Pediatrics · Medical Literature
Exploring associations between maternal vitamin D-binding protein, vitamin D, and offspring asthma/recurrent wheeze.
Abstract
Vitamin D-binding protein (DBP) transports vitamin D metabolites and regulates vitamin D levels in circulation. Additionally, maternal vitamin D levels during pregnancy play an important role in lung development and childhood asthma occurrence. In this post hoc analysis of a randomized controlled trial, the joint associations of maternal 25-hydroxyvitamin-D (25OHD) and DBP with offspring asthma/recurrent wheeze, for both the full cohort and stratified by maternal asthma status, are analyzed. Additionally, associations between estimated maternal free 25OHD and offspring asthma/recurrent wheeze are investigated. A total of 518 participants from the Vitamin D Antenatal Asthma Reduction Trial were included in this analysis. The primary outcome was offspring asthma/recurrent wheeze by age 3 y. Maternal plasma DBP levels were measured for 517 and 516 participants at 10-18 and 32-38 wk of gestation, respectively. Total 25OHD and DBP levels were compared across haplotypes of the group-specific component (GC) gene, which codes for DBP. Logistic regression models estimated the relationships between maternal DBP, total 25OHD, and offspring asthma/recurrent wheeze. In addition, offspring asthma/recurrent wheeze was modeled as a function of estimated maternal free 25OHD. Maternal DBP levels generally increased as pregnancy progressed. Maternal DBP and total 25OHD levels varied significantly across GC haplotypes. A significant positive interaction effect between maternal DBP and total 25OHD on offspring asthma/recurrent wheeze risk was observed for the subset of mothers without asthma. For the subset of mothers with asthma, we observed a significant negative association between estimated maternal free 25OHD and offspring asthma/recurrent wheeze, surpassing the effects of DBP or total 25OHD individually. Our study gives insight into the interplay between vitamin D metabolites during pregnancy and their associations with offspring asthma/recurrent wheeze. These results also suggest that maternal free vitamin D during pregnancy may be more biologically relevant than total vitamin D for offspring's respiratory health. This trial was registered at clinicaltrials.gov for VDAART as NCT00920621.
Clinical relevance
Among pregnant women enrolled in the VDAART trial, do maternal vitamin D-binding protein (DBP) levels, individually or jointly with total 25-hydroxyvitamin D (25OHD), and estimated free 25OHD modify the risk of offspring asthma or recurrent wheeze by age 3 years, and does this relationship differ by maternal asthma status? __SNAPSHOT__ {"studyDesign":"Post hoc secondary analysis of a randomized controlled trial (VDAART; NCT00920621)","setting":"Three academic medical centers in the United States: Boston Medical Center, Washington University in St. Louis, and Kaiser Permanente Southern California Region (San Diego, CA)","sampleSize":"n=518 (DBP measured in 517 participants at 10–18 weeks and 516 at 32–38 weeks of gestation)","population":"Pregnant women aged 18–39 years at 10–18 weeks gestation with a personal or partner history of asthma, eczema, or allergic rhinitis; non-smokers; English- or Spanish-speaking; analyzed here as a sub-cohort with available DBP measurements from the parent VDAART trial","intervention":"Measurement of maternal plasma DBP levels at two gestational timepoints (10–18 weeks and 32–38 weeks), joint analysis of total 25OHD and DBP, and estimation of free (unbound) 25OHD calculated from existing DBP and total 25OHD values; cohort also stratified by GC gene haplotype","comparator":"Analyses conducted across the full cohort and stratified by maternal asthma status (mothers with asthma vs. mothers without asthma); no separate control arm for the DBP exposure itself","primaryOutcome":"Offspring asthma or recurrent wheeze by age 3 years, defined by parental report of physician-diagnosed asthma or recurrent wheeze meeting pre-specified VDAART criteria","followUp":"3 years (offspring followed to their third birthday)","fundingSource":"Government (National Heart, Lung, and Blood Institute; NHLBI grants R01 HL091528, U01 HL091528, UH3 OD023268)"}
Key findings
This post hoc analysis of 518 VDAART participants found that maternal DBP and total 25OHD levels varied significantly across GC gene haplotypes, confirming the genetic regulation of circulating DBP concentrations. In the full cohort, the joint effect of maternal DBP and total 25OHD on offspring asthma/recurrent wheeze risk was examined using logistic regression with bootstrap-derived confidence intervals. In the subset of mothers without asthma, a significant positive interaction between maternal DBP and total 25OHD on offspring asthma/recurrent wheeze risk was observed, suggesting that the protective association of higher total 25OHD may be attenuated (or even reversed) at higher DBP levels in this group. In the subset of mothers with asthma, a significant negative association between estimated free 25OHD and offspring asthma/recurrent wheeze was identified, and this association was stronger than those observed for total 25OHD or DBP individually. Specific odds ratios, confidence intervals, and p-values for logistic regression models are not reported in the available abstract; quantitative effect sizes are therefore not reported here. Estimated free 25OHD was calculated from measured DBP and total 25OHD rather than directly assayed. No clinically important adverse events were reported, as this was an observational secondary analysis. NNT and NNH are not calculable from the available data.
Practice implications
This study establishes that maternal DBP and total 25OHD interact in their association with offspring asthma/recurrent wheeze risk, and that this relationship is modified by whether the mother herself has asthma. The finding that estimated free 25OHD showed a stronger inverse association with offspring asthma than total 25OHD in the maternal-asthma subgroup provides mechanistic support for the free hormone hypothesis — that biologically active, unbound vitamin D may be more relevant to respiratory outcomes than total circulating levels measured by standard clinical assays. What this study does not establish: it does not demonstrate that measuring or targeting free 25OHD in clinical practice will reduce childhood asthma; no causal inference can be drawn from a post hoc secondary analysis; the observed interactions were hypothesis-generating and should be considered exploratory. Patients represented are predominantly those with atopic risk factors (personal or family history of asthma/eczema/allergic rhinitis) at U.S. academic centers, limiting generalizability to lower-risk or more ethnically diverse populations. Because only an abstract and conference-derived data are accessible, quantitative effect sizes, full covariate adjustment details, and potential subgroup multiplicity corrections cannot be evaluated — a significant limitation of this summary. Practice change is not warranted on the basis of this analysis alone. __LIMITATIONS__ Post hoc secondary analysis of the VDAART RCT: not pre-specified, raising risk of spurious findings due to multiple comparisons and hypothesis inflation.,Reduced sample size (n=518) compared to the full VDAART trial (n≈806 mother-child pairs with live births), introducing potential selection bias and limiting statistical power for subgroup analyses.,Free 25OHD was estimated (calculated) from total 25OHD and DBP concentrations rather than directly measured, introducing methodological uncertainty; immunoassay-based estimation of free vitamin D has known imprecision.,Stratification by maternal asthma status further reduces subgroup sizes, increasing the risk of false-positive or unstable interaction estimates.,The parent VDAART trial's primary intention-to-treat analysis found no statistically significant effect of vitamin D supplementation on offspring asthma at 3 years, and a 6-year follow-up showed no effect at all — the broader clinical context is one of null or modest intervention benefit.,Confounding by GC haplotype and other genetic or environmental determinants of DBP levels cannot be fully excluded in an observational framework.,Follow-up to 3 years only; asthma/wheeze phenotypes in early childhood are unstable, and associations may not persist to school age.,Childhood asthma/recurrent wheeze was based on parental report of physician diagnosis, which may introduce misclassification.,Only an abstract and partial text are available for this summary; full results including adjusted odds ratios, confidence intervals, and sensitivity analyses have not been independently verified.,Government-funded (NHLBI); no apparent industry conflict, though two senior co-authors receive UpToDate royalties. __BOTTOM_LINE_CATEGORY__ promising_but_preliminary __BOTTOM_LINE__ This post hoc secondary analysis suggests that maternal vitamin D-binding protein and estimated free 25OHD interact with total vitamin D to modulate the risk of offspring asthma/recurrent wheeze by age 3, with the most notable signal being a stronger protective association of free 25OHD among mothers with asthma. These findings are hypothesis-generating and support further investigation of free 25OHD as a potentially more biologically relevant marker than total 25OHD, but they do not justify changes to current clinical practice regarding vitamin D monitoring or supplementation in pregnancy. Confirmatory prospective studies with directly measured free 25OHD and pre-specified analyses are needed before clinical translation.