The retinol isotope dilution (RID) method has been used to estimate vitamin A stores, but questions remain concerning the ideal times for sampling in order to provide accurate estimates not only for groups, but for individuals.  Model-based compartmental analysis (MCBA) has been used in conjunction with RID to study vitamin A kinetics, while also estimating total body stores.  MCBA can also be used to estimate the equation coefficients needed for the RID equation.  Using data generated for theoretical adults and children, Green and Green worked to refine the proposed sampling times for RID analyses.  Their results are published in the July 2021 issue of The Journal of Nutrition.

A set of data from hypothetical adults and children with a wide range of assigned values for total body stores and vitamin A kinetics were used for the analyses.  Simulation software (Simulation, Analysis and Modeling) was used to develop individual responses and those data were used to create geometric means for the RID equation coefficients.  In addition, the data were used to calculate individual plasma retinol activity at various sampling times and that information was used to predict individual and group means.

Group mean total body stores were accurately estimated for all sampling times (1-30 days) for both the adults and children.  Correlations between predicted total body stores and assigned values increased with time after tracer administration for the adult and child datasets, with the greatest R2 obtained at day 28 for adults and days 21 and 28 for children.  These outcomes led the authors to suggest there is no unique time that provides the most accurate prediction of total body stores for all individuals.  However, they concluded that sampling between 21 and 28 days for adults would yield predictions within 25% of assigned values for more than 90% of individuals.  Although the predictions for individual children were lower, they still provided predictions that accurately represented predictions for 85% when sampling occurred between 21 and 28 days.  They also concluded that sampling prior to day 14 for adults and prior to day 10 in children provided less accurate predictions.  In a commentary, Lopez-Teros suggests that the values for equation coefficients for various sampling times provided by Green and Green will make it feasible for most investigators to obtain realistic total body stores of vitamin A for groups and individuals, without having to perform a kinetic study.


Green MH, Green JB.  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. The Journal of Nutrition, Volume 151, Issue 7, July 2021, Pages 2068–2074,

Lopez-Teros V. Advances in vitamin A assessment:  From mathematical modeling to field application. The Journal of Nutrition, Volume 151, Issue 7, July 2021, Pages 1680–1681,

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