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

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/m] and 50 % overweight/obese [BMI≥25 kg/m]), 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 free25OHDtotal25OHDx100. 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.