Avoiding maternal vitamin D deficiency may lower blood glucose in pregnancy

Background Vitamin D status is hypothesised to play a role in gestational glucose control. No studies to date have examined vitamin D in relation to changes in blood glucose in pregnancy. Thus, the aim was to examine if vitamin D in early pregnancy and vitamin D trajectory associate with blood glucose trajectory over pregnancy in a Swedish cohort. We also investigated the relation between maternal vitamin D status and excessive fetal growth. Methods In 2013–2014, pregnant women were recruited to the GraviD cohort study when registering at the antenatal clinics in south-west Sweden. In the present analysis, 1928 women were included. Women with preexisting diabetes and multifetal pregnancy were excluded. Random blood glucose was assessed according to routine practice, in first trimester (T1, gestational week 4–16), second trimester (T2, gestational week 17–27), early (T3a, gestational week 28–35) and late third trimester (T3b, gestational week 36–41). In T1 and T3a, serum 25-hydroxyvitamim D (25OHD) was analyzed by liquid chromatography tandem mass spectrometry. Large for gestational age (LGA), as a proxy of excessive fetal growth, was defined as body weight at birth above 2 standard deviations of the gender specific population mean. Adjusted linear regression, linear mixed models analysis and logistic regression analysis were used to study 25OHD in relation to T1 blood glucose, glucose trajectory and LGA, respectively. Results Mean blood glucose increased during pregnancy (5.21 mmol/L in T1, 5.27 mmol/L in T2, 5.31 mmol/L in T3a and 5.34 mmol/L in T3b; p = 0.003). In T1, 25OHD was negatively associated with blood glucose, i.e. 25OHD ≥ 30 nmol/L was associated with 0.25-0.35 mmol/L lower glucose. T1 25OHD was also negatively associated with blood glucose trajectory. Higher T3 25OHD was associated with higher odds of LGA (p = 0.032). Conclusion Avoiding maternal vitamin D deficiency in early pregnancy is associated with lower blood glucose in early pregnancy and throughout pregnancy. Higher 25OHD in late pregnancy was associated with higher odds of LGA at birth. Keywords: Hyperglycaemia; Large for gestational age; 25-hydroxyvitamin