Model-based Compartmental Analysis Research Articles

Use of Theoretical Women and Model-Based Compartmental Analysis to Evaluate the Impact of Vitamin A Intake with or without a Daily Vitamin A Supplement on Vitamin A Total Body Stores and Balance During Lactation

BackgroundInadequate vitamin A (VA) intake is common among lactating women in many communities worldwide, but high-dose VA supplementation for postpartum women is not recommended by the World Health Organization as an effective intervention. ObjectivesTo simulate the impact of VA intake via diet and daily VA supplements on VA total body stores (TBS) and balance in theoretical lactating women with low/moderate TBS. MethodsWe studied 6 theoretical subjects with assigned values for TBS from 219–624 μmol. Using Simulation, Analysis, and Modeling software and a previously published compartmental model for whole-body VA metabolism, we simulated TBS over 6 mo of established lactation for each subject under 4 conditions: 1) prelactation VA intake was increased to maintain VA balance (LSS); 2) prelactation VA intake was maintained (NLSS); 3) VA intake was the same as 2) but a daily VA supplement (2.8 μmol/d) was added (NLSS+S); and 4) VA intake was as 1) and the daily VA supplement was included (LSS+S). ResultsTo compensate for the loss of VA via milk while VA balance was maintained (LSS) over 6 mo of lactation, VA intake had to increase by 0.8–1.87 μmol/d (n = 6) compared with NLSS. Over 6 mo of NLSS treatment, VA balance was negative (geometric mean, −0.77 μmol/d) compared with LSS, whereas balance was positive under NLSS+S and LSS+S conditions (0.75 and 1.5 μmol/d, respectively). For LSS, the proportion of total VA disposal was 37% via breastmilk, 32% from VA stores, and 32% from nonstorage tissues. ConclusionsAdding a daily VA supplement (2.8 μmol/d) to the diet of lactating women with suboptimal VA intake may effectively counterbalance the negative VA balance resulting from the output of VA via breastmilk and thus benefit both mother and infant by maintaining or increasing VA stores and breastmilk VA concentration.

A systematic review on maternal-to-infant transfer of drugs through breast milk during the treatment of malaria, tuberculosis, and neglected tropical diseases.

Exclusive breastfeeding of infants under 6 months of age is recommended by the World Health Organization. In 2021, over 300 million combined incident cases of malaria, tuberculosis, and neglected tropical diseases (NTDs) were reported, predominantly in low-income countries. For many of the drugs used as first-line treatments for these conditions, there is limited knowledge on infant exposure through breastfeeding with poorly understood consequences. This review summarized available knowledge on mother-to-infant transfer of these drugs to inform future lactation pharmacokinetic studies. A list of first-line drugs was generated from the latest WHO treatment guidelines. Using standard online databases, 2 independent reviewers searched for eligible articles reporting lactation pharmacokinetics studies and extracted information on study design, participant characteristics, and the mathematical approach used for parameter estimation. A third reviewer settled any disagreements between the 2 reviewers. All studies were scored against the standardized "ClinPK" checklist for conformity to best practices for reporting clinical pharmacokinetic studies. Simple proportions were used to summarize different study characteristics. The most remarkable finding was the scarcity of lactation pharmacokinetic data. Only 15 of the 69 drugs we listed had lactation pharmacokinetics fully characterized. Most studies enrolled few mothers, and only one evaluated infant drug concentrations. Up to 66% of the studies used non-compartmental analysis to estimate pharmacokinetic parameters rather than model-based compartmental analysis. Unlike non-compartmental approaches, model-based compartmental analysis provides for dynamic characterization of individual plasma and breast milk concentration-time profiles and adequately characterizes variability within and between individuals, using sparsely sampled data. The "ClinPK" checklist inadequately appraised the studies with variability in the number of relevant criteria across different studies. A consensus is required on best practices for conducting and reporting lactation pharmacokinetic studies, especially in neglected diseases such as malaria, tuberculosis, and NTDs, to optimize treatment of mother-infant pairs.

P01-003-23 Use of Model-Based Compartmental Analysis To Evaluate the Impact of Vitamin A Intake During Lactation on Vitamin A Balance and Total Body Stores in Theoretical Lactating Women

P01-003-23 Use of Model-Based Compartmental Analysis To Evaluate the Impact of Vitamin A Intake During Lactation on Vitamin A Balance and Total Body Stores in Theoretical Lactating Women

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

We review recent work in which model-based compartmental analysis has been applied to data for theoretical human subjects in order to study questions related to vitamin A kinetics and metabolism. Using model simulations in this way, one can validate experimental designs, evaluate or improve vitamin A assessment methods, study the influence of perturbations on assessment methods, and/or advance information related to retinol kinetics. We also provide some information on the rationale for assigning physiologically appropriate values for specified characteristics [e.g., plasma retinol concentration, vitamin A total body stores (TBS), vitamin A intake] to hypothetical individuals, and in addition, we outline how one might first select an appropriate compartmental model for whole-body vitamin A metabolism and then specify physiologically reasonable values for the associated retinol kinetic parameters. In the studies discussed here, the Simulation, Analysis, and Modeling software was used to simulate responses in key model compartments for hypothetical subjects so that model predictions could be compared to assigned values or projected outcomes. For example, in the case of the retinol isotope dilution (RID) method that is used to assess vitamin A status, application of this approach has provided a way to evaluate the accuracy of TBS predictions under different steady state and non-steady state conditions, thus increasing confidence about the validity of RID results obtained in the field. Although datasets for theoretical subjects have been used to evaluate protocols in pharmacokinetics, to our knowledge, other nutrition researchers have not previously used approaches such as those described here. Our results to date indicate that this strategy has the potential to provide useful information related not only to vitamin A but perhaps to other nutrients as well.

Pregnancy and Lactation Alter Vitamin A Metabolism and Kinetics in Rats under Vitamin A-Adequate Dietary Conditions.

Background: Vitamin A (VA) plays critical roles in prenatal and postnatal development; however, limited information is available regarding maternal VA metabolism during pregnancy and lactation. Objectives: We investigated the impact of pregnancy and lactation on VA metabolism and kinetics in rats, hypothesizing that changes in physiological status would naturally perturb whole-body VA kinetics. Methods: Eight-week old female rats (n = 10) fed an AIN-93G diet received an oral tracer dose of 3H-labeled retinol to initiate the kinetic study. On d 21 after dosing, six female rats were mated. Serial blood samples were collected from each female rat at selected times after dose administration until d 14 of lactation. Model-based compartmental analysis was applied to the plasma tracer data to develop VA kinetic models. Results: Our compartmental model revealed that pregnancy resulted in a gradual increase in hepatic VA mobilization, presumably to support different stages of fetal development. Additionally, the model indicates that during lactation, VA derived from dietary intake was the primary source of VA delivered to the mammary gland for milk VA secretion. Conclusion: During pregnancy and lactation in rats with an adequate VA intake and previous VA storage, the internal redistribution of VA and increased uptake from diet supported the maintenance of VA homeostasis.

Use of Model-Based Compartmental Analysis and Theoretical Data to Further Explore Choice of Sampling Time for Assessing Vitamin A Status in Groups and Individual Human Subjects by the Retinol Isotope Dilution Method.

ABSTRACTBackgroundAn optimal blood sampling time for application of the retinol isotope dilution (RID) method for predicting vitamin A total body stores (TBS) (i.e., vitamin A status) has not been established.ObjectivesObjectives were to identify sampling times that provide accurate estimates of TBS by RID in groups and individuals by applying compartmental modeling to data for theoretical adults and children.MethodsWe selected previously generated hypothetical adults and children (20 per group) that had a wide range of assigned values for TBS and vitamin A kinetic parameters. We used the Simulation, Analysis and Modeling software to simulate individual kinetic responses; then we calculated geometric mean values for the RID equation coefficients and each individual's plasma retinol specific activity at various times, using those values to predict group mean and individual subject TBS. Predicted values for TBS were compared with assigned values.ResultsAccurate estimates of group mean TBS were obtained at all sampling times from 1 to 30 d in both adults and children. For individuals, correlations between RID-predicted TBS and assigned values increased with time in the adults (R2 = 0.80 at day 14, 0.96 at day 21, and 0.99 at day 28); a similar trend was observed for the children, with R2 = 0.82 at day 7 and increasing to 0.97 at days 21 and 28 (P < 0.001 for all comparisons).ConclusionsAlthough no single, unique time provided the most accurate prediction of TBS for all individuals within these groups, applying the RID method at 21 or 28 d yielded predictions that were within 25% of assigned values for 90% or 95% of adults, respectively; corresponding values for children were 80% from 10 to 20 d, and 85% at 21 and 28 d. For most subjects, early times (<14 d for adults and <10 d for children) provided less accurate predictions.

A Compartmental Model Describing the Kinetics of β-Carotene and β-Carotene-Derived Retinol in Healthy Older Adults

Descriptive and quantitative information on β-carotene whole-body kinetics in humans is limited. Our objective was to advance the development of a physiologically based, working hypothesis compartmental model describing the metabolism of β-carotene and β-carotene-derived retinol. We used model-based compartmental analysis (Simulation, Analysis and Modeling software) to analyze previously published data on plasma kinetics of [2H8]β-carotene, [2H4]β-carotene-derived retinol, and [2H8]retinyl acetate-derived retinol in healthy, older US adults (3 female, 2 male; 50-68 y); subjects were studied for 56 d after consuming doses of 11μmol [2H8]β-carotene and, 3 d later, 9μmol [2H8]retinyl acetate in oil. We developed a complex model for labeled β-carotene and β-carotene-derived retinol, as well as preformed vitamin A, using simulations to augment observed data during model calibration. The model predicts that mean (range) β-carotene absorption (bioavailability) was 9.5% (5.2-14%) and bioefficacy was 7.3% (3.6-14%). Of the absorbed β-carotene, 41% (25-58%) was packaged intact in chylomicrons and the balance was converted to retinol, with 58% (42-75%) transported as retinyl esters in chylomicrons and 0-2% by retinol-binding protein. Most (95%) chylomicron β-carotene was cleared by the liver. Later data revealed differences in the metabolism of retinyl acetate- versus β-carotene-derived retinol; data required that both β-carotene and derived retinol be recycled from extrahepatic tissues (e.g. adipose) in HDL. Of total bioconversion [73% (47-99%)], 82% occurred in the intestine, 17% in the liver, and 0.83% in other tissues. Our model advances knowledge about whole-body β-carotene metabolism in healthy adults, including the kinetics of transport in all lipoprotein species, and suggests hypotheses to be tested in future studies, such as the possibility that retinol derived from hepatic conversion over a long period of time might contribute to plasma retinol homeostasis and total body vitamin A stores.

Dietary Iron Repletion Stimulates Hepatic Mobilization of Vitamin A in Previously Iron-Deficient Rats as Determined by Model-Based Compartmental Analysis

Iron deficiency can result in hyporetinolemia and hepatic vitamin A (VA) sequestration. We used model-based compartmental analysis to determine the impact of iron repletion on VA metabolism and kinetics in iron-deficient rats. At weaning, Sprague-Dawley rats were assigned to either a VA-marginal diet (0.35mg retinol equivalent/kg) with adequate iron (35ppm, control group [CN]) or reduced iron (3ppm, iron-deficient group [ID-]), with an equivalent average body weight for each group. After 5 wk, n=4 rats from each group were euthanized for baseline measurements of VA and iron indices, and the remaining rats (n=6 CN, n=10 ID-) received an intravenous injection of 3H-labeled retinol in an emulsion as tracer to initiate the kinetic study. On day 21 after dosing, half of the ID- rats were switched to the CN diet to initiate iron repletion, referred to as the iron-repletion group (ID+). From the time of dosing, 34 serial blood samples were collected from each rat over a 92-d time course. Plasma tracer and tissue tracee data were fitted to 6- and 4-compartment models, respectively, to analyze the kinetic behavior of VA in all groups. Our mathematical model indicated that ID- rats exhibited a nearly 6-fold decrease in liver VA secretion and >4-fold reduction in whole-body VA utilization, compared with CN rats, whereas these perturbed kinetic behaviors were notably corrected in ID+rats, close to those from the CN group. Iron repletion can remove the inhibitory effect that iron deficiency exerts on hepatic mobilization of VA and restore retinol kinetic parameters to values similar to that of never-deficient CN rats. Together with improvements in iron and VA indices, our results suggest that restoration of an iron-adequate diet is sufficient to improve VA kinetics after a previous state of iron deficiency.

The “Super-Child” Approach Is Applied To Estimate Retinol Kinetics and Vitamin A Total Body Stores in Mexican Preschoolers

Background:Retinol isotope dilution (RID) and model-based compartmental analysis are recognized techniques for assessing vitamin A (VA) status. Recent studies have shown that RID predictions of VA total body stores (TBS) can be improved by using modeling and that VA kinetics and TBS in children can be effectively studied by applying population modeling (“super-child” approach) to a composite data set. Objectives:The objectives were to model whole-body retinol kinetics and predict VA TBS in a group of Mexican preschoolers using the super-child approach and to use model predictions of RID coefficients to estimate TBS by RID in individuals. Methods:Twenty-four healthy Mexican children (aged 3–6 y) received an oral dose (2.96 µmol) of [13C10]retinyl acetate in corn oil. Blood samples were collected from 8 h to 21 d after dosing, with each child sampled at 4 d and at 1 other time. Composite data for plasma labeled retinol compared with time were analyzed using a 6-component model to obtain group retinol kinetic parameters and pool sizes. Model-predicted TBS was compared with mean RID predictions at 4 d; RID estimates at 4 d were compared with those calculated at 7–21 d. Results:Model-predicted TBS was 1097 µmol, equivalent to ~2.4 y-worth of VA; using model-derived coefficients, group mean RID-predicted TBS was 1096 µmol (IQR: 836–1492 µmol). TBS at 4 d compared with a later time was similar (P = 0.33). The model predicted that retinol spent 1.5 h in plasma during each transit and recycled to plasma 13 times before utilization. Conclusions:The super-child modeling approach provides information on whole-body VA kinetics and can be used with RID to estimate TBS at any time between 4 and 21 d postdose. The high TBS predicted for these children suggests positive VA balance, likely due to large-dose VA supplements, and warrants further investigation.

Pregnancy and Lactation Alter Vitamin A Metabolism and Kinetics in Rats under Vitamin A-Adequate Dietary Conditions

Abstract Objectives We investigated the impact of pregnancy and lactation on vitamin A (VA) metabolism and kinetics in rats, hypothesizing that this changed physiological status would perturb whole-body VA kinetics. Such information may be informative for future dietary recommendations. Methods Ten female rats (7 wk of age) and 6 male rats (9 wk of age) were fed an AIN-93 G diet upon arrival. After 1 week of acclimation, female rats received an oral dose of 3H-labeled retinol as the tracer to initiate the kinetic study. On d 21 after dosing (when 3H-retinol was expected to reach a log-linear terminal slope), 6 female rats were mated and checked daily for a vaginal plug to determine the date of pregnancy. On the day of delivery, litter size was adjusted to 10 pups/dam. Serial blood samples were collected from each female rat at 27–28 time points after dose administration until dams and pups were euthanized on d 14 of lactation. Hematocrit was measured, plasma tracer level was determined, and plasma fraction of dose vs. time was plotted. Model-based compartmental analysis will be applied to the plasma tracer data to develop VA kinetic models. Results All mated female rats became pregnant (pregnant group, PG, n = 6). Non-mated female rats were studied as non-pregnant controls (CN, n = 4). No difference was observed in hematocrit between PG and CN rats, suggesting no significant change in plasma volume expansion. Before breeding, plasma tracer response profiles were similar to CN rats. However, a consistent decline in plasma tracer levels was observed in PG rats during the middle of pregnancy, followed by a rise in late pregnancy, whereas such a change did not occur in CN rats. Moreover, during lactation, PG rats exhibited a steeper terminal slope compared to CN rats, indicating a more rapid utilization of VA in these lactating rats. Conclusions Pregnancy and lactation resulted in altered VA metabolism and kinetics in rats. Further analysis using mathematical modeling will explore the changes in kinetic parameters that underlie the perturbations we have observed in VA kinetics. Funding Sources National Institutes of Health.

Better Predictions of Vitamin A Total Body Stores by the Retinol Isotope Dilution Method Are Possible with Deeper Understanding of the Mathematics and by Applying Compartmental Modeling

Retinol isotope dilution (RID) is a well-accepted technique for assessing vitamin A status [i.e., total body stores (TBS)]. Here, in an effort to increase understanding of the method, we briefly review RID equations and discuss their included variables and their coefficients (i.e., assumptions that account for the efficiency of absorption of an orally administered tracer dose of vitamin A, mixing of the dose with endogenous vitamin A, and loss due to utilization). Then, we focus on contributions of another technique, model-based compartmental analysis and especially the “super-person” approach, that advance the RID method. Specifically, we explain how adding this modeling component, which involves taking 1 additional blood sample from each subject, provides population-specific estimates for the RID coefficients that can be used in the equation instead of values derived from the literature; using model-derived RID coefficients results in improved confidence in predictions of TBS for both a group and its individuals. We note that work is still needed to identify the optimal time for applying RID in different groups and to quantify vitamin A absorption efficiency. Finally, we mention other contributions of modeling, including the use of theoretical data to verify the accuracy of RID predictions and the additional knowledge that model-based compartmental analysis provides about whole-body vitamin A kinetics.

Use of Model-Based Compartmental Analysis and a Super-Child Design to Study Whole-Body Retinol Kinetics and Vitamin A Total Body Stores in Children from 3 Lower-Income Countries

ABSTRACTBackgroundModel-based compartmental analysis has been used to describe and quantify whole-body vitamin A metabolism and estimate total body stores (TBS) in animals and humans.ObjectivesWe applied compartmental modeling and a super-child design to estimate retinol kinetic parameters and TBS for young children in Bangladesh, Guatemala, and the Philippines.MethodsChildren ingested [13C10]retinyl acetate and 1 or 2 blood samples were collected from each child from 6 h to 28 d after dosing. Temporal data for fraction of dose in plasma [13C10]retinol were modeled using WinSAAM software and a 6-component model with vitamin A intake included as weighted data.ResultsModel-predicted TBS was 198, 533, and 1062 μmol for the Bangladeshi (age, 9–17 mo), Filipino (12–18 mo), and Guatemalan children (35–65 mo). Retinol kinetics were similar for Filipino and Guatemalan groups and generally faster for Bangladeshi children, although fractional transfer of plasma retinol to a larger exchangeable storage pool was the same for the 3 groups. Recycling to plasma from that pool was ∼2.5 times faster in the Bangladeshi children compared with the other groups and the recycling number was 2–3 times greater. Differences in kinetics between groups are likely related to differences in vitamin A stores and intakes (geometric means: 352, 727, and 764 μg retinol activity equivalents/d for the Bangladeshi, Filipino, and Guatemalan children, respectively).ConclusionsBy collecting 1 or 2 blood samples from each child to generate a composite plasma tracer data set with a minimum of 5 children/time, group TBS and retinol kinetics can be estimated in children by compartmental analysis; inclusion of vitamin A intake data increases confidence in model predictions. The super-child modeling approach is an effective technique for comparing vitamin A status among children from different populations. These trials were registered at www.clinicaltrials.gov as NCT03000543 (Bangladesh), NCT03345147 (Guatemala), and NCT03030339 (Philippines).

Perturbed Whole-body Vitamin a Kinetics Induced by Iron Deficiency Is Corrected by Dietary Iron Repletion in Rats (OR05-03-19)

ObjectivesWe investigated the impact of iron repletion on vitamin A (VA) metabolism and kinetics in iron-deficient rats, hypothesizing that iron repletion can remove the inhibitory effect of iron deficiency on hepatic mobilization of VA and improve the hyporetinolemia of iron deficiency. MethodsAt weaning, rats were divided into two groups and fed either a VA-marginal diet (control group, CN) with normal levels of iron (37 ppm) or the same diet with reduced iron (5 ppm, iron deficient group, ID-). After 5 weeks, n = 4 rats from each group were euthanized for baseline measurements, and the remaining rats (n = 6 CN, n = 10 ID-) received an i.v. dose of 3H-labeled retinol in an emulsion as tracer to initiate the kinetic study. On day 21 after dosing, half of the ID- rats were switched to the CN diet, thus creating an iron repletion group (ID+). Serial blood samples were collected from each rat at 34 preselected time points after dosing. On day 92, all rats were euthanized. Iron and VA status were determined, and model-based compartmental analysis was applied to the plasma tracer data to develop VA kinetic models for rats from all groups. ResultsRats fed the iron-deficient diet developed iron deficiency, and exhibited reduced plasma retinol (0.67 vs. 1.19 μmol/L), with higher liver VA (265 vs. 187 nmol) at the start of the kinetic study, suggesting impaired hepatic VA mobilization. At the study conclusion, ID+ rats had recovered from iron deficiency, as indicated by hematological indices including hemoglobin, hematocrit, and serum iron. While liver VA storage was more than doubled in ID- rats (456 nmol) vs. CN rats (190 nmol), hyporetinolemia was maintained in ID- rats. In contrast, VA in the liver of ID+ rats was mobilized, reflected by the reduced liver VA mass (276 nmol) and increased hepatic output of VA into plasma based on predictions of the compartmental model, and plasma retinol in ID + rats returned to normal. ConclusionsIron repletion restored plasma retinol concentrations and increased liver vitamin A mobilization in iron deficient rats. Funding SourcesNational Institutes of Health.

A Population-Based (Super-Child) Approach for Predicting Vitamin A Total Body Stores and Retinol Kinetics in Children Is Validated by the Application of Model-Based Compartmental Analysis to Theoretical Data

BackgroundPublic health nutritionists need accurate and feasible methods to assess vitamin A status and to evaluate efficacy of interventions, especially in children. The application of population-based designs to tracer kinetic data is an effective approach that reduces sample burden for each child. ObjectivesObjectives of the study were to use theoretical data to validate a population-based (super-child) approach for estimating group mean vitamin A total body stores (TBS) and retinol kinetics in children and to use population-based data to improve individual TBS predictions using retinol isotope dilution (RID). MethodsWe generated plasma retinol kinetic data from 6 h to 56 d for 50 theoretical children with high vitamin A intakes, assigning values within physiologically reasonable ranges for state variables and kinetic parameters (“known values”). Mean data sets for all subjects at extensive (n = 36) and reduced (n = 11) sampling times, plus 5 data sets for reduced numbers (5/time, except all at 4 d) and times, were analyzed using Simulation, Analysis and Modeling software. Results were compared with known values; population RID coefficients were used to calculate TBS for individuals. ResultsFor extensive and reduced data sets including all subjects, population TBS predictions were within 1% of the known value. For 5 data sets reflecting numbers and times being used in ongoing super-child studies, predictions were within 1–17% of the known group value. Using RID equation coefficients from population modeling, TBS predictions at 4 d were within 25% of the known value for 66–80% of subjects and reflected the range of assigned values; when ranked, predicted and assigned values were significantly correlated (Rs = 0.93, P < 0.0001). Results indicate that 7 d may be better than 4 d for applying RID in children. For all data sets, predictions for kinetic parameters reflected the range of known values. ConclusionThe population-based (super-child) approach provides a feasible experimental design for quantifying retinol kinetics, accurately estimating group mean TBS, and predicting TBS for individuals reasonably well. Curr Dev Nutr 2018;2:nzy071.

A Simple Plasma Retinol Isotope Ratio Method for Estimating β-Carotene Relative Bioefficacy in Humans: Validation with the Use of Model-Based Compartmental Analysis

Background: Provitamin A carotenoids are an important source of dietary vitamin A for many populations. Thus, accurate and simple methods for estimating carotenoid bioefficacy are needed to evaluate the vitamin A value of test solutions and plant sources. β-Carotene bioefficacy is often estimated from the ratio of the areas under plasma isotope response curves after subjects ingest labeled β-carotene and a labeled retinyl acetate reference dose [isotope reference method (IRM)], but to our knowledge, the method has not yet been evaluated for accuracy.Objectives: Our objectives were to develop and test a physiologically based compartmental model that includes both absorptive and postabsorptive β-carotene bioconversion and to use the model to evaluate the accuracy of the IRM and a simple plasma retinol isotope ratio [(RIR), labeled β-carotene-derived retinol/labeled reference-dose-derived retinol in one plasma sample] for estimating relative bioefficacy.Methods: We used model-based compartmental analysis (Simulation, Analysis and Modeling software) to develop and apply a model that provided known values for β-carotene bioefficacy. Theoretical data for 10 subjects were generated by the model and used to determine bioefficacy by RIR and IRM; predictions were compared with known values. We also applied RIR and IRM to previously published data.Results: Plasma RIR accurately predicted β-carotene relative bioefficacy at 14 d or later. IRM also accurately predicted bioefficacy by 14 d, except that, when there was substantial postabsorptive bioconversion, IRM underestimated bioefficacy. Based on our model, 1-d predictions of relative bioefficacy include absorptive plus a portion of early postabsorptive conversion.Conclusion: The plasma RIR is a simple tracer method that accurately predicts β-carotene relative bioefficacy based on analysis of one blood sample obtained at ≥14 d after co-ingestion of labeled β-carotene and retinyl acetate. The method also provides information about the contributions of absorptive and postabsorptive conversion to total bioefficacy if an additional sample is taken at 1 d.

Retinol Isotope Dilution Is Applied during Restriction of Vitamin A Intake to Predict Individual Subject Total Body Vitamin A Stores at Isotopic Equilibrium

Background: Retinol isotope dilution (RID) equations are used to determine vitamin A status and the efficacy of vitamin A intervention programs. Recent work related to RID methods has focused on modifying the “Olson equation” to improve the accuracy of predictions of vitamin A total body stores (TBS) in individual subjects.Objective: We investigated the hypothesis that short-term restriction of vitamin A intake would result in accurate RID prediction of vitamin A TBS in individuals.Methods: We applied model-based compartmental analysis to a 6-component model derived from published retinol kinetic studies on 12 individuals with a wide range of vitamin A stores and determined vitamin A TBS in the steady state. Then we simulated the impact of eliminating or strictly limiting vitamin A intake at the time of isotope administration, while maintaining plasma retinol homeostasis, on retinol specific activity in plasma (SAp; fraction of dose/μmol retinol) and stores, and we calculated TBS using the simplified RID equation TBS = 0.75 × 1/SAp, where the fractional absorption of tracer was set at 0.75 and SAp was simulated 5 d after dosing.Results: When vitamin A intake was zero or strictly limited (0.25 μmol/d), mean TBS predicted by the equation at 5 d after dose administration divided by TBS determined by using the model was 1.00 (range: 0.959–1.04) or 1.02 (range: 0.983 – 1.06), respectively.Conclusions: By eliminating or strictly limiting vitamin A input, isotopic equilibrium was reached by 5 d. At isotopic equilibrium, SAp is the same as that in the body’s exchangeable vitamin A pools; under these conditions, SAp may be measured at any time from 5 d on and used to calculate TBS.

Plasma Retinol Kinetics and β-Carotene Bioefficacy Are Quantified by Model-Based Compartmental Analysis in Healthy Young Adults with Low Vitamin A Stores

Background: Model-based compartmental analysis of data on plasma retinol kinetics after administration of labeled retinol provides unique information about whole-body vitamin A metabolism. If labeled β-carotene is coadministered, its bioefficacy relative to the retinol reference dose can also be estimated.Objectives: The objectives were to model plasma retinol kinetics after administration of labeled preformed vitamin A and provitamin A β-carotene and to determine relative β-carotene bioefficacy.Methods: We used the Simulation, Analysis and Modeling software (WinSAAM version 3.0.8; http://www.WinSAAM.org) to analyze previously collected data on plasma [13C10]- and [13C5]retinol kinetics for 14 d after oral administration of 1 mg [13C10]retinyl acetate and 2 mg [13C10]β-carotene in oil to 30 healthy young adults of European ancestry [13 men, 17 women; mean ± SD age: 24.5 ± 4.2 y; mean ± SD body weight: 65.2 ± 10 kg; mean ± SD body mass index (in kg/m2): 22.5 ± 1.9] with moderate vitamin A intakes.Results: A 6-component model provided the best fit to the data, including compartments for initial metabolism of vitamin A, plasma retinol, and extravascular vitamin A storage. The disposal rate was 6.7 ± 3.1 μmol/d, fractional catabolic rate was 6.0% ± 2.3%/d, and vitamin A stores were 123 ± 71 μmol. Relative β-carotene bioefficacy, based on the ratio of the areas under the fraction of dose curves calculated by WinSAAM, averaged 13.5% ± 6.02% (retinol activity equivalents = 7.7:1.0 μg). Interindividual variation in relative β-carotene bioefficacy was high (CV: 44%).Conclusions: Vitamin A kinetics in these young adults were best described by essentially the same model that had been previously developed by using data for older adults with higher vitamin A stores; differences in parameter values reflected differences in vitamin A status. Estimated β-carotene bioefficacy was relatively low but similar to previously reported estimates obtained by graphical methods. This trial was registered at the UK Clinical Research Network as UKCRN 7413.

Current Capabilities and Limitations of Stable Isotope Techniques and Applied Mathematical Equations in Determining Whole-Body Vitamin A Status.

Retinol isotope dilution (RID) methodology provides a quantitative estimate of total body vitamin A (VA) stores and is the best method currently available for assessing VA status in adults and children. The methodology has also been used to test the efficacy of VA interventions in a number of low-income countries. Infections, micronutrient deficiencies (eg, iron and zinc), liver disease, physiological age, pregnancy, and lactation are known or hypothesized to influence the accuracy of estimating total body VA stores using the isotope dilution technique. Our objectives were to review the strengths and limitations of RID methods, to discuss what is known about the impact of various factors on results, and to summarize contributions of model-based compartmental analysis to assessing VA status. Relevant published literature is reviewed and discussed. Various equations and compartmental modeling have been used to estimate the total body VA stores using stable isotopes, including a newer 3-day equation that provides an estimate of total body VA stores in healthy adults. At present, there is insufficient information on absorption of the isotope tracer, and there is a need to further investigate how various factors impact the application of RID techniques in field studies. Isotope dilution methodology can provide useful estimates of total body VA stores in apparently healthy populations under controlled study conditions. However, more research is needed to determine whether the method is suitable for use in settings where there is a high prevalence of infection, iron deficiency, and/or liver disease.

β‐carotene (BC) bioconversion is similar in men and women

Gender differences in plasma retinol and provitamin A carotenoids have been previously reported, with retinol being higher in men and BC higher in women(1). Furthermore, lower conversion efficiency is correlated with BMI in women but not in men(2). We investigated BC conversion efficiency in 23 female and 19 male volunteers by co-administering 2 mg [13C10] BC and 1 mg [13C10]-retinyl acetate. Data on fraction of the administered doses in plasma vs. time for each subject were analyzed by model-based compartmental analysis using the Simulation, Analysis and Modeling software WinSAAM. Relative absorption of BC in the first 24 hours represented 11% of total ingested [13C10]BC, with an inter-individual coefficient of variation of 49%. Although significantly higher plasma concentrations of preformed [13C10]-retinol and bio-converted [13C5]-retinol were found in men compared to women, differences in retinoid concentrations were independent of [13C10]BC cleavage, in that model-calculated bioconversion was not significantly different between men and women (Figure 1). Fig 1: Effect of gender on provitamin A conversion efficiency. In summary, application of [13C10]-retinyl acetate with [13C10]BC confirms that there are no differences in BC conversion efficiency between men and women.

A modified isotope dilution equation predicts vitamin A total body stores in individuals

Isotope dilution is used to successfully predict vitamin A (VA) total body stores (TBS) in populations but does not typically work for individuals. Using plasma kinetic data from 33 healthy young adults dosed with [13C10]retinyl acetate, we determined TBS using model‐based compartmental analysis (Simulation, Analysis and Modelling software [SAAM]) vs the equation TBS = F x S x (1/SA), where F= fraction of the orally administered stable isotope dose in stores, S= ratio of specific activity of retinol in plasma to that in stores, and SA = VA specific activity in plasma at 3 d post dosing. A high correlation (R=0.92) was found for the population. We found that the fractional catabolic rate (FCR) was a proxy for F and developed a mono‐exponential equation using 7 and 14 d data to predict F in individual subjects (R=0.999). Since S is ~1 at 3 d (when VA SA in stores crosses over plasma), the equation for d 3 simplifies to TBS = F x (1/SA). That is, by collecting blood samples at 3, 7 and 14 d and then calculating FCR based on 7 and 14 d data, we can accurately estimate an individual's TBS as individual F x (1/SA) at 3 d (R=0.94).This study was supported by BBSRC.