict TBS in the group and its individuals.

Methods

Women (n = 89) ingested a dose of [2H6]retinyl acetate and blood samples (3/woman) were collected from 6 h to 91 d, with all participants sampled at 14 d, about half at either 21 or 28 d, and each at one other time. Composite data (plasma retinol fraction of dose; FDp) were analyzed using Simulation, Analysis and Modeling software to obtain kinetic parameters, TBS, and other state variables as well as model-derived values for the RID composite coefficient FaS. The latter were used in the RID equation TBS (μmol) = FaS × 1/SAp (where SAp is plasma retinol specific activity) to predict TBS at various times.

Results

Model-predicted TBS was 973 μmol (n = 87). Geometric mean RID-predicted TBS was 965, 926, and 1006 μmol at 14, 21, and 28 d, respectively, with wide ranges [for example, 252-3848 μmol on day 14 (n = 86)]; TBS predictions were similar at later times. Participants had a mean 2 y of vitamin A in stores and estimated liver vitamin A concentrations in the normal range. Model-predicted vitamin A disposal rate was 1.3 μmol/d and plasma recycling number was 37.

Conclusions

Super-subject modeling provides an estimate of group mean TBS as well as group-specific values for the RID coefficient FaS; the latter can be used to confidently predict TBS by RID for individual participants in the group under study or in similar individuals at 14 d or more after isotope ingestion.

Trial registration number

Trial is registered (NCT04632771) at https://clinicaltrials.gov.

Use of the Paired Retinol Isotope Dilution Test, but with a Single Isotope Dose, to Assess the Impact of a Vitamin A Intervention on Vitamin A Stores in Theoretical Children with Low Stores

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 2 doses of stable isotope-labeled vitamin A.

OBJECTIVES

The objectives of this study 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 6 theoretical children with assigned values for TBS ranging from 82 to 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 × 1/SAp in both the unsupplemented steady state at day 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 day 150 compared with day 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 2 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, because 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 after a vitamin A intervention in individuals from the same or a similar group.

Use of Compartmental Modeling and Retinol Isotope Dilution to Determine Vitamin A Stores in Young People with Sickle Cell Disease Before and After Vitamin A Supplementation

BACKGROUND

Suboptimal plasma retinol concentrations have been documented in US children with sickle cell disease (SCD) hemoglobin SS type (SCD-HbSS), but little is known about vitamin A kinetics and stores in SCD.

OBJECTIVES

The objectives were to quantify vitamin A total body stores (TBS) and whole-body retinol kinetics in young people with SCD-HbSS and use retinol isotope dilution (RID) to predict TBS in SCD-HbSS and healthy peers as well as after vitamin A supplementation in SCD-HbSS subjects.

METHODS

Composite plasma [13C10]retinol response data collected from 22 subjects with SCD-HbSS for 28 d after isotope ingestion were analyzed using population-based compartmental modeling ("super-subject" approach); TBS and retinol kinetics were quantified for the group. TBS was also calculated for the same individuals using RID, as well as for healthy peers (n = 20) and for the subjects with SCD-HbSS after 8 wk of daily vitamin A supplements (3.15 or 6.29 μmol retinol/d [900 or 1800 μg retinol activity equivalents/d]).

RESULTS

Model-predicted group mean TBS for subjects with SCD-HbSS was 428 μmol, equivalent to ∼11 mo of stored vitamin A; vitamin A disposal rate was 1.3 μmol/d. Model-predicted TBS was similar to that predicted by RID at 3 d postdosing (mean, 389 μmol; ∼0.3 μmol/g liver); TBS predictions at 3 compared with 28 d were not significantly different. Mean TBS in healthy peers was similar (406 μmol). RID-predicted TBS for subjects with SCD-HbSS was not significantly affected by vitamin A supplementation at either dose.

CONCLUSIONS

Despite differences in plasma retinol concentrations, TBS was the same in subjects with SCD-HbSS compared with healthy peers. Because 56 d of vitamin A supplementation at levels 1.2 to 2.6 times the Recommended Dietary Allowance did not increase TBS in these subjects with SCD-HbSS, further work will be needed to understand the effects of SCD on retinol metabolism. This trial was registered as NCT03632876 at clinicaltrials.gov.

Development of a Compartmental Model for Studying Vitamin A Kinetics and Status in Theoretical Lactating Women

BACKGROUND

Low vitamin A status and suboptimal milk vitamin A concentrations are problems in many populations worldwide. However, limited research has been done on whole-body vitamin A kinetics in women of reproductive age, especially during lactation.

OBJECTIVES

Goals were to develop compartmental models describing retinol kinetics in theoretical nonlactating (NL) and lactating (L) women and to determine whether the retinol isotope dilution (RID) method accurately predicted vitamin A total body stores (TBS) in the groups and individuals.

METHODS

We adapted 12 previously-used theoretical females with assigned values for retinol kinetic parameters and TBS (225-1348 μmol); subjects were NL or L (nursing one 3- to 6-mo-old infant) during 49-d kinetic studies after isotope dosing. We used an established compartmental model, adding a compartment for chylomicrons and, for L, another for mammary gland milk with inputs from holo-retinol-binding protein and chylomicron retinyl esters and output to milk. Using compartmental analysis, we simulated tracer responses in compartments of interest and calculated TBS using the RID equation TBS =   FaS/SAp [Fa, fraction of dose in stores; S, retinol specific activity in plasma/specific activity in stores; SAp, specific activity of retinol in plasma].

RESULTS

Models for both groups were well identified. Simulated plasma tracer responses were similar for NL and L, with L always below NL; milk tracer paralleled plasma from 10 d postdosing. Geometric mean FaS ratios (L/NL) were ∼0.75 during days 2-30. Using appropriate group FaS, RID provided accurate TBS predictions for >80% of NL and L subjects after day 18 when CV% for FaS was ∼10%.

CONCLUSIONS

These new physiologically-based models for vitamin A kinetics may be useful for future research in women of reproductive age. Results indicate that, in groups like these, RID to assess an individual's vitamin A status should be done at 21-28 d after isotope dosing.