Research has identified reduced circulating 25-hydroxyvitamin D [25(OH)D] in individuals with the rs7041 (c.1296T>G) T allele in the vitamin D binding protein gene ( GC); however, the effects of the T allele on vitamin D biomarkers during pregnancy and lactation are unknown. Thus, we examined the metabolic effects of GC rs7041 on vitamin D biomarkers among third-trimester pregnant ( n = 26), lactating ( n = 28), and nonpregnant/nonlactating ( n = 21) women consuming a single amount of vitamin D (511 IU/d) and related nutrients for 10-12 wk. T allele carriers had less circulating 25(OH)D, regardless of reproductive state [thymine-thymine (TT): 80% of guanine-guanine (GG), P = 0.05; guanine-thymine (GT): 85% of GG, P = 0.1]. Among pregnant women, the T allele attenuated the expected increase in vitamin D binding protein (DBP). Specifically, although GG pregnant women exhibited greater DBP (216%, P < 0.0001) than did GG nonpregnant women, that difference was lessened among GT women, and TT pregnant women did not exhibit greater DBP than TT nonpregnant women. Furthermore, TT pregnant women had greater placental 25(OH)D3 to 24,25-dihydroxyvitamin D ratios (251% of GG, P = 0.07) and less osteocalcin, a bone formation marker, in the cord blood of their neonates (24% of GT, P = 0.02). Overall, the GC rs7041 genotype modified the effects of pregnancy on maternal and placental vitamin D metabolism, with possible functional consequences for fetal bone development and infant health.-Ganz, A. B., Park, H., Malysheva, O. V., Caudill, M. A. Vitamin D binding protein rs7041 genotype alters vitamin D metabolism in pregnant women.

Background: Little is known about placental vitamin D metabolism and its impact on maternal circulating vitamin D concentrations in humans.Objective: This study sought to advance the current understanding of placental vitamin D metabolism and its role in modulating maternal circulating vitamin D metabolites during pregnancy.Design: Nested within a feeding study, 24 healthy pregnant women (26-29 wk of gestation) consumed a single amount of vitamin D (511 IU/d from diet and a cholecalciferol supplement) for 10 wk. Concentrations of placental and blood vitamin D metabolites and placental messenger RNA (mRNA) abundance of vitamin D metabolic pathway components were quantified. In addition, cultured human trophoblasts were incubated with 13C-cholecalciferol to examine the intracellular generation and secretion of vitamin D metabolites along with the regulation of target genes.Results: In placental tissue, 25-hydroxyvitamin D3 [25(OH)D3] was strongly correlated (r = 0.83, P < 0.001) with 24,25-dihydroxyvitamin D3 Moreover, these placental metabolites were strongly correlated (r ≤ 0.85, P ≤ 0.04) with their respective metabolites in maternal circulation. Positive associations (P ≤ 0.045) were also observed between placental mRNA abundance of vitamin D metabolic components and circulating vitamin D metabolites [i.e., LDL-related protein 2 (LRP2, also known as megalin) with 25(OH)D3 and the C3 epimer of 25(OH)D3 [3-epi-25(OH)D3]; cubilin (CUBN) with 25(OH)D3; 25-hydroxylase (CYP2R1) with 3-epi-25(OH)D3; 24-hydroxylase (CYP24A1) with 25(OH)D3, 3-epi-25(OH)D3, and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]; and 1α-hydroxylase [(CYP27B1) with 3-epi-25(OH)D3 and 1,25(OH)2D3]. Notably, in vitro experiments with trophoblasts showed increased production and secretion of 25(OH)D3 and higher CYP24A1 gene transcript abundance in response to cholecalciferol treatment.Conclusions: The numerous associations of many of the placental biomarkers of vitamin D metabolism with circulating vitamin D metabolites among pregnant women [including a CYP27B1-associated increase in 1,25(OH)2D3] and the evidence of trophoblast production and secretion of vitamin D metabolites, especially 25(OH)D3, suggest that the placenta may play an active role in modulating the vitamin D metabolite profile in maternal circulation in human pregnancy. This trial was registered at clinicaltrials.gov as NCT03051867.

BACKGROUND:The impact of the reproductive state on vitamin D metabolism and requirements is uncertain in part because of a lack of studies with controlled dietary intakes of vitamin D and related nutrients.OBJECTIVE:We aimed to quantify the impact of the reproductive state on a panel of vitamin D biomarkers among women of childbearing age consuming equivalent amounts of vitamin D and related nutrients.METHODS:Nested within a feeding study providing 2 doses of choline, healthy pregnant (26-29 wk gestation; n = 26), lactating (5 wk postpartum; n = 28), and control (nonpregnant/nonlactating; n = 21) women consumed a single amount of vitamin D (511 ± 48 IU/d: 311 ± 48 IU/d from diet and 200 IU/d as supplemental cholecalciferol) and related nutrients (1.6 ± 0.4 g Ca/d and 1.9 ± 0.3 g P/d) for 10 wk. Vitamin D biomarkers were measured in blood obtained at baseline and study end, and differences in biomarker response among the reproductive groups were assessed with linear mixed models adjusted for influential covariates (e.g., body mass index, season, race/ethnicity).RESULTS:At study end, pregnant women had higher (P < 0.01) circulating concentrations of 25-hydroxyvitamin D [25(OH)D; 30%], 1,25-dihydroxyvitamin D [1,25(OH)2D; 80%], vitamin D binding protein (67%), and C3 epimer of 25(OH)D3 (100%) than control women. Pregnant women also had higher (P ≤ 0.04) ratios of 25(OH)D to 24,25-dihydroxyvitamin D [24,25(OH)2D; 40%] and 1,25(OH)2D to 25(OH)D (50%) than control women. In contrast, no differences (P ≥ 0.15) in vitamin D biomarkers were detected between the lactating and control groups. Notably, the study vitamin D dose of 511 IU/d achieved vitamin D adequacy in most participants (95%) regardless of their reproductive state.CONCLUSIONS:The higher concentrations of vitamin D biomarkers among pregnant women than among control women suggest that metabolic adaptations, likely involving the placenta, transpire to enhance vitamin D supply during pregnancy. The study findings also support the adequacy of the current vitamin D RDA of 600 IU for achieving serum 25(OH)D concentrations ≥50 nmol/L among women differing in their reproductive state. This trial was registered at clinicaltrials.gov as NCT01127022.

This 10‐wk feeding study sought to advance understanding of the role of the placenta in maternal vitamin D status. To achieve this aim, we assessed associations between placental vitamin D metabolites [25‐hydroxyvitamin D3 (25[OH]D3), 24,25‐dihydroxyvitamin D3 (24,25[OH]2D3)], placental expression of vitamin D‐related genes [25‐hydroxylase (CYP2R1), 1a‐hydroxylase (CYP27B1), 24‐hydroxylase (CYP24A1)] and maternal blood vitamin D metabolites [25(OH)D3, 24,25(OH)2D3, 1,25‐dihydroxyvitamin D (1,25[OH]2D)] among healthy third‐trimester pregnant women (n=26). Participants consumed equivalent and rigorously controlled vitamin D (515 IU/d) and calcium intakes (1445 mg/d) throughout the third trimester, and blood samples were obtained at study‐baseline (beginning of third trimester) and study‐end (term), while placenta tissues were collected at delivery. Blood and placental vitamin D metabolites were measured by LC‐MS/MS, and placental gene transcript abundance was quantified by qRT‐PCR with Taqman probes. In Pearson correlation analyses, 25(OH)D3 and 24,25(OH)2D3 concentrations showed a strong positive correlation in placenta [r=0.83, P<0.001], and in maternal blood [r=0.89, P<0.001 at baseline; r=0.84, P<0.001 at study‐end]. Remarkably, placental 25(OH)D3 concentrations correlated positively with maternal serum 25(OH)D3 levels [r=0.63, P<0.001 at baseline; r=0.85, P<0.001 at study‐end], and also with maternal circulating 24,25(OH)2D3 levels [r=0.63, P=0.001 at baseline; r=0.81, P<0.001 at study‐end]. Similarly, placental 24,25(OH)2D3 concentrations were positively correlated with maternal serum 25(OH)D3 [r=0.45, P=0.03 at baseline; r=0.68, P<0.001 at study‐end] and maternal blood 24,25(OH)2D3 levels [r=0.52, P=0.01 at baseline; r=0.67, P<0.001 at study‐end]. Moreover, positive associations were observed between 25(OH)D3 and 1,25(OH)2D concentrations in maternal circulation [r=0.47, P=0.02 at baseline; r=0.36, P=0.068 at study‐end], and circulating 1,25(OH)2D and 24,25(OH)2D levels were correlated only at baseline [r=0.44, P=0.02]. Of note, in multivariate‐adjusted regression models, baseline concentrations of maternal serum 25(OH)D3 correlated positively with both placental CYP27B1 [R2=0.86 ; β=0.007; P=0.04] and CYP24A1 expression [R2 = 0.70; β=0.012; P=0.01]. In addition, maternal serum 25(OH)D at study‐end and placental CYP2R1 expression showed an inverse relationship with a borderline significance [R2=0.92; β=−0.006; P=0.06], and placental CYP24A1 expression positively correlated with 1,25(OH)2D levels at both study‐time points [R2=0.70; β=0.0022; P=0.01 at baseline; R2=0.67; β=0.0017; P=0.03 at study‐end]. To the best our knowledge, this is the first study to quantify vitamin D metabolites in human placenta tissue. The strong correlations between maternal circulating vitamin D metabolites and placental metabolic and genomic readouts suggest that the placenta may modulate the vitamin D metabolite profile in the maternal compartment.Support or Funding InformationGERBER FOUNDATION 1843‐3882, NIH R03 HD080824‐01A1

Vitamin D plays a central role in calcium homeostasis; however, its relationship with bone turnover during pregnancy remains unclear due to a lack of studies that have rigorously controlled for vitamin D and other nutrients known to influence bone metabolism. Similarly, prior investigations of the effect of pregnancy on bone turnover relative to the nonpregnant state may have been confounded by varying intakes of these nutrients. Nested within a controlled intake study, the present investigation sought to quantify associations between maternal vitamin D biomarkers and biochemical markers of bone turnover among pregnant (versus nonpregnant) women and their fetuses under conditions of equivalent and adequate intakes of vitamin D and related nutrients. Changes in markers of bone turnover across the third trimester were also examined. Healthy pregnant (26-29 wk gestation; n=26) and nonpregnant (n=21) women consumed 511IU vitamin D/d, 1.6g calcium/d, and 1.9g phosphorus/d for 10weeks while participating in a controlled feeding study featuring two choline doses. Based on linear mixed models adjusted for influential covariates (e.g., BMI, ethnicity, and season), pregnant women had 50-150% higher (P<0.001) concentrations of bone resorption markers than nonpregnant women. Among pregnant women, increases in maternal 25(OH)D across the study period were associated (P<0.020) with lower osteocalcin and deoxypyridinoline at study-end, and higher fetal osteocalcin. In addition, maternal free 25(OH)D, 1,25(OH)2D and 24,25(OH)2D tended to be negatively associated (P≤0.063) with maternal NTx at study-end, and maternal free 25(OH)D and 24,25(OH)2D were positively associated (P≤0.021) with fetal CTx. Similarly, maternal 3-epi-25(OH)D3 was negatively related (P≤0.037) to maternal NTx and deoxypyridinoline at study-end. These declines in bone resorption markers resulting from higher vitamin D biomarker concentrations among pregnant women coincided with increases in their albumin-corrected serum calcium concentrations, indicating that calcium transfer to the fetus was uncompromised. Notably, none of these associations achieved statistical significance among nonpregnant women. Overall, our study findings suggest that achieving higher maternal concentrations of vitamin D biomarkers might attenuate third-trimester bone resorption while ensuring sufficient calcium delivery to the fetus.

As part of a 10‐wk controlled feeding study, we investigated the impact of reproductive state on circulating levels of vitamin D metabolites among pregnant (n=26, 27wk gestation), lactating (n=28, 5wk postpartum), and nonpregnant (n=21) women consuming a daily intake of 300 IU vitamin D derived from food (90IU/d) and a prenatal supplement (200IU/d). At study‐end, serum 25‐hydroxyvitamin D (25[OH]D, mean±SD nmol/L) was higher in pregnant women (98±32) than lactating (81±19, P =0.044) and nonpregnant (78±25, P =0.032) women. Similar results were achieved after controlling for confounding factors such as season in a multivariate model. Notably, 300 IU (half of the current RDA) achieved 25(OH)D levels above the RDA target value of 50nmol/L in most of study participants (92%). Study‐end plasma 1,25‐dihydroxyvitamin D (mean [95% CI] pmol/L) was ~2 times higher in pregnant women (303 [252‐364]) than lactating (139 [116‐167], P <.0001) and nonpregnant (163 [139‐191], P <.0001) women. Similarly, study‐end vitamin D binding protein levels (mean [95% CI] mcg/mL) were ~1.8 times higher in pregnant women (370 [291‐470]) than lactating (161 [131‐198], P <.0001) and nonpregnant (205[166‐255], P =0.001) women. In sum, pregnancy dramatically influenced circulating vitamin D metabolites under controlled intakes of vitamin D and related nutrients (e.g., calcium and phosphorus). Grant Funding Source : USDA, AEB‐ENC and the Beef Checkoff