Vitamin A Status Assessment Methods from Deficiency to Excess

Abstract

Objectives: Methods to assess vitamin A status of groups and populations were reviewed. Methods: Various vitamin A status indicators were aligned with the liver reserve concentration where they have the most usability to predict liver stores of vitamin A if a positive result is found with the test.  Both human and animal studies were used to compare these variables. Results: Clinical signs only have utility when liver reserves are almost exhausted and serum retinol concentrations have utility in the zone of overt deficiency. Dose response tests offer a little more coverage, but they also lack utility when vitamin A reserves are above adequate and are not able to distinguish between an adequate, sub-toxic and toxic vitamin A status. As different countries continue or begin to add preformed vitamin A to a variety of foods in the process of fortification, indicators that work in the excessive to toxic range of liver reserves are needed.  To date the only indicator, excluding liver biopsy, that has been validated in this range of liver reserves is the retinol isotope dilution test. Conclusions: Preformed retinyl esters are added to a variety of staple foods; provitamin A biofortified crops are currently being released in several countries; and vitamin A supplementation programs are still being distributed in many countries, especially to preschool children. The ability to assess population vitamin A status needs to move beyond serum retinol concentrations. Retinol isotope dilution tests, either using deuterium or 13C, are the only methods that span the entire liver reserve continuum.