Changes in vitamin D biomarkers across pregnancy and by maternal BMI: A secondary analysis of data and biospecimens from the National Children’s Study

Abstract

Obesity during pregnancy is associated with increased risk for vitamin D deficiency in both the mother and offspring. Free or unbound 25-hydroxyvitamin D (25(OH)D) is the biologically active form as compared to total 25(OH)D, which may be important in the assessment of vitamin D status during conditions like pregnancy where vitamin D binding protein (VDBP) is affected. Little is known about how pre-pregnancy BMI affects changes in vitamin D markers during pregnancy. This is a secondary analysis of data and biospecimens from the 2009–2014 National Children’s Study (NCS). The current analysis included 50 participants (50 % normal weight [18.5 ≥BMI<25 kg/m²] and 50 % overweight/obese [BMI≥25 kg/m²]), recruited across four US Census regions (Northeast, South, West, and Midwest), with serum samples available from three time points during pregnancy: 1st half (<25 weeks), 2nd half (>25 weeks) and birth. Total 25(OH)D was quantified via liquid chromatography-tandem mass spectrometry (LC-MS/MS), and commercially available assays were used to measure free 25(OH)D and VDBP. Percent free 25(OH)D was calculated as $\frac{\mathrm{free}25\left(\mathrm{OH}\right)D}{\mathrm{total}25\left(\mathrm{OH}\right)D}x100$. Linear mixed effect models, including quadratic gestational age term were employed to exam the change in vitamin D metabolites by quadratic gestational age, as well as interactions with pre-pregnancy BMI and season of birth. A positive linear trend was observed for total 25(OH)D levels across gestation (p = 0.002), while quadratic relationships were observed for both VDBP (p < 0.001) and % free 25(OH)D (p = 0.001). A significant interaction was observed between gestational age and season of birth for total 25(OH)D (p < 0.001) and free 25(OH)D (p = 0.006). Furthermore, the interactive effect of gestational age and pre-pregnancy BMI was statistically significant for both total 25(OH)D (p = 0.002) and % free 25(OH)D (p = 0.007). Our results suggest that among a sample of US women both season of birth and maternal pre-pregnancy BMI affected changes in vitamin D metabolites across pregnancy. The effects of maternal BMI on changes in total 25(OH)D and % free 25(OH)D across pregnancy suggest maternal obesity may differentially affect vitamin D metabolism in pregnancy. Future research is necessary to compare differences in vitamin D metabolism among obesity affected pregnancies as compared to healthy-weight counterparts.