Evaluation of the 'Olson equation', an isotope dilution method for estimating vitamin a stores

Isotope dilution methods have been successfully used to estimate vitamin A status in human populations as well as to evaluate the impact of vitamin A interventions. The most commonly applied isotope dilution method is the retinol isotope dilution technique, which is based on the 1989 "Olson equation" for estimating total body vitamin A stores (sometimes equated to liver vitamin A) after an oral dose of labeled vitamin A. The equation relies on several factors related to absorption and retention of the dose, the equilibration of label in plasma vs. liver, and timing of a blood sample for measurement of labeled vitamin A. Here, the assumptions underlying these factors are discussed, and new results based on applying model-based compartmental analysis [specifically, the Simulation, Analysis and Modeling software (WinSAAM)] to data on retinol kinetics in humans are summarized. A simplification of the Olson equation, in which plasma tracer is measured 3 days after administration of the oral dose and several factors are eliminated, is presented. The potential usefulness of the retinol isotope dilution technique for setting vitamin A requirements and assessing vitamin A status in children, as well as the confounding effects of inflammation and likely variability in vitamin A absorption, are also discussed.