Vitamin A plays an essential role in animal biology and has negative effects associated with both hypo- and
hypervitaminosis A. Many notable interventions are being done globally to eliminate
vitamin A deficiency, including supplementation, fortification, and biofortification. At the same time, it is important to monitor
vitamin A status in nations where preformed
vitamin A intake is high because of consumption of animal source foods (e.g., liver, dairy, eggs), fortified foods (e.g., milk, cereals, oil,
sugar,
margarine), or
vitamin supplements (e.g., one-a-day multivitamins) to ensure the population does not reach
hypervitaminosis A. To accurately assess population status and evaluate interventions aimed at improving
vitamin A status, accurate assessment methods are needed. The primary storage site of
vitamin A is the liver; however, routinely obtaining liver samples from humans is impractical and unethical.
Isotope dilution using
deuterium- or (13)C-labeled
retinol is currently the most sensitive indirect
biomarker of
vitamin A status across a wide range of liver reserves. The major drawback to its application is the increased technicality in sample analysis and data calculations when compared to less sensitive methodology, such as serum
retinol concentrations and dose response tests. Two main equations have emerged for calculating
vitamin A body pool size or liver concentrations from
isotope dilution data: the "Olson equation" and the "mass balance equation." Different applications of these equations can lead to
confusion and lack of consistency if the underlying principles and assumptions used are not clarified. The purpose of this focused review is to describe the evolution of the equations used in
retinol stable-
isotope work and the assumptions appropriate to different applications of the test. Ultimately, the 2 main equations are shown to be fundamentally the same and differ only in assumptions made for each specific research application.