Retinol isotope dilution accurately predicts liver reserves in piglets but overestimates reserves in lactating sows

Development of a Compartmental Model for Studying Vitamin A Kinetics and Status in Theoretical Lactating Women

BACKGROUND

Low vitamin A status and suboptimal milk vitamin A concentrations are problems in many populations worldwide. However, limited research has been done on whole-body vitamin A kinetics in women of reproductive age, especially during lactation.

OBJECTIVES

Goals were to develop compartmental models describing retinol kinetics in theoretical nonlactating (NL) and lactating (L) women and to determine whether the retinol isotope dilution (RID) method accurately predicted vitamin A total body stores (TBS) in the groups and individuals.

METHODS

We adapted 12 previously-used theoretical females with assigned values for retinol kinetic parameters and TBS (225-1348 μmol); subjects were NL or L (nursing one 3- to 6-mo-old infant) during 49-d kinetic studies after isotope dosing. We used an established compartmental model, adding a compartment for chylomicrons and, for L, another for mammary gland milk with inputs from holo-retinol-binding protein and chylomicron retinyl esters and output to milk. Using compartmental analysis, we simulated tracer responses in compartments of interest and calculated TBS using the RID equation TBS =   FaS/SAp [Fa, fraction of dose in stores; S, retinol specific activity in plasma/specific activity in stores; SAp, specific activity of retinol in plasma].

RESULTS

Models for both groups were well identified. Simulated plasma tracer responses were similar for NL and L, with L always below NL; milk tracer paralleled plasma from 10 d postdosing. Geometric mean FaS ratios (L/NL) were ∼0.75 during days 2-30. Using appropriate group FaS, RID provided accurate TBS predictions for >80% of NL and L subjects after day 18 when CV% for FaS was ∼10%.

CONCLUSIONS

These new physiologically-based models for vitamin A kinetics may be useful for future research in women of reproductive age. Results indicate that, in groups like these, RID to assess an individual's vitamin A status should be done at 21-28 d after isotope dosing.

Use of Compartmental Modeling and Datasets for Theoretical Lactating Women to Determine Conditions under Which Vitamin A-Specific Activity in Breast Milk Provides Accurate Estimates of Vitamin A Total Body Stores by Retinol Isotope Dilution

BACKGROUND

Vitamin A concentrations in breast milk are related to maternal vitamin A intake and status.

OBJECTIVES

Our objective was to identify conditions under which vitamin A specific activity in breast milk (SAm) could be used instead of retinol specific activity in plasma (SAp) to predict vitamin A total body stores (TBS) by retinol isotope dilution (RID).

METHODS

We used 12 previously-studied theoretical lactating women with assigned values for TBS (219-1348 μmol) and retinol kinetic parameters; we assumed subjects ingested a dose of stable isotope-labeled vitamin A. We expanded a 9-compartment steady state tracer model to include a parallel model for tracee (unlabeled retinol) and then adapted that model so vitamin A intake entered the system in 3 meals each day. Using compartmental analysis, we first simulated SAm and SAp after an overnight fast (as in actual RID experiments) and then with vitamin A intake also restricted in sequential meals on the day before sampling for RID.

RESULTS

After an overnight fast, SAm at day 21 postdosing was lower than SAp. However, if vitamin A intake was also restricted in 1, 2, or 3 meals before sampling, SAm/SAp (mean ± SD) was 0.92 ± 0.042,  0.96 ± 0.016,  or 0.99 ± 0.004,  respectively; results for days 14 and 28 were similar. When either SAp or SAm was used to predict TBS by RID on day 21 after 1-d restriction, predictions for all subjects were within 25% of assigned TBS.

CONCLUSIONS

Results indicate that, for theoretical lactating women with a wide range of vitamin A status, SAm will accurately predict TBS by RID at 2-4 wk postdosing if vitamin A intake is restricted for 1 d before sampling. If confirmed in community settings, results suggest that vitamin A status in lactating women can be determined without collecting blood.

Time Since Dose and Dietary Vitamin A Intake Affect Tracer Mixing in the 13C-Retinol Isotope Dilution Test in Male Rats

BACKGROUND

Retinol isotope dilution (RID) estimates total liver vitamin A reserves (TLRs), the gold-standard vitamin A (VA) biomarker. RID equation assumptions are based on limited data.

OBJECTIVES

We measured the impact of tracer choice, mixing period, and VA intake on tracer mixing [ratio of tracer enrichment in serum to that in liver stores (S)] in VA-deficient, -adequate, and hypervitaminotic rats.

METHODS

Study 1 was a 3 × 2 × 3 design (18 groups, n = 5/group). Male Sprague-Dawley rats (21 d old) received 50, 100, or 3500 nmol VA/d for 21 d, were administered 52 nmol 13C2- or 13C10-retinyl acetate orally, and killed 5, 10, or 15 d later. Unlabeled VA (50 nmol/d) was given on days 11-14. Study 2 used 100 nmol VA/d for 21 d with 3 groups (n = 6-7): 52 nmol 13C2- or 13C10-retinyl acetate and 100 nmol VA/d throughout 14-d mixing, or 13C2-retinyl acetate without VA. Repeated-measures, 1-factor, and 3-factor ANOVAs were used for analysis.

RESULTS

Mean ± SD TLRs (μmol/g liver) reflected intake: 0.11 ± 0.04 (50 nmol VA/d), 0.16 ± 0.04 (100 nmol VA/d), and 5.07 ± 1.58 (3500 nmol VA/d) in Study 1 and 0.24 ± 0.08 (100 nmol VA/d) in Study 2. In Study 1, mean ± SD S was 1.65 ± 0.26 (5 d), 1.16 ± 0.09 (10 d), and 0.92 ± 0.08 (15 d). The interactions tracer*VA intake and time*VA intake were significant between days 10 and 15 (P < 0.05). In Study 2, mean ± SD S was 1.07 ± 0.02 without VA during mixing, and 0.81 ± 0.04 (13C2) and 0.79 ± 0.03 (13C10) with VA intake throughout. Estimated:measured TLRs varied by VA intake and time in Study 1 but not between groups in Study 2.

CONCLUSIONS

The 13C-content effect on RID through S is inconsistent. S is highly variable at 5 d, contraindicating early-time point RID. VA intake effects on S vary with timing and quantity. Assuming S = 0.8 at 14 d with consistent VA intake in human studies is likely appropriate.

Vitamin A-fortified rice increases total body vitamin A stores in lactating Thai women measured by retinol isotope dilution: a double-blind, randomized, controlled trial

BACKGROUND

Lactating women are at increased risk for vitamin A (VA) deficiency due to demands for breast milk content and limited hepatic stores for women in some countries. Previously, consumption of triple-fortified rice, which included VA, iron, and zinc, successfully improved the VA status of Thai children in whom their total body VA stores (TBSs) were doubled in 2 mo.

OBJECTIVE

This study assessed the efficacy of consuming VA-fortified rice, which delivered 500 µg retinol activity equivalents (RAEs)/d, on TBSs and estimated total liver VA reserves (TLRs) in Thai lactating women using the retinol isotope dilution (RID) test.

METHODS

A randomized controlled trial was conducted with 70 lactating women (n = 35/group) who received either VA-fortified rice (500 µg RAEs/d) or unfortified rice for 14 wk on weekdays only. Serum retinol concentrations (SRs), C-reactive protein, and TBSs were assessed before and after the intervention. The paired 13C-RID test was used to measure TBSs. After a baseline blood sample, 2.0 µmol [14,15]-13C2-retinyl acetate was administered orally. A follow-up blood sample was drawn 14 d later. The RID test was repeated after the intervention.

RESULTS

TBSs increased significantly (P < 0.05) in the intervention group from 240 (182, 316) to 331 (251, 447) [geometric means (95% CIs)] µmol retinol, and this change in TBSs was significantly higher (P < 0.05) than that in the control group [+52.9 (-74, 453) compared with -4.3 (-106, 275) µmol retinol]. Estimated TLRs indicated a high prevalence of VA deficiency among these lactating women. Initial and final SRs did not differ by group and did not change over the course of the intervention.

CONCLUSION

VA-fortified rice improved the VA status of lactating women by increasing TBSs. A targeted approach to disseminate VA interventions among vulnerable groups should be considered in some contexts. This trial was registered at clinicaltrials.gov as NCT03056625.

Breast Milk-Derived Retinol Is a Potential Surrogate for Serum in the 13C-Retinol Isotope Dilution Test in Zambian Lactating Women with Vitamin A Deficient and Adequate Status

BACKGROUND

Vitamin A (VA) deficiency (VAD) affects ∼19 million pregnant women worldwide. The extent of VAD in Zambian women of reproductive age is unknown owing to lack of survey inclusion or the use of static serum retinol concentrations, a low-sensitivity biomarker.

OBJECTIVES

This cross-sectional study employed isotopic techniques to determine VA status with serum and milk among women aged 18-49 y (n = 197) either lactating with infants aged 0-24 mo or nonlactating with or without infants.

METHODS

Assistants were trained and piloted data collection. Demographic data, anthropometry, and relevant histories were obtained including malaria and anemia. For retinol isotope dilution (RID), baseline fasting blood and casual breast milk samples were collected before administration of 2.0 μmol 13C2-retinyl acetate and 24-h dietary recalls. On day 14, blood (n = 144) and milk (n = 66) were collected. Prevalence of total liver VA reserves (TLR) ≤0.10 μmol/g was defined as VAD with comparison to the DRI assumption of 0.07 μmol/g as minimally acceptable for North Americans.

RESULTS

When a 20% adjustment for dose lost to milk was made in the RID equation for lactation, mean total body VA stores (TBS) for lactating women were 25% lower than for nonlactating women (P < 0.01), which was not the case without adjustment (P = 0.3). Mean ± SD TLR for all women were 0.15 ± 0.11 μmol/g liver. Using retinol purified from breast milk instead of serum for RID analysis yielded similar TBS and TLR, which were highly correlated between methods (P < 0.0001). Serum retinol ≤0.70 μmol/L had 0% sensitivity using either VAD liver cutoff and milk retinol ≤1.0 μmol/L had 42% sensitivity for VAD at 0.10 μmol/g.

CONCLUSIONS

Determining accurate VA status among women of reproductive age, especially lactating women, forms a basis for extrapolation to the general population and informing policy development and program implementation.

Metabolism of Neonatal Vitamin A Supplementation: A Systematic Review

A systematic review was conducted to summarize the absorption, transport, storage, and metabolism of oral neonatal vitamin A supplementation (NVAS). This review focused specifically on the neonatal period (first 28 d of life for humans) to inform guidance by WHO on recommendations related to NVAS. A systematic search of international and regional databases was conducted. Inclusion criteria were human or animal studies that gave oral vitamin A as a single or limited number of doses to apparently healthy neonates. Studies evaluating fortification or food-based approaches, dosing with retinoic acid, or studies of neonatal models of disease were excluded. The search retrieved 8847 unique records. After screening by title and abstract, 88 were screened using the full text, and 35 records met inclusion criteria: 13 human and 22 animal studies. Studies indicate that high-dose NVAS is absorbed well by neonates, typically mirroring fat absorption. Doses were primarily stored in the liver and transiently increased in the lung, kidney, spleen, adrenal glands, brain, skin, and adipose tissue, generally with a dose-response. Serum retinol and retinyl esters also transiently increased following NVAS. Although minimal acute adverse effects are noted, there is a lack of data supporting NVAS for improving organ maturation or sustained delivery to target organs. Research gaps include the physiological effects of the short-term increase of vitamin A concentrations in extrahepatic tissues, or whether there are unknown adverse effects over time.

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.

Liver retinol estimated by 13C-retinol isotope dilution at 7 versus 14 days in Burkinabe schoolchildren

Vitamin A status assessment is not straightforward. Retinol isotope dilution (RID) testing requires time for the tracer dose to mix with the total body stores of vitamin A (TBS). Researchers are interested in shortening the time interval between tracer administration and follow-up blood draws, and in re-examining current assumptions about liver mass for calculation of total liver vitamin A reserves (TLR, in µmol/g liver). Schoolchildren (aged 7–12 years; n =  72) were recruited from one school in Burkina Faso. After a baseline blood draw, 1.0 µmol [14,15]-13C2-retinyl acetate was administered to estimate TBS and TLR by retinol isotope dilution with follow-up blood samples at days 7 and 14. Correlations were determined to evaluate if sampling at day 7 could be used to predict TLR compared with day 14. Liver mass was estimated using body surface area and compared with the currently used assumption of liver weight equivalent to 3% of body weight. (This trial was registered at Pan African Clinical Trial Registry: PACTR201702001947398). Liver mass calculated using body surface area did not differ from the standard assumption of 3% of body weight and yielded similar TLR values. The children in this study had mean TLR (0.67 ± 0.35 µmol/g) in the adequate range, while serum retinol concentrations (0.92 ± 0.33 µmol/L) predicted 25% vitamin A deficiency. TLR values at seven days were highly correlated with, but significantly different from day 14 (P  <  0.0001, r  =  0.85) and needed a correction factor added to the equation to yield equivalency. Blood drawing at day 7, using correction factors in the prediction equation and the current assumption of liver mass as 3% of body weight, can be used to estimate TLR in schoolchildren with adequate vitamin A status in 13 C2-RID applications, but further investigations are needed to verify the seven-day predictive equation.