Effect of prenatal calcium supplementation on bone during pregnancy and 1 y postpartum

Abstract

Background: Low calcium intake during pregnancy may cause maternal skeletal calcium mobilization to meet fetal needs. The Recommended Dietary Allowance (RDA) for calcium in nonpregnant, pregnant, or lactating women aged 19-50 y is 1000 mg/d; most women in the United States report consuming 60-80% of the calcium RDA. An insufficient calcium intake could increase maternal bone loss during pregnancy and reduce bone recovery postpartum.

Objectives: The aim of this study was to determine the effect of maternal calcium supplementation on peripheral cortical and trabecular bone loss during pregnancy and bone gain postpartum.

Methods: A total of 64 women were enrolled in the study at 16 wk of gestation and randomly assigned to receive 1000 mg Ca/d or placebo for the remainder of the pregnancy. Measurements were performed at 16, 26, and 36 wk of pregnancy and at 4 and 12 mo postpartum for serum 25-hydroxyvitamin D and markers of bone turnover. Trabecular and cortical bone mineral density (BMD) and content were assessed at the tibia and radius by peripheral quantitative computed tomography.

Results: Mean ± SD daily calcium intake at baseline was 733 ± 350 mg; only 25% of the women met the RDA. Thirty women (47% of those enrolled) remained in the study at 12 mo postpartum. After controlling for baseline bone value, serum 25-hydroxyvitamin D concentrations, length of breastfeeding, and body mass index, the calcium group had significantly greater increases in radial total BMD (P = 0.02) and tibial cortical BMD (P = 0.03) at 12 mo postpartum than the placebo group. Trabecular and total BMD at the tibia trended toward higher values (P < 0.06) in the calcium group than in the placebo group in the same models.

Conclusions: These data show that supplemental calcium provided during pregnancy may improve bone recovery postpartum in women consuming a typical US diet. A larger study is warranted to solidify the conclusions. This trial was registered at clinicaltrials.gov as NCT01145573.

FIGURE 1

Flowchart of subject recruitment, enrollment, and retention from early pregnancy to 12 mo postpartum.

FIGURE 2

Total calcium intake (dietary and prenatal supplementation) during pregnancy in subjects provided placebo or calcium supplementation. In each box plot, the line represents the median, the outside edges of each box represent the IQR, hash marks represent the 5–95% range, and any values outside are represented by single points. The line in the graph represents the Recommended Dietary Allowance for calcium intake for pregnant women aged 19–50 y. Calcium intake (milligrams per day) was estimated by validated FFQ. Total intake = calcium by FFQ + prenatal supplemental intake (200 mg/d). These estimates do not include the calcium intervention supplement (1000 mg/d) provided to those in the calcium intervention group. No significant difference in total calcium intake was observed by group at any time point. Baseline visit: calcium, n = 32; placebo, n = 32; visit 2: calcium, n = 28; placebo, n = 29; visit 3: calcium, n = 26; placebo, n = 27. FFQ, food-frequency questionnaire.

FIGURE 3

Serum 25(OH)D status from baseline to 12 mo postpartum. In each box plot, the line represents the median, the outside edges of each box represent the IQR, hash marks represent the 5–95% range, and any values outside are represented by single points. 25(OH)D decreased by 17.7% from baseline to 36 wk of gestation and increased slightly postpartum, although 12-mo-postpartum vitamin D concentrations never returned to baseline concentrations (P < 0.001, repeated-measures ANOVA). These changes did not differ by supplementation group. Baseline visit: calcium, n = 32; placebo, n = 32; visit 2: calcium, n = 28; placebo, n = 29; visit 3: calcium, n = 26; placebo, n = 27; visit 4: calcium, n = 22; placebo, n = 23; visit 5: calcium, n = 15; placebo, n = 15. 25(OH)D, 25-hydroxyvitamin D.