Maternal Vitamin D Status During Pregnancy and Bone Mass in Offspring at 20 Years of Age: A Prospective Cohort Study

Authors

  • Kun Zhu,

    Corresponding author
    1. Department of Endocrinology and Diabetes, Sir Charles Gairdner Hospital, Nedlands, Australia
    2. School of Medicine and Pharmacology, University of Western Australia, Crawley, Australia
    • Address correspondence to: Kun Zhu, PhD, Department of Endocrinology and Diabetes, Sir Charles Gardiner Hospital, Nedlands, WA 6009, Australia. E-mail: kun.zhu@uwa.edu.au

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  • Andrew JO Whitehouse,

    1. Telethon Institute for Child Health Research, Centre for Child Health Research, University of Western Australia, Perth, Australia
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    • AJOW, PHH, and MK contributed equally to this work.
  • Prue H Hart,

    1. Telethon Institute for Child Health Research, Centre for Child Health Research, University of Western Australia, Perth, Australia
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    • AJOW, PHH, and MK contributed equally to this work.
  • Merci Kusel,

    1. Telethon Institute for Child Health Research, Centre for Child Health Research, University of Western Australia, Perth, Australia
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    • AJOW, PHH, and MK contributed equally to this work.
  • Jenny Mountain,

    1. Telethon Institute for Child Health Research, Centre for Child Health Research, University of Western Australia, Perth, Australia
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  • Stephen Lye,

    1. Samuel Lunenfeld Research Institute, Mount Sinai Hospital, Toronto, Canada
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  • Craig Pennell,

    1. School of Women's and Infants' Health, University of Western Australia, Crawley, Australia
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  • John P Walsh

    1. Department of Endocrinology and Diabetes, Sir Charles Gairdner Hospital, Nedlands, Australia
    2. School of Medicine and Pharmacology, University of Western Australia, Crawley, Australia
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ABSTRACT

It is uncertain whether the vitamin D status of pregnant women influences bone mass of their children. Cohort studies have yielded conflicting results; none have examined offspring at skeletal maturity. This longitudinal, prospective study investigated the association between maternal vitamin D status and peak bone mass of offspring in 341 mother and offspring pairs in the Western Australian Pregnancy Cohort (Raine) Study. Maternal serum samples collected at 18 weeks gestation were assayed for 25-hydroxyvitamin D (25OHD). Outcomes were total body bone mineral content (BMC) and bone mineral density (BMD) measured by dual-energy X-ray absorptiometry in offspring at 20 years of age. The mean (± SD) maternal serum 25OHD concentration was 57.2 ± 19.2 nmol/L; 132 women (38.7%) were vitamin D-deficient (25OHD <50 nmol/L). After adjustment for season of sample collection, maternal factors, and offspring factors (sex, birth weight, and age, height, lean mass, and fat mass at 20 years), maternal 25OHD concentration was positively associated with total body BMC and BMD in offspring, with a mean difference of 19.2 (95% confidence interval [CI], 5.6–32.7) g for BMC and 4.6 (95% CI, 0.1–9.1) mg/cm2 for BMD per 10.0 nmol/L of maternal 25OHD. Maternal vitamin D deficiency was associated with 2.7% lower total body BMC (mean ± SE) (2846 ± 20 versus 2924 ± 16 g, p = 0.004) and 1.7% lower total body BMD (1053 ± 7 versus 1071 ± 5 mg/cm2, p = 0.043) in the offspring. We conclude that vitamin D deficiency in pregnant women is associated with lower peak bone mass in their children. This may increase fracture risk in the offspring in later life. © 2014 American Society for Bone and Mineral Research.

Introduction

Bone mass at any time during adult life is determined by peak bone mass achieved at skeletal maturity and the rate of bone loss during aging. An optimal peak bone mass is considered the best protection against age-related bone loss and subsequent fracture risk.[1] It has been estimated that 5% higher bone mineral density (BMD) could translate to a 20% reduction in osteoporotic fracture and 50% reduction in hip fracture in later life.[2] Determinants of peak bone mass include genetic and environmental factors, such as diet and physical activity. However, a proportion of the variance in peak bone mass is not explained by these factors and it has been suggested that part of the residual variance in bone mass may be explained by intrauterine growth and/or fetal programming.[3] In epidemiological studies, higher birth weight is associated with greater hip and spine bone mineral content (BMC) in adulthood,[4] whereas low birth length and poor childhood growth are predictors of hip fracture risk in later life,[5] and maternal dietary factors during pregnancy are associated with bone mass in children at 9 years[6] and 16 years.[7]

A potentially important maternal nutrition factor related to bone health of offspring is maternal vitamin D status, which has been shown to be related to neonatal calcium homeostasis.[8] However, it is not clear whether maternal vitamin D status is associated with bone mineral accretion in offspring. Two cohort studies from the United Kingdom evaluated the association between maternal vitamin D status and BMC of offspring and yielded conflicting results. In the Southampton Women's Survey, maternal 25-hydroxyvitamin D (25OHD) levels during late pregnancy (mean 34 weeks) was positively associated with whole-body and lumbar spine BMC in children at 9 years of age.[9] By contrast, in the Avon Longitudinal Study of Parents and Children (ALSPAC), there was no significant association between maternal vitamin D status assessed at various stages of pregnancy and total body or spine BMC in late childhood (age 9–10 years).[10]

Peak bone mass assessed at the time of skeletal maturity is almost certainly a better predictor of long-term fracture risk than bone mass measured at 9 or 10 years of age, but to our knowledge, the relationship between maternal vitamin D status during pregnancy and peak bone mass has not been examined. We therefore examined this relationship in 341 mother-offspring pairs from the Western Australian Pregnancy Cohort (Raine) Study, with measurement of maternal serum 25OHD at 18 weeks gestation, and total body bone mass in the offspring at 20 years of age, hypothesizing that maternal vitamin D deficiency during pregnancy would be associated with lower bone mass in offspring. In a longitudinal study of Canadian boys and girls to 30 years of age, total body bone mass reached a plateau at 18 years in girls and 20 years in boys,[11] suggesting that 20 years is an appropriate age to assess peak bone mass.

Subjects and Methods

Participants

This longitudinal, prospective study included data from 341 offspring (137 males and 204 females) and mother pairs from the Western Australian Pregnancy Cohort (Raine) Study. The Raine study recruited 2900 pregnant women from the antenatal clinic at King Edward Memorial Hospital and near private clinics in Perth, Western Australia, between May 1989 and November 1991. Inclusion criteria were a gestational age between 16 and 20 weeks, English language skills sufficient to understand the study demands, an expectation to deliver at King Edward Memorial Hospital, and an intention to remain in Western Australia to enable future follow-up of their child.[12] All offspring have been invited to attend periodic follow-up surveys, the most recent being at 20 years of age (2010–2012). Compared with the general Western Australian population, the Raine cohort at birth was characterized by higher proportions of high-risk births and fathers employed in managerial and professional positions, but comparison of participants remaining in the study at the 14-year follow-up suggested that attrition resulted in a cohort comparable with the general population.[13] The current study is restricted to mother-child pairs for whom maternal 25OHD concentrations were available and whose offspring underwent whole-body dual-energy X-ray absorptiometry (DXA) scanning as part of the 20-year follow-up visit. Thirteen subjects with intrauterine growth restriction were excluded. The original study and follow-up studies of the offspring were approved by Human Research Ethics Committees at King Edward Memorial Hospital, Princess Margaret Hospital for Children, and the University of Western Australia. Informed consent was obtained from mothers and offspring.

Whole-body DXA at 20 years

Whole-body scanning was performed in offspring at the 20-year follow-up visit using DXA on a Norland XR-36 densitometer (Norland Medical Systems, Inc., Fort Atkinson, WI, USA), according to manufacturer-recommended procedures. Analysis of scans was performed using built-in machine software (version 4 · 3 · 0) that provided estimates of whole-body BMC (g), bone area (cm2), and areal BMD (mg/cm2), as well as whole-body fat mass (g) and lean mass (bone free) (g). Daily calibration was performed prior to each scanning session, and the interscan coefficient of variation was less than 2%.

Other assessments of offspring

At birth, weight, crown-heel length, and head circumference were measured by standard techniques. At 20 years, body weight was measured to the nearest 0.1 kg with subjects dressed in light clothes, and height measured with a hypsometer to the nearest 0.1 cm. Body mass index (BMI) was calculated as weight (kg)/height (m)2.

Maternal 25OHD

From 1989 to 1991, venous blood was collected at 16 to 20 (mean 18) weeks gestation at the time of antenatal ultrasound scan from a randomly generated subset of 929 study participants and serum was then securely stored at –80°C. Serum 25OHD concentrations were measured in 2011 using an enzyme immunoassay (EIA) kit from Immunodiagnostic Systems (IDS) Ltd (Scottsdale, AZ, USA). Serum 25OHD concentrations in stored sera have been shown to remain stable for three decades.[14, 15] Twenty-eight samples were also assayed using isotope-dilution liquid chromatography/tandem mass spectrometry (LC-MS/MS) by RMIT Drug Discovery Technologies (Melbourne, Australia) according to published methodology.[16] There was a strong correlation between 25OHD concentrations assayed by the two techniques (r2 = 0.87), with no evidence of analytical interference (for example by vitamin D metabolites) in the sera from the pregnant women.

Other maternal measurements

Sociodemographic factors including maternal race/ethnicity, mother's height, mother's weight before pregnancy, maternal education, and family income were recorded at 18 weeks gestation. Antenatal data including maternal smoking and alcohol consumption during pregnancy were recorded at 34 weeks' pregnancy. Obstetric data including gestational age, offspring gender, and parity were collected at birth.

Sample size calculation

At 80% power and 5% level of significance, our study was powered to detect differences of 4.7% in total body BMC, 2.3% in total body bone area, and 3.2% in total body BMD between offspring whose mothers had serum 25OHD <50 nmol/L and ≥50 nmol/L in the unadjusted analysis.

Data analysis

Variables are presented as mean (SD) unless otherwise stated. The characteristics of participants included in the present study were compared with those of the whole Raine Study cohort to determine whether participants were representative of the broader cohort. These comparisons as well as those between male and female offspring were made using Student's t test and chi-square test.

Linear regression analysis was used to evaluate the relationships between maternal vitamin D status during pregnancy and total body BMC, bone area, and BMD. The initial models included the following covariates:

  • Model 1 (minimal adjusted model): maternal age at delivery, season of blood sample collection, offspring sex, and offspring age at time of DXA scan;
  • Model 2 (maternal factors further adjusted model): Model 1 plus maternal education, parity, ethnicity, smoking during pregnancy, height, and weight before pregnancy;
  • Model 3 (offspring factors further adjusted model): Model 2 plus birth weight, gestational age, plus offspring's height, lean mass, and fat mass at 20 years.

Interaction terms for sex were included in each model to determine if associations differed between male and female offspring. As there were no significant interactions between sex and maternal 25OHD for any bone outcome variables, data for males and females were analyzed together with sex included in the model as covariate. Maternal age, smoking during pregnancy, and gestational age were not significant predictors of bone outcome variables in any regression models and were not included in the final linear regression models as covariates. Collinearity was tested in each regression model, with a variance inflation factor (VIF) value larger than 10 considered as showing the existence of collinearity or near collinearity.[17] The adequacy of the assumption of linearity of associations for maternal 25OHD adjusted for other confounders was assessed by examination of the partial regression plots for each model. The normality of the residuals and the homogeneity of variance of each model were checked using residual plots (normal probability plot and plot of residuals versus predicted values). Further comparisons were made by grouping offspring according to maternal vitamin D status using 50 nmol/L, the recommended desirable level in adults and pregnant women,[18, 19] as the cutoff point using analysis of covariance (ANCOVA). Statistical significance level was set at p < 0.05 (two-tailed). All analyses were performed using IBM SPSS (version 20; IBM, Chicago, IL, USA).

Results

Characteristics of mothers and offspring included in the analysis

Maternal serum 25OHD concentrations and DXA results for offspring at 20 years were available for 341 mother and offspring pairs, the offspring comprising 137 males (40.2%) and 204 females (59.8%). A participant disposition chart is shown in Fig. 1. In comparison with Raine Study participants who were not included in this analysis (n = 2527), included mothers were older (29.3 ± 5.6 versus 27.9 ± 5.9 years, p < 0.001), more likely to be Caucasian (89.1% versus 85.8%, p = 0.010), to have completed high school (46.0% versus 37.9%, p = 0.004), to have family income above median disposable household income (59.5% versus 50.7%, p < 0.001), and less likely to smoke during pregnancy (70.8% versus 75.7%, p = 0.017). Offspring included in the analysis were born at slightly later gestation than those not included (276 ± 13 versus 274 ± 17 days, p = 0.002), but did not differ with regard to other maternal and neonatal factors, including maternal height or weight before pregnancy and offspring birth weight, crown-heel length, or head circumference. Maternal serum 25OHD concentrations did not differ significantly between the 341 mothers included in the study and the 588 mothers who had blood samples drawn during pregnancy but were not included (57.2 ± 19.2 versus 58.5 ± 19.2 nmol/L, p = 0.350). For data at 20 years, compared with offspring not included in the analysis (n = 842), offspring included were slightly older (20.1 ± 0.5 versus 20.0 ± 0.4 years, p = 0.001), but height, body weight, and bone and body composition measures did not differ significantly.

Figure 1.

Participant disposition chart showing how the study population of 341 mother-offspring pairs was derived. 25OHD = 25-hydroxyvitamin D; DXA = dual-energy X-ray absorptiometry.

Characteristics of the mothers and offspring included in the analysis are presented in Tables 1 and 2, respectively. The mean (± SD) maternal serum 25OHD concentration at 18 weeks gestation was 57.2 ± 19.2 nmol/L; 132 women (38.7%) were vitamin D–deficient (25OHD <50 nmol/L), comprising 121 women (35.5%) with 25OHD between 25 and 50 nmol/L and 11 women (3.2%) with 25OHD levels <25 nmol/L. Compared with mothers with serum 25OHD <50 nmol/L, mothers with higher vitamin D levels were taller, had higher parity, were more likely to be white and to have had blood samples taken during summer or fall, but the two groups did not differ significantly in education, family income, and smoking habit during pregnancy (Table 1). The mean age of offspring at the time of DXA was 20.1 ± 0.5 years.

Table 1. Characteristics of Mothers
 All (n = 341)Grouped by serum 25-hydroxyvitamin D levels
<50 nmol/L (n = 132)≥50 nmol/L (n = 209)pa
  1. Values are mean ± SD or percentage as stated.
  2. aStudent's t test or chi-square test.
Age at delivery (years)29.0 ± 5.629.0 ± 5.229.4 ± 5.90.505
Height (cm)164.1 ± 6.2162.6 ± 5.6165.0 ± 6.4<0.001
Body weight before pregnancy (kg)59.0 ± 11.058.1 ± 10.959.6 ± 11.00.232
Ethnic origin (%)   0.006
Caucasian89.183.392.8 
Other10.916.77.2 
Completed high school (%)   0.536
Yes46.056.152.6 
No54.043.947.4 
Family income (%)   0.362
<$24,000 per year38.741.736.7 
≥$24,000 per year61.358.363.3 
Smoking during pregnancy (%)   0.998
None75.775.875.6 
1–10 cigarettes/d13.813.613.9 
>10 cigarettes/d10.610.610.5 
Parity (%)   0.005
146.352.342.6 
233.436.431.6 
≥320.211.425.8 
Serum 25-hydroxyvitamin D (nmol/L)57.2 ± 19.238.4 ± 8.469.2 ± 13.7<0.001
Season of blood collection (%)   <0.001
Summer19.99.826.3 
Fall20.512.125.8 
Winter26.437.919.1 
Spring33.140.228.7 
Table 2. Characteristics of Offspring at Birth and at 20-Year Follow-up Visit
 Male (n = 137)Female (n = 204)pa
  1. Values are mean ± SD.
  2. BMI = body mass index; BMC = bone mineral content; BMD = bone mineral density.
  3. aStudent's t test.
Birth
Birth weight (g)3325 ± 4673290 ± 5210.538
Crown heel length (cm)49.2 ± 2.348.7 ± 2.30.065
Head circumference (cm)34.7 ± 1.634.1 ± 1.60.003
Gestational age (days)278 ± 12275 ± 140.105
20 years
Age (years)20.2 ± 0.520.1 ± 0.50.041
Height (cm)177.8 ± 7.5166.2 ± 5.9<0.001
Weight (kg)75.1 ± 12.765.0 ± 12.5<0.001
BMI (kg/m2)23.7 ± 3.723.6 ± 4.60.726
Lean body mass (kg)56.2 ± 8.636.2 ± 4.9<0.001
Fat mass (kg)16.4 ± 8.626.8 ± 10.7<0.001
Total body BMC (g)3194 ± 4432693 ± 326<0.001
Total body bone area (cm2)2803 ± 2032648 ± 175<0.001
Total body BMD (mg/cm2)1137 ± 1101016 ± 86<0.001

Maternal 25OHD as a predictor of bone mass in offspring

After adjustment for season of maternal blood collection and sex and age at DXA of offspring (Model 1), maternal serum 25OHD (as a continuous variable) was a significant predictor of total body BMC in offspring at 20 years of age, with each additional 10 nmol/L of maternal 25OHD associated with an additional 37.6 (95% confidence interval [CI], 16.0–59.2) g of BMC (Fig. 2). Further adjustment for maternal factors (Model 2) and offspring factors (Model 3) attenuated the association, but it remained significant, with differences of 27.1 (95% CI, 6.0–48.3) g and 19.2 (95% CI, 5.6–32.7) g per 10 nmol/L of 25OHD, respectively. When bone area was further adjusted in the model, the difference in bone and body size–adjusted BMC was 13.8 (95% CI, 1.5–26.2) g per 10 nmol/L of 25OHD.

Figure 2.

Expected differences (mean, 95% CI) in offspring bone measures in relation to maternal 25OHD concentrations during pregnancy. Model 1 (minimal adjusted model): adjusted for season of maternal blood sample collection and offspring's sex and age at DXA; Model 2 (maternal factors further adjusted model): Model 1 plus maternal education, parity, ethnic origin, mother's height and weight before pregnancy; Model 3 (offspring factors further adjusted model): Model 2 plus offspring birth weight; and offspring height, lean mass and fat mass at 20 years. BMC = bone mineral content; BMD = bone mineral density; CI = confidence interval; 25OHD = 25-hydroxyvitamin D; DXA = dual-energy X-ray absorptiometry.

Maternal 25OHD was also significantly associated with total body BMD in the offspring, with each additional 10 nmol/L of 25OHD associated with an additional 6.7 (95% CI, 1.1–12.3) mg/cm2 of BMD in the minimally adjusted model (Model 1), 6.7 (95% CI, 0.9–12.4) mg/cm2 further adjusted for maternal factors (Model 2) and 4.6 (95% CI, 0.1–9.1) mg/cm2 further adjusted for offspring factors (Model 3). Maternal 25OHD was positively associated with total body bone area in the minimally adjusted model (Model 1), with each 10 nmol/L of 25OHD associated with an additional 17.9 (95% CI, 7.2–28.6) cm2 of bone area, but after further adjustment for maternal factors (Model 2) and offspring factors (Model 3), the association was no longer significant (Fig. 2).

When season of maternal blood collection was removed from the above models, the associations of maternal 25OHD and total body BMC and BMD only changed slightly, with each additional 10 nmol/L of maternal 25OHD associated with an additional 37.4 (95% CI, 16.6–58.2) g of BMC in Model 1, 27.9 (95% CI, 7.7–48.2) g in Model 2 and 17.9 (95% CI, 4.9–30.9) g in Model 3; and an additional total body BMD of 6.7 (95% CI, 1.4–12.1) mg/cm2 for Model 1, 6.7 (95% CI, 1.2–12.2) mg/cm2 for Model 2 and 4.3 (95% CI, 0.0–8.6) mg/cm2 for Model 3.

There were no significant associations between maternal serum 25OHD and lean and fat body mass of offspring at 20 years. The regression coefficients were 201.9 (95% CI, –374.2 to 778.0) g for fat mass and 207.8 (95% CI, –113.8 to 529.4) g for lean mass per additional 10 nmol/L of 25OHD in the model adjusted for season of maternal blood collection, and offspring's sex and age and height at DXA; and 125.1 (95% CI, –440.4 to 690.5) g for fat mass and 161.4 (95% CI, –178.3 to 501.1) g for lean mass when maternal factors and birth weight were further adjusted in the model.

Maternal vitamin D deficiency and peak bone mass in offspring

In the 132 offspring whose mothers were vitamin D–deficient (25OHD <50 nmol/L) at 18 weeks gestation, total body BMC and BMD were lower than in the offspring of women whose serum 25OHD concentrations were 50 nmol/L or higher, with significant differences in all three models (Fig. 3). In Model 1 (which accounted for seasonality, sex and age of offspring at time of DXA), maternal vitamin D deficiency was associated with 5.5% lower total body BMC (estimated mean ± SE: 2800 ± 33 versus 2954 ± 26 g, p < 0.001) and 2.4% lower total body BMD (1048 ± 8 versus 1074 ± 7 mg/cm2, p = 0.019). In Model 2, which further adjusted for maternal factors including maternal height, parity, and ethnicity, maternal vitamin D deficiency was associated with 3.7% lower total body BMC (2829 ± 32 versus 2935 ± 25 g, p = 0.012) and 2.4% lower total body BMD (1049 ± 9 versus 1074 ± 7 mg/cm2, p = 0.030). In Model 3, which further adjusted for offspring factors, maternal vitamin D deficiency was associated with 2.7% lower total body BMC (2846 ± 20 versus 2924 ± 16 g, p = 0.004) and 1.7% lower total body BMD (1053 ± 7 versus 1071 ± 5 mg/cm2, p = 0.043). Total body bone area was significantly lower in offspring of vitamin D–deficient mothers after adjustment for seasonality and sex and age at DXA of offspring (Model 1), or when both maternal and offspring factors were further adjusted in the model (Model 3), but not in the maternal factors additionally adjusted model (Model 2).

Figure 3.

Expected means (by analysis of covariance) in offspring bone measures in mothers with serum 25OHD <50 nmol/L or ≥50 nmol/L at 18 weeks pregnancy. Error bars represent SEM. Model 1 (minimal adjusted model): adjusted for season of maternal blood sample collection and offspring's sex and age at DXA; Model 2 (maternal factors further adjusted model): Model 1 plus maternal education, parity, ethnic origin, mother's height and weight before pregnancy; Model 3 (offspring factors further adjusted model): Model 2 plus offspring birth weight; and offspring height, lean mass and fat mass at 20 years. BMC = bone mineral content; BMD = bone mineral density; 25OHD = 25-hydroxyvitamin D; DXA = dual-energy X-ray absorptiometry.

Discussion

This study examines, for the first time, the relationship between vitamin D status of pregnant women and peak bone mass achieved at early adulthood in their children. After adjustment for relevant covariates, there was a significant positive association between maternal serum 25OHD and total body BMC and BMD at 20 years in offspring, and significantly lower bone mass in the offspring of women who were vitamin D–deficient during pregnancy. The difference in total body BMC associated with vitamin D deficiency was between 2.7% and 5.5% in different models, whereas the BMD difference ranged from 1.7% to 2.4%. Because a 5% difference in BMD is associated with a 20% difference in the risk of osteoporotic fracture and a 50% difference in the risk of hip fracture,[2] the magnitude of the differences observed may be clinically relevant, with implications for the fracture risk of the offspring in later life.

A possible explanation for these results is that maternal vitamin D status during pregnancy influences fetal skeletal development and mineralization. Several lines of evidence make this biologically plausible. The physiology of fetal skeletal growth and mineralization is complex, but maternal 25OHD plays a key role as a substrate for synthesis of the active form of vitamin D (1,25(OH)2-vitamin D) in maternal and fetal kidney, placenta, and probably extrarenal maternal and fetal tissues.[20] Severe maternal vitamin D deficiency causes neonatal rickets and fractures,[21] whereas mild maternal vitamin D deficiency (25OHD 25 to 50 nmol/L) is associated with changes in distal femoral morphology[22] and reduced fetal femoral volume[23] in utero as measured by ultrasound. Furthermore, in a study of 87 mother and offspring pairs, maternal serum 25OHD levels below 42.6 nmol/L in the first trimester of pregnancy were associated with lower tibial total BMC and cross-sectional bone area assessed using peripheral computed tomography (pQCT) in offspring at birth, with the difference in bone area persisting at 14 months.[24] Maternal vitamin D status may also influence fetal bone mass accrual in utero by epigenetic effects on the gene encoding the Retinoid-X Receptor-alpha, an essential cofactor in the action of 1,25 dihydroxyvitamin D.[25] It is now generally accepted that the intrauterine environment during periods of critical development has long-term influences on adult health via fetal programming,[3] and it is plausible that suboptimal bone development during intrauterine life due to mild maternal vitamin D deficiency could reduce the subsequent trajectory of bone mineral accretion during growth, thus affecting peak bone mass attained by offspring at skeletal maturity.

Our results are consistent with those of the Southampton Women's Survey of 198 mother-offspring pairs, in which maternal vitamin D insufficiency during pregnancy was associated with lower bone mass in children,[9] but not with those of the larger ALSPAC cohort (3960 pairs), in which no such association was observed.[10] The discrepancy between the three studies is not fully explained, but the studies differ in terms of timing of maternal blood sampling, age of offspring at DXA assessment, and data analysis techniques employed. In our study and the Southampton study, maternal blood samples were drawn at defined time points (18 weeks and 34 weeks gestation, respectively), whereas in ALSPAC, vitamin D status was assessed at a range of time points, and estimations made of predicted vitamin D status in late pregnancy or at standardized time points in each trimester. We think that 18 weeks gestation is an appropriate time to evaluate maternal vitamin D, as the second trimester is considered a critical period for long bone growth.[3] The importance of vitamin D status at 18 weeks gestation is supported by our finding that the positive association between maternal serum 25OHD concentration and offspring's peak bone mass remained the same when season of blood sample collection was not adjusted in the models. However, assessment during third trimester (as in the Southampton study) is also reasonable, because 80% of fetal BMC is accrued during this trimester,[26] and it is possible that maternal 25OHD has differential effects at different points during pregnancy. In both the Southampton study and ALSPAC, BMC was assessed during childhood (age 9–10 years), whereas our cohort was assessed at skeletal maturity,[11] which should reduce the influence of confounders arising from interindividual differences in rate of growth and development. Our sample size is larger than that of the Southampton Women's Survey but smaller than that of ALSPAC. Power calculations indicate that our sample size was adequate to detect clinically meaningful differences in total body bone measures between offspring of mothers with high and low vitamin D status; it therefore seems unlikely that sample size alone accounts for the different results of the three studies. It is noteworthy that the recent ALSPAC study[10] contradicted a previous study from the same cohort with a larger sample size (6995 offspring), in which estimated maternal ultraviolet B exposure in pregnancy (a surrogate measure of vitamin D) was a significant predictor of BMC in offspring at 9.9 years.[27, 28] The authors state that this discrepancy arose because age of offspring at the time of DXA scan was included as a covariate in the second but not the first study.[10] In our study, we included offspring age in all models and adjusted for similar covariates as the second ALSPAC study, but our results still differ from those of that study.

In the present study, 38.7% of pregnant women were vitamin D–deficient (25OHD <50 nmol/L), including 3.2% with 25OHD <25 nmol/L. These results are consistent with two other studies of Australian pregnant women,[29, 30] demonstrating that vitamin D deficiency is common during pregnancy despite the relative abundance of sunshine. Current Australian and U.S. guidelines do not recommend routine vitamin D screening during pregnancy.[19, 31] A recent review noted the lack of evidence from randomized trials on the effects of maternal vitamin D supplementation on offspring bone mass,[32] and therefore the results of randomized controlled trials of vitamin D supplementation during pregnancy are keenly awaited.[33]

Strengths of our study include the long-term follow-up of the birth cohort, allowing the use of peak bone mass as the outcome measure, and the comprehensive assessment of mothers and offspring, allowing adjustment for multiple potential confounders. Our study also has limitations. First, its observational nature means we cannot assume that the relationships demonstrated are causal; the associations detected could, for example, reflect shared lifestyle and nutritional factors (such as sunshine exposure, calcium nutrition and exercise), which affect both maternal vitamin D status and bone mineral accretion in offspring. Second, although we adjusted for multiple maternal and offspring-related covariates, we were not able to include every potentially relevant covariate, such as interactions with maternal obesity prior to pregnancy and weight gain during pregnancy; and calcium nutrition, physical activity, tobacco and alcohol use, and onset of puberty in the offspring. Third, only a small proportion of the original cohort was included in the analysis. However, although included participants tended to be of higher socioeconomic class than nonparticipants, detailed comparison of included mothers and offspring with those excluded did not identify significant confounders. Female offspring were somewhat overrepresented (being 60% of the study sample) but there were no significant interactions between sex and the associations of maternal vitamin D status with measures of bone mass in the offspring, suggesting this is not an important source of bias. Fourth, we measured 25OHD by immunoassay, and in recent years commercial vitamin D assays have been criticized for inaccuracy. However, the immunoassay used showed an excellent correlation with the reference method of LC-MS/MS in a subgroup of participants, and in a recent published study.[34] The results we obtained are therefore comparable to those seen in clinical practice. We selected a cutoff of 50 nmol/L to define vitamin D deficiency, as recommended by the Institute of Medicine for general use in adults,[18] but we acknowledge that the definition of optimal vitamin D status in pregnant and nonpregnant adults is controversial. Most study participants were white, and the study findings may not be applicable to other ethnic groups.

In conclusion, maternal serum 25OHD concentration at 18 weeks gestation has a positive association with peak bone mass in offspring assessed at 20 years of age, and vitamin D deficiency in pregnant women is associated with lower bone mass in their children at the time of skeletal maturity. Randomized, controlled trials of vitamin D supplementation in pregnancy are indicated to determine whether this is beneficial for skeletal development in offspring.

Disclosures

All authors state that they have no conflicts of interest.

Acknowledgments

The 20 year cohort follow-up assessment was funded by project grants from the Australian National Health and Medical Research Council and funding from the Lions Eye Institute, Nedlands, Western Australia. The Canadian Institutes of Health Research funded the DXA data collection. Core funding for the Raine Study is provided by the University of Western Australia, the Raine Medical Research Foundation, Telethon Institute for Childhood Health Research, Women and Infants Research Foundation, and Curtin University. We acknowledge with thanks the Raine Study participants and families for their participation, the Raine Study Team for cohort management and data collection, the Department of Pulmonary Physiology and Sleep Medicine, Sir Charles Gairdner Hospital for providing the DXA machine, and S. Brown for statistical advice. None of the funding agencies had any role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.

Authors' roles: Study concept and design: KZ, CP, and JPW. Acquisition of data: AJOW, PHH, MK, JM, CP, and SL. Data analysis: KZ. Data interpretation: All the authors. Drafting manuscript: KZ and JPW. Revising manuscript content: All the authors. Approving final version of manuscript: All the authors. KZ takes responsibility for the integrity of the data analysis.

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