Comparison of methods of assessing vitamin A status in children

Systematic Review and Meta-Analysis of the Relative Dose-Response Tests to Assess Vitamin A Status

Vitamin A (VA) is an essential nutrient often lacking in the diets of people in developing countries. Accurate biomarkers of VA status are vital to inform public health policy and monitor interventions. The relative dose-response (RDR) and modified-RDR (MRDR) tests are semi-quantitative screening tests for VA deficiency that have been used in Demographic and Health Surveys and VA intervention studies. A systematic review and meta-analysis of sensitivity and specificity were conducted to summarize the physiological evidence to support the RDR tests as methods to assess VA status and investigate the impact of different pathological and physiological states on the tests. A total of 190 studies were screened for inclusion, with 21 studies comparing the RDR tests with the gold-standard biomarker, liver VA concentration (68% and 80% sensitivity and 85% and 69% specificity for the RDR and MRDR, respectively). Nearly all studies with VA interventions in VA-deficient populations demonstrated a response of the tests to VA intake that would be expected to improve VA status. The impacts of chronic liver disease, protein malnutrition, age, pregnancy and lactation, infection and inflammation, and various other conditions were examined in 51 studies. The RDR and MRDR tests were reported to have been used in 39 observational studies, and the MRDR has been used in at least 6 national micronutrient surveys. The RDR and MRDR are sensitive tests for determining population VA status and assessing VA interventions. Although they are robust to most physiological and pathological states, caution may be warranted when using the tests in neonates, individuals with chronic liver disease, and those with protein or iron malnutrition. Research on further improvements to the tests to increase accessibility, such as sampling breast milk instead of blood or using intramuscular doses in subjects with malabsorption, will allow wider adoption. This review was registered with PROSPERO as CRD42019124180.

Physiologic indicators of vitamin A status

Physiologic indicators reflect the functional consequences of vitamin A deficiency and may be particularly useful for detecting early perturbations in vitamin A status. In conjunctival impression cytology (CIC), epithelial morphology and the presence or absence of mucin spots and goblet cells allow samples, obtained by applying filter paper to the temporal conjunctiva, to be characterized as normal or typical of vitamin A-deficient keratinizing metaplasia. The validity of CIC has been established with reference to other indicators of vitamin A status, and a prevalence of > or =20% abnormal results has been suggested as indicative of a public health problem. However, interpretation of specimens requires considerable training, and nonresponsiveness to supplementation is a frequent problem, which limits the utility of CIC as a method for evaluating the impact of intervention programs. Several simplified field protocols for dark adaptation have been developed, including one in which dark adaptation is assessed by the responsiveness of the pupil to light. Night blind subjects have consistently shown abnormal results on this test, and a significant response to placebo-controlled dosing has been demonstrated for children and pregnant women. Scores have correlated significantly with serum retinol and relative dose response. Pupillary dark adaptation testing is acceptable to most children as young as 2 y old. Limitations of this technique include a time course for recovery after dosing as long as 4-6 wk, a testing time of 20 min, and the need for 1-3 d of training. Given its low cost, noninvasive nature, and lack of the need to transport samples, pupillary dark adaptation offers advantages over other techniques for assessing a population's vitamin A status.

Vitamin A and zinc supplementation of preschool children

OBJECTIVE

To determine whether supplementation of vitamin A and/or zinc (Zn) improved serum levels of these nutrients and/or height and weight gains in preschool children, 22 to 66 months, living in Belize, Central America.

METHODS

Subjects received either Zn, vitamin A, Zn and vitamin A or a placebo, (70 mg Zn and/or 3030 RE vitamin A, once per week) for 6 months in a 2x2 factorial design. Forty-three children, from a population of 104 prescreened, completed the study; they were selected, prior to treatment, for low/marginal serum concentrations of these micronutrients.

RESULTS

Serum Zn levels were greater (16%, p<0.001) for those who received Zn. In contrast, after vitamin A treatment there were no differences in serum vitamin A among groups. Although increases in height (+4.4 cm, p<0.001) and weight (+0.79 kg, p<0.001), compared with baseline values, were numerically greatest for children who received both supplements, only the vitamin A supplementation effect was significant, resulting in increased height (+1.4 cm, p<0.002) and greater weight gain (+0.15 kg, p<0.03) compared to those receiving no vitamin A. Vitamin A supplementation alone significantly increased (p<0.001) hemoglobin concentration.

CONCLUSION

The results suggest that the preschool children in this study, prescreened for low/marginal serum concentrations from a larger population prior to treatment, were enduring inadequate vitamin A and, to a lesser degree, Zn nutriture. Height and weight gain were significantly increased in the subjects who received a single weekly supplement 3030 RE of vitamin A.

Acute inflammation induces hyporetinemia and modifies the plasma and tissue response to vitamin A supplementation in marginally vitamin A-deficient rats

Plasma retinol is reduced during numerous infections, and inflammation alters the hepatic synthesis of retinol-binding protein (RBP). In this study, we have investigated the effects of endotoxin-induced inflammation on vitamin A (VA) supplementation in a rat model of marginal VA deficiency. Marginally VA-deficient rats received an intraperitoneal dose of lipopolysaccharide (LPS, n = 14) or saline (n = 10); 6 h later, six LPS + VA and six saline + VA rats received 7.1 micromol VA orally. Twenty-four hours after endotoxin administration, rats with inflammation (LPS) had lower plasma retinol, RBP, and hepatic RBP than saline rats (37, 31 and 44%, respectively, P < 0.05). Inflammation did not affect VA concentrations in liver and perirenal adipose tissue, although kidney VA was reduced relative to saline rats. However, urinary VA was not detected. Eighteen hours after VA supplementation, inflammation reduced the plasma unesterified retinol response (P < 0. 05) in LPS + VA relative to saline + VA rats, although total VA increased as a result of the presence of retinyl esters in LPS + VA rats. Hepatic esterified retinol concentration was reduced (P < 0. 01) in LPS + VA compared with saline + VA rats; however, hepatic unesterified retinol did not differ. Renal total retinol increased in VA-supplemented rats, but urinary retinol excretion, when observed, was low, independently of inflammation. These findings indicate that inflammation-induced hyporetinemia does not necessarily imply a loss of VA, but rather represents a redistribution of tissue VA brought about by a reduced hepatic synthesis of RBP. Practical implications from these collective results are to recommend the determination of both unesterified and esterified retinol to fully assess the plasma response to VA supplementation and to caution the use of VA assessment methodologies that depend on the hepatic synthesis of RBP during acute inflammation.