Estimation of the maternal vitamin D intake that maintains circulating 25-hydroxyvitamin D in late gestation at a concentration sufficient to keep umbilical cord sera ≥25-30 nmol/L: a dose-response, double-blind, randomized placebo-controlled trial in pregnant women at northern latitude

Abstract

Background: In the absence of dose-response data, Dietary Reference Values for vitamin D in nonpregnant adults are extended to pregnancy.

Objective: The aim was to estimate vitamin D intake needed to maintain maternal 25-hydroxyvitamin D [25(OH)D] in late gestation at a concentration sufficient to prevent newborn 25(OH)D <25-30 nmol/L, a threshold indicative of increased risk of nutritional rickets.

Design: We conducted a 3-arm, dose-response, double-blind, randomized placebo-controlled trial in Cork, Ireland (51.9oN). A total of 144 white-skinned pregnant women were assigned to receive 0, 10 (400 IU), or 20 (800 IU) µg vitamin D3/d from ≤18 wk of gestation. Vitamin D metabolites at 14, 24, and 36 wk of gestation and in cord sera, including 25(OH)D3, 3-epi-25(OH)D3, 24,25(OH)2D3, and 25(OH)D2 were quantified by liquid chromatography-tandem mass spectrometry. A curvilinear regression model predicted the total vitamin D intake (from diet and antenatal supplements plus treatment dose) that maintained maternal 25(OH)D in late gestation at a concentration sufficient to maintain cord 25(OH)D at ≥25-30 nmol/L.

Results: Mean ± SD baseline 25(OH)D was 54.9 ± 10.7 nmol/L. Total vitamin D intakes at the study endpoint (36 wk of gestation) were 12.1 ± 8.0, 21.9 ± 5.3, and 33.7 ± 5.1 µg/d in the placebo and 10-µg and 20-µg vitamin D3 groups, respectively; and 25(OH)D was 24.3 ± 5.8 and 29.2 ± 5.6 nmol/L higher in the 10- and 20-µg groups, respectively, compared with placebo (P < 0.001). For maternal 25(OH)D concentrations ≥50 nmol/L, 95% of cord sera were ≥30 nmol/L and 99% were >25 nmol/L. The estimated vitamin D intake required to maintain serum 25(OH)D at ≥50 nmol/L in 97.5% of women was 28.9 µg/d.

Conclusions: Thirty micrograms of vitamin D per day safely maintained serum 25(OH)D concentrations at ≥50 nmol/L in almost all white-skinned women during pregnancy at a northern latitude, which kept 25(OH)D at >25 nmol/L in 99% and ≥30 nmol/L in 95% of umbilical cord sera. This trial was registered at www.clinicaltrials.gov as NCT02506439.

FIGURE 1

CONSORT flow diagram of participant enrollment, random assignment, and biochemical analysis throughout the study by treatment group, where n is based on the total number of samples available for each visit. Any participant who did not provide a blood sample at baseline was included in the descriptive and biochemical analysis at later time points but excluded from the dose-response analysis, whereas women missing a midpoint sample only were included in both the dose-response analysis and the analysis at endpoint, if a blood sample was collected at this time point. The numbers of women who provided both a baseline and ≥1 follow-up sample (midpoint or endpoint) were 43, 42, and 43 for the placebo and 10- and 20-µg groups, respectively, which left a final number of 128 for the dose-response analysis. CONSORT, Consolidated Standards of Reporting Trials; inclu., including.

FIGURE 2

Mean maternal serum 25(OH)D concentrations achieved at each time point by intervention group. n = 40, 35, and 43 for the placebo and 10- and 20-µg groups, respectively, in which the analysis includes women who provided a blood sample at all 3 time points. Total mean ± SD vitamin D intakes (including habitual diet and supplementation plus treatment dose) were 12.1 ± 8.0, 21.9 ± 5.3, and 33.7 ± 5.1 µg/d in the placebo and 10- and 20-µg groups, respectively. Mean gestational age = 14, 24, and 36 wk at baseline, midpoint, and endpoint, respectively. Differences in mean serum 25(OH)D within groups from baseline to endpoint were all significantly different (P < 0.01 in all cases, paired-samples t test for each group). 25(OH)D, 25-hydroxyvitamin D.

FIGURE 3

The relation between achieved serum 25(OH)D concentrations and total vitamin D intake in pregnant women living at 51.9°N, assessed by using a curvilinear regression model. The mean response is indicated by the central line, and the outer lines are its 95% prediction intervals; n = 128. Horizontal lines represent serum 25(OH)D thresholds of 25, 30, and 50 nmol/L, respectively. 25(OH)D, 25-hydroxyvitamin D.

FIGURE 4

Association of maternal 25(OH)D at 36 wk of gestation and umbilical cord 25(OH)D concentrations (linear trendline: y = 0.4391x + 5.6922; R² = 0.62; n = 96; r = 0.79, P < 0.001) (A). The vertical line represents maternal 25(OH)D at 50 nmol/L, and the horizontal lines represent cord 25(OH)D at 25 and 30 nmol/L. When maternal 25(OH)D concentration was ≥50 nmol/L, cord concentrations were ≥25 nmol/L in all but 1 and ≥30 nmol/L in 95% of newborns. Association of maternal 24,25(OH)₂D₃ at 36 wk of gestation and umbilical cord 24,25(OH)₂D₃ (power fit trendline: y = 0.9715x^0.6905; R² = 0.68; n = 95; r = 0.80, P < 0.001) (B). Association of the ratio of maternal 25(OH)D₃ to 24,25(OH)₂D₃ at 36 wk of gestation and the ratio of umbilical cord 25(OH)D₃ to 24,25(OH)₂D₃ (power fit trendline: y = 2.9938x^0.5346; R² = 0.33; n = 95; r = 0.55, P < 0.001) (C). 25(OH)D, 25-hydroxyvitamin D; 25(OH)D₃, 25-hydroxyvitamin D₃; 24,25(OH)²D₃, 24,25-dihydroxyvitamin D₃.

FIGURE 5

Association of maternal 25(OH)D₃ and 24,25(OH)₂D₃ concentrations at 36 wk of gestation (power fit trendline: y = 0.0162x^1.3309; R² = 0.80; n = 121; r = 0.86, P < 0.001) (A); association of maternal 25(OH)D₃ concentrations and the ratio of 25(OH)D₃ to 24,25(OH)₂D₃ at 36 wk of gestation (power fit trendline: y = 61.575x^−0.331; R² = 0.20; n = 121; r = –0.48, P < 0.001) (B). 25(OH)D₃, 25-hydroxyvitamin D₃; 24,25(OH)²D₃, 24,25-dihydroxyvitamin D₃.

FIGURE 6

Association of umbilical cord 25(OH)D₃ and 24,25(OH)₂D₃ concentrations (power fit trendline: y = 0.0794x^1.0036; R² = 0.71; n = 95; r = 0.82, P < 0.001) (A); association of the ratio of umbilical cord 25(OH)D₃ concentrations and the ratio of 25(OH)D₃ to 24,25(OH)₂D₃ (linear trendline: y = –0.0353x + 14.473; R² = 0.02; n = 95; r = –0.14, P = 0.173) (B). 25(OH)D₃, 25-hydroxycholecalciferol; 24,25(OH)²D₃, 24,25-dihydroxycholecalciferol.

FIGURE 7

Association of maternal vitamin D intake and 24,25(OH)₂D₃ concentrations (power fit trendline: y = 1.604x^0.4247; R² = 0.20; n = 121; r = 0.401, P < 0.001) (A); association of maternal vitamin D intake and the ratio of 25(OH)D₃ to 24,25(OH)₂D₃ at 36 wk of gestation (power fit trendline: y = 21.431x^−0.135; R² = 0.08; n = 121; r = −0.26, P = 0.004) (B). 25(OH)D₃, 25-hydroxycholecalciferol; 24,25(OH)²D₃, 24,25-dihydroxycholecalciferol.