Vitamin A Absorption Determined in Rats Using a Plasma Isotope Ratio Method

Direct Use of Plasma or Milk Vitamin A Specific Activity Data to Model Retinol Kinetics and Predict Vitamin A Stores in Theoretical Lactating Women

BACKGROUND

Many applications of the Simulation, Analysis and Modeling software use data on the fraction of an orally administered tracer dose (FD) in plasma; thus, researchers must scale-up measured analyte concentration to the total plasma pool. For studies in lactating women, estimating breast milk pool size is challenging.

OBJECTIVES

The objectives were to determine whether the standard vitamin A modeling approach using FD data could be modified to use vitamin A specific activity in milk (SAm) and/or plasma (SAp) for compartmental analysis of vitamin A kinetics and status in theoretical lactating women.

METHODS

Using 12 previously studied theoretical subjects with a wide range of assigned values for vitamin A total body stores (TBS) and the coefficient ("FaS") needed to predict TBS using a retinol isotope dilution equation, we simulated data for SAp and SAm for 49 d after oral administration of labeled vitamin A. Then we modeled datasets for SAp and SAm, as well as only SAp or SAm, incorporating a linear scaling factor to automatically convert SA to FD and including several physiologically reasonable constraints as input data. As outcomes, we compared model-predicted TBS and FaS to assigned values.

RESULTS

Scaling factors effectively adjusted SA data to adequately predict vitamin A mass in plasma and breast milk pools. Data for SAp and SAm provided model predictions of TBS that were comparable to assigned values (range: 85-107%); using only SAp, ratios ranged from 92% to 108% and for SAm from 85% to 108%. Parallel results were obtained for simulated FaS.

CONCLUSIONS

Results show that SA data from plasma and/or milk can be used directly for modeling vitamin A during lactation in theoretical subjects, providing accurate estimates of TBS and FaS. Results suggest that, in free-living lactating women, researchers might measure only SAp or only SAm and adequately describe whole-body vitamin A metabolism and status.

Vitamin A Absorption Efficiency Determined by Compartmental Analysis of Postprandial Plasma Retinyl Ester Kinetics in Theoretical Humans

BACKGROUND

Better methods are needed for determining vitamin A absorption efficiency in humans to support development of dietary recommendations and to improve the accuracy of predictions of vitamin A status.

OBJECTIVES

We developed and evaluated a method for estimating vitamin A absorption efficiency based on compartmental modeling of theoretical data on postprandial plasma retinyl ester (RE) kinetics.

METHODS

We generated data on plasma RE and retinol kinetics (30 min to 8 h or 56 d, respectively) after oral administration of labeled vitamin A for 12 theoretical adults with a range of values assigned for vitamin A absorption (55-90%); we modeled all data to obtain best-fit values for absorption and other parameters using Simulation, Analysis, and Modeling software. We then modeled RE data only (16 or 10 samples), with or without added random error, and compared assigned to predicted absorption values. We also compared assigned values to areas under RE response curves (RE AUCs).

RESULTS

We confirmed that a unique value for vitamin A absorption cannot be identified by modeling plasma retinol tracer kinetics. However, when RE data were modeled, predicted vitamin A absorptions were within 1% of assigned values using data without error and within 12% when 5% error was included. When the sample number was reduced, predictions were still within 13% for 10 of the 12 subjects and within 23% overall. Assigned values for absorption were not correlated with RE AUC (P = 0.21).

CONCLUSIONS

We describe a feasible and accurate method for determining vitamin A absorption efficiency that is based on compartmental modeling of plasma RE kinetic data collected for 8 h after a test meal. This approach can be used in a clinical setting after fasting subjects consume a fat-containing breakfast meal with a known amount of vitamin A or a stable isotope label.