Validity of the relative-dose-response test and the modified-relative-dose-response test as indicators of vitamin A stores in liver

Control by the brain of vitamin A homeostasis

Vitamin A is a micronutrient essential for vertebrate animals maintained in homeostatic balance in the body; however, little is known about the control of this balance. This study investigated whether the hypothalamus, a key integrative brain region, regulates vitamin A levels in the liver and circulation. Vitamin A in the form of retinol or retinoic acid was stereotactically injected into the 3rd ventricle of the rat brain. Alternatively, retinoids in the mouse hypothalamus were altered through retinol-binding protein 4 (Rbp4) gene knockdown. This led to rapid change in the liver proteins controlling vitamin A homeostasis as well as vitamin A itself in liver and the circulation. Prolonged disruption of Rbp4 in the region of the arcuate nucleus of the mouse hypothalamus altered retinol levels in the liver. This supports the concept that the brain may sense retinoids and influence whole-body vitamin A homeostasis with a possible "vitaminostatic" role.

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.

Interpretation of Serum Retinol Data From Latin America and the Caribbean

BACKGROUND

In recent decades, the general socioeconomic situation in Latin America and the Caribbean countries has improved, and many vitamin A programs have been implemented in an attempt to reduce vitamin A deficiency in the region.

OBJECTIVE

To examine vitamin A status in Latin America and the Caribbean based on serum retinol concentrations and to contrast available data published before and after 1998.

METHODS

A systematic review was performed. National surveys or representative studies that reported vitamin A status were selected.

RESULTS

Ten national surveys and six representative studies were identified. Data for children under 6 years of age indicate that Guatemala and Nicaragua have practically eradicated vitamin A deficiency (less than 2% prevalence of serum retinol < 20 μg/dL). In Costa Rica, Cuba, El Salvador, and Panama, the prevalence of vitamin A deficiency ranged from 2.8% to 9.4%. In Peru, Honduras, Argentina, Ecuador, and Brazil, vitamin A deficiency is a moderate public health problem (prevalence from 14.0% to 17.4%), while in Colombia, Mexico, and Haiti it is a severe public health problem (prevalence from 24.3% to 32.0%). Disadvantaged groups (indigenous people and those of Afro-Colombian descent) have the highest rates of deficiency. The prevalence of vitamin A deficiency is under 20% in school-children and adult women. When data published before and after 1998 for children under 6 years of age were compared, most Central American countries had a reduction in the prevalence of vitamin A deficiency (p < .05), whereas in South American countries, the prevalence of vitamin A deficiency increased over time (p < .05).

CONCLUSIONS

The prevalence of vitamin A deficiency in children under 6 years of age has decreased in many Central American countries, but vitamin A deficiency still remains a public health problem in numerous Latin America and Caribbean countries, especially among disadvantaged and vulnerable groups. Because of issues with the accuracy of the serum retinol biomarker reflecting body stores, these results must be interpreted with caution.

Proxy markers of serum retinol concentration, used alone and in combination, to assess population vitamin A status in Kenyan children: a cross-sectional study

BACKGROUND

Serum retinol concentration determined by high-performance liquid chromatography (HPLC) is recommended by the World Health Organization to assess population vitamin A status. This assay is expensive, technically demanding and rarely available in developing countries. Our objective was a) to assess the diagnostic performance of proxy markers in detecting vitamin A deficiency and b) to derive decision rules based on these markers to estimate vitamin A deficiency prevalence.

METHODS

A survey was conducted in 15 rural primary schools in Eastern Province, Kenya, with 375 children aged 6 to 12 years (25 randomly selected per school). Serum retinol concentration <0.70 μmol/L by HPLC was used to define vitamin A deficiency. Proxy markers for vitamin A deficiency were serum concentrations of retinol binding protein (RBP), transthyretin, retinol measured by fluorometry and RBP:transthyretin molar ratio.

RESULTS

The prevalence of vitamin A deficiency (HPLC) was 18%. Transthyretin and RBP showed the best diagnostic performance individually, with area-under-the-curve (AUC) values of 0.96 and 0.93. When combined, and with C-reactive protein added, the AUC increased to 0.98. A simple decision rule {(-15.277 × [RBP, μmol/L] - 7.013 × [Transthyretin, μmol/L] + 0.367 × [C-reactive protein, mg/L] + 24.714) > 0.496} yielded prevalence estimates of vitamin A deficiency that is unbiased by diagnostic error.

CONCLUSIONS

The combination of transthyretin, RBP and C-reactive protein concentrations could eventually replace retinol concentration by HPLC in resource-poor settings as the preferred method to assess the population burden of vitamin A deficiency.

Vitamin A intake and infection are associated with plasma retinol among pre-school children in rural Zambia

OBJECTIVE

To determine the prevalence of vitamin A deficiency, infection and adequacy of vitamin A intakes among Zambian children, and the contribution of dietary vitamin A and infection to vitamin A status.

DESIGN

A cross-sectional survey of vitamin A intakes by the 24 h recall method, vitamin A status by plasma retinol and the modified relative dose-response test, and infection by acute-phase proteins.

SETTING

Rural communities in Central and Eastern Provinces of Zambia.

SUBJECTS

Children 2-5 years of age.

RESULTS

The prevalence of vitamin A deficiency was 56 % by plasma retinol, 48 % with infection-adjusted plasma retinol and 22 % by the modified relative dose-response test. The majority of children (61 %) had a current infection. Vitamin A intakes were relatively high (331 to 585 μg retinol activity equivalents/d in the harvest/early post-harvest and late post-harvest seasons, respectively) and the prevalence of inadequate intakes was <1 % when compared with the Estimated Average Requirement (210 and 275 μg retinol activity equivalents/d for children aged 1-3 and 4-8 years, respectively). Elevated α-1-acid glycoprotein was negatively associated with plasma retinol (P < 0·0 0 1) and vitamin A intake was positively associated with plasma retinol (P < 0·05), but only when estimated assuming a 26:1 retinol equivalence for provitamin A from green and yellow vegetables.

CONCLUSIONS

Infection and vitamin A intakes were significant determinants of plasma retinol. We cannot conclude which indicator more accurately represents the true vitamin A status of the population. Reasons for the persistent high prevalence of vitamin A deficiency in the presence of adequate vitamin A intakes are unclear, but the high rates of infection may play a role.

Vitamin A: biomarkers of nutrition for development

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.

Assessment of the relative dose-response test based on serum retinol-binding protein instead of serum retinol in determining low hepatic vitamin A stores

BACKGROUND

The relative dose-response (RDR) test, which measures the percentage of change in serum retinol concentration in response to an oral vitamin A (VA) dose, is a functional reference method to assess low hepatic VA stores. However, problems due to the relative instability of retinol, which is measured in the traditional RDR test, could be circumvented if retinol-binding protein (RBP), a more stable marker of VA, could be measured instead of retinol to provide the RDR value.

OBJECTIVE

The objective was to compare classification of VA status assessed by retinol-RDR with that assessed by using RBP-RDR.

DESIGN

With the use of serum samples from 57 lactating women in northern Kenya collected in August-September 2006, we assessed the accuracy of RBP-RDR in predicting low hepatic VA stores through receiver operator characteristic (ROC) analysis using retinol-RDR values as the gold standard. By using regression analysis, we explored the effects of 1) body mass index (BMI) on RBP-RDR performance and 2) the oral VA dose on the retinol-RBP molar ratio.

RESULTS

The classificatory accuracy of RBP-RDR was low to moderate (n = 50; ROC area: 0.56-0.72) depending on the cutoffs used. RBP-RDR systematically overestimated VA deficiency with higher BMI, although it was superior to a single measurement of serum retinol. The discrepancy between RBP-RDR and retinol-RDR appears to originate in a retinol concentration-dependent alteration of the retinol-RBP molar ratio triggered by the oral dose.

CONCLUSIONS

RBP-RDR has the potential to serve as a moderately accurate surrogate measure of retinol-RDR if the variation associated with BMI is understood and adjusted. Further studies should clarify the dynamics of the retinol-RBP molar ratio and its link to RBP-RDR performance.

Assessment of vitamin A nutritional status in newborn preterm infants

OBJECTIVE

This study assessed the vitamin A nutritional status of preterm infants determined by the vitamin A relative dose-response test (RDR) compared with serum levels of vitamin A, retinol-binding protein (RBP), transthyretin (TTR), and retinol relations with carrier proteins.

METHODS

Serum levels of retinol, RBP, and TTR and retinol/RBP, retinol/TTR, and RBP/TTR molar ratios were determined in 120 infants at 7 d and in 92 at 28 d. For the determination of the performance of the tests, the RDR was considered the reference method. The sensitivity and specificity for all possible cutoff values were determined by constructing receiver operator characteristic curves. The areas under the curves were used to estimate the overall accuracy of the tests. The best cutoff values to be used for the calculation of sensitivity and specificity were determined with 95% confidence intervals.

RESULTS

RDR indicated vitamin A deficiency in 60% of the infants at 7 d and in 51.1% at 28 d. In the receiver operator characteristic curves, the best area under the curve was 0.710 obtained for serum retinol at 28 d of postnatal age and considered moderately accurate. The least inadequate cutoff level was set at 25 mg/dL, but no value was considered adequate due to low sensitivity and/or low specificity.

CONCLUSION

Compared with RDR, the determination of serum levels of retinol, RBP, and TTR and their molar ratios are not adequate to assess nutritional vitamin A status in preterm infants.

Evaluation of vitamin A supplementation regimens in Ghanaian postpartum mothers with the use of the modified-relative-dose-response test

BACKGROUND

Vitamin A deficiency is an important public health problem in many developing countries. Women of childbearing age and children are documented as the most affected groups.

OBJECTIVE

The objective was to determine the length of time mothers are protected postpartum against vitamin A depletion after receiving either 400,000 IU vitamin A in 2 divided doses or 200,000 IU as a single dose plus a placebo 24 h apart.

DESIGN

Mothers (n = 168) were recruited by trained fieldworkers 7-10 d after delivery. Modified-relative-dose-response (MRDR) tests were performed at baseline in 167 women, and vitamin A was administered within 6 wk after delivery. The women were randomly assigned to 2 main treatment groups, and each treatment group was divided into 3 follow-up subgroups. Each subgroup was invited back once at month 1, 3, or 5 for a second MRDR test.

RESULTS

The serum retinol concentration and the MRDR value were 1.4 +/- 0.5 micromol/L and 0.048 +/- 0.037, respectively, at baseline. A significant improvement in vitamin A status occurred after vitamin A treatment as assessed by the MRDR test (P < 0.0001). Serum retinol concentrations were not different after vitamin A treatment (P = 0.87).

CONCLUSIONS

The mothers had marginally depleted liver reserves of vitamin A at baseline on the basis of MRDR test results. Liver reserves of vitamin A significantly improved in both treatment groups, and the improvement was maintained for >or= 5 mo.

The modified-relative-dose-response values in serum and milk are positively correlated over time in lactating sows with adequate vitamin A status

The modified-relative-dose-response (MRDR) test, which requires a blood sample after dosing with 3,4-didehydroretinyl acetate (DRA), has been used to determine vitamin A (VA) status of individuals and groups worldwide. Less invasive methods using milk are in development in a swine model. Swine are a good choice for studying VA metabolism because their gastrointestinal anatomy, morphology, physiology, and VA requirements are similar to those of humans. In this study, DRA was used as a VA tracer in lactating sows to follow the metabolism of newly ingested VA. Lactating sows (n = 6) were administered 35 micromol DRA after overnight food deprivation. Blood and milk were collected at 0, 1.5, 3, 5, 7, 9, 24, and 48 h; livers were obtained at the time of killing. Samples were analyzed for didehydroretinol (DR), retinol (R), and didehydroretinyl esters (DRE). Serum DR:R was compared with that in milk and other VA indicators. DRE rapidly increased in serum, corresponding to chylomicra, whereas DR increased at a slower rate corresponding to the holo-DR:retinol-binding protein complex released from the liver. An estimated 10-20% of the dose was irreversibly lost in milk over 48 h. The mean MRDR value was 0.018 +/- 0.013 at 5 h and the mean liver VA was 0.73 +/- 0.21 micromol/g, both signifying sufficient stores. Milk and serum DR:R values were directly correlated (r = 0.64, P < 0.0001). Thus, DR:R values in milk may be a potential alternative to serum in determining VA status in lactating women. Future work is required in VA-deficient sows and women of varying VA status to determine DR trafficking and to compare DR:R values in milk with those in serum.