(P44-27-26) Determinants of Serum 25-Hydroxyvitamin D Concentrations During Pregnancy in Rural Bangladesh

Objectives: To identify determinants of serum 25-hydroxyvitamin D [25(OH)D] concentrations across pregnancy in rural Bangladesh.

Methods: We analyzed data from a double-blind randomized trial (MiNDR) testing four incremental levels of 18 micronutrients, including vitamin D, among pregnant women in northwestern Bangladesh. Blood samples were collected in early (pre-supplementation, < 16 wk gestation, n=337), mid (22-28 wk, n=269), and late (~34 wk, n=261) pregnancy. Serum 25(OH)D, calcium, and parathyroid hormone (PTH) were measured at each visit. Determinants of 25(OH)D were assessed using linear mixed-effects models with participant random intercepts. Fixed effects included supplement arm, gestational age, dietary vitamin D and calcium intakes, early pregnancy vitamin D sufficiency status, BMI, season, α-1-acid glycoprotein, C-reactive protein, fasting duration, gestational weight change, age, height, parity, education, living standards index, and adherence. Covariate importance was assessed by change in marginal R² after removal of each covariate (ΔR²). Spearman correlations examined associations among 25(OH)D, calcium, and PTH across pregnancy.

Results: Mean (SD) serum 25(OH)D increased from 46.5 (11.9) nmol/L in early to 80.6 (23.6) in mid and 84.2 (30.7) in late pregnancy. Serum calcium decreased from 87.3 (4.8) to 84.1 (5.0) mg/L as did PTH from 14.0 (10.8) to 12.2 (8.9) pg/mL. The model explained 53.8% of fixed-effect and 62.6% of total variance. The strongest determinants (ΔR²) were gestational age, vitamin D status in early pregnancy, season, age and BMI, together accounting for ~16% of the explained variance. 25(OH)D was inversely correlated with PTH at all visits (r=-0.13 to -0.17, p< 0.05). Calcium was inversely correlated with PTH, with stronger correlations later in pregnancy (r=-0.18 to -0.35, p< 0.001).

Conclusions: Gestational age, early pregnancy vitamin D sufficiency status, and season were the strongest determinants of 25(OH)D levels during pregnancy after accounting for supplementation. Calcium and PTH declined across pregnancy, and their inverse correlation strengthened later in gestation, suggesting tighter calcium-PTH coupling.

Funding Sources: Gates Foundation, Seattle, WA