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

Background: 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. Objective: 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. Methods: Relevant published literature is reviewed and discussed. Results: 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. Conclusions: 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.