Vitamin A: biomarkers of nutrition for development

Abstract

Vitamin A is essential for multiple functions in mammals. Without vitamin A, mammals cannot grow, reproduce, or fight off disease. Because of its numerous functions in humans, biomarkers of vitamin A status are quite diverse. Assessment of liver reserves of vitamin A is considered the gold standard because the liver is the major storage organ. However, this measure is not feasible in human studies. Alternative biomarkers of status can be classified as biological, functional, histologic, and biochemical. Historically, signs of xerophthalmia were used to determine vitamin A deficiency. Before overt clinical damage to the eye, individuals who suffer from vitamin A deficiency are plagued by night blindness and longer vision-restoration times. These types of assessments require large population-based evaluations. Therefore, surrogate biochemical measures of vitamin A status, as defined by liver reserves, have been developed. Serum retinol concentrations are a common method used to evaluate vitamin A deficiency. Serum retinol concentrations are homeostatically controlled until liver reserves are dangerously low. Therefore, other biochemical methods that respond to liver reserves in the marginal category were developed. These included dose-response tests and isotope dilution assays. Dose-response tests work on the principle that apo-retinol-binding protein builds up in the liver as liver reserves become depleted. A challenge dose of vitamin A binds to this protein, and serum concentrations increase within a few hours if liver vitamin A concentrations are low. Isotope dilution assays use stable isotopes as tracers of total body reserves of vitamin A and evaluate a wide range of liver reserves. Resources available and study objectives often dictate the choice of a biomarker.

FIGURE 1.

Chemical structures of important functional forms of vitamin A. Retinol and retinyl esters (palmitate shown) are the dietary forms of preformed vitamin A. Retinal is essential in vision, and retinoic acid is involved in growth and cellular differentiation.

FIGURE 2.

Biomarkers of vitamin A status relative to qualitative liver reserves of vitamin A. These relations were proposed as part of the James Allen Memorial symposium in 2001 (adapted from reference 3). RAG, retinoyl β-glucuronide; RBP, retinol-binding protein; RDR, relative dose response.

FIGURE 3.

Top: Recommended biomarkers of vitamin A (VA) status by the Vitamin A Tracer Task Force in 2004 (adapted from reference 31) in relation to liver-reserve concentrations (μmol VA/g liver). Bottom: Replacing the “?” in 2010 regarding the utility of isotope dilution testing in the hypervitaminotic state.

FIGURE 4.

The relation of the modified relative dose-response (MRDR) value to liver retinol concentrations in piglets. Below 17 μg retinol/g liver, the MRDR value was invariably positive (ie, ≥0.060); between 17 and 29 μg retinol/g liver, the response was split; and at >29 μg retinol/g liver, the MRDR value was usually <0.060. The solid lines are the current accepted cutoffs to define vitamin A deficiency. The dashed lines define the areas where the MRDR value had the best sensitivity to reflect deficient and adequate vitamin A status defined by liver reserves.