Vitamin D status of pregnant women with obesity in the United Kingdom and its association with pregnancy outcomes: a secondary analysis of the UPBEAT study.

Prenatal vitamin D deficiency is widely reported and may affect perinatal outcomes. In this secondary analysis of the UK Pregnancies Better Eating and Activity Trial (UPBEAT), we examined vitamin D status and its relationship with selected pregnancy outcomes in women with obesity (BMI≥30kg/m2 ) from multi-ethnic inner-city settings in the UK. Determinants of vitamin D status at a mean of 17±1 weeks' gestation were assessed using multivariable linear regression and reported as percent differences in serum hydroxyvitamin D (25(OH)D). Associations between 25(OH)D and clinical outcomes were examined using logistic regression. Among 1089 participants, 67% had 25(OH)D <50nmol/L and 26% had concentrations <25nmol/L. In fully adjusted models accounting for socio-demographic and anthropometric characteristics, 25(OH)D was lower among women of Black (% difference = -33; 95%CI: -39 to -27), Asian (% difference= -43; 95%CI: -51 to -35) and other non-White (% difference= -26; 95%CI: -35 to -14) ethnicity compared to women of White ethnicity ( n =1086; P <0.001 for all). In unadjusted analysis, risk of gestational diabetes was greater in women with 25(OH)D <25nmol/L compared to ≥50nmol/L (OR=1.58; 95%CI: 1.09 to 2.31), but the magnitude of effect estimates was attenuated in the multivariable model (OR=1.33; 95%CI: 0.88 to 2.00). There were no associations between 25(OH)D and risk of preeclampsia, preterm birth, or SGA or LGA delivery. These findings demonstrate low 25(OH)D among pregnant women with obesity and highlight ethnic disparities in vitamin D status in the UK. However, evidence for a greater risk of adverse perinatal outcomes among women with vitamin D deficiency was limited.