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.

Effects of a large-scale micronutrient powder and young child feeding education program on the micronutrient status of children 6–24 months of age in the Kyrgyz Republic

“…These findings are similar to those of three other studies of home fortification products in which there was no significant impact of the intervention on vitamin A status. 3,9,17,18 In our population, the lack of impact might be partially attributable to concurrent vitamin A capsule distribution, the insensitivity of current indicators to assess vitamin A status, 19 and the relatively low prevalence of vitamin A deficiency in the population without inflammation.…”

Ophthalmic Complications of Bariatric Surgery

“…Although progress has been made globally to alleviate overt clinical signs of VAD, marginal vitamin A status is still prevalent and difficult to diagnose [22] (Fig. 8).…”

Research Recommendations for Applying Vitamin A-Labelled Isotope Dilution Techniques to Improve Human Vitamin A Nutrition

“…The liver concentration to designate defi ciency (< 0.07 μmol / g liver) was based upon mathematical assumptions to prevent clinical ocular manifestations [10]. A newer cutoff for defi ciency of 0.1 μmol / g liver was proposed and is based on biological response in the liver, including the up-regulation of storage enzymes [11] and the accumulation of retinolbinding protein [3]. Other categories of vitamin A status that have been suggested, such as excessive, sub-toxic, and toxic, need further data, potentially using VALID techniques, to better understand the relationship of high liver stores to health.…”

“…The VALID techniques have been applied to assess vitamin A status, to evaluate the effi cacy of vitamin A interventions (e. g. supplementation, food fortifi cation, or dietary changes), to model human vitamin A requirements, and to determine the effi ciency of the body's conversion of provitamin A carotenoids (from plant foods) to retinol [1]. Unlike methods such as the measurement of serum retinol, which is tightly regulated and can really only detect very low body stores [3], the VALID techniques provide the sensitivity needed to quantitatively evaluate the effectiveness of different vitamin A dosing and timing regimens for public health interventions [1]. This paper summarizes recommendations (Figure 1) resulting from a technical meeting on applying VALID techniques to improve human vitamin A nutrition, and includes topics related to the further development of the method as well as use of VALID techniques to meet public health needs for evaluating the safety, effi cacy, and effectiveness of vitamin A intervention programs.…”