Who Report: Priorities in the Assessment of Vitamin a and Iron Status in Populations

n Vitamin A is essential for growth, reproduction and immunity. Biomarkers of vitamin A status are diverse, in part, due to its functions. Liver reserves of vitamin A are considered the gold standard but this measure is not feasible for population evaluation. Biomarkers of status can be grouped into two categories: (1) biological, functional and histological indicators; and (2) biochemical indicators. Historically, signs of xerophthalmia were used to determine vitamin A deficiency. Before overt clinical damage to the eye, individuals with vitamin A deficiency are plagued by night blindness and longer vision restoration times. Surrogate biochemical measures of vitamin A status, as defined by liver reserves, have been developed. Serum retinol concentration is a common method used to evaluate vitamin A deficiency, but it is homeostatically controlled until liver reserves become dangerously low. Therefore, other biochemical methods that respond to liver reserves in the marginal category have been developed, such as dose response tests and isotope dilution assays. Dose response tests work on the principle that as liver reserves become depleted, apo-retinol-binding protein builds up in the liver. A challenge dose of vitamin A binds to this protein and serum concentrations increase within a few hours if liver vitamin A is low. Isotope dilution assays use stable isotopes to trace total body reserves of vitamin A. Different biomarkers have utility across a range of liver values.