Lower vitamin D levels at first trimester are associated with higher risk of developing gestational diabetes mellitus

The progressive increase of insulin resistance observed in pregnancy contributes to the pathophysiology of gestational diabetes mellitus (GDM). There is controversy whether vitamin D deficiency contributes to abnormal glycemic regulation in pregnancy. We tested the associations between first trimester 25-hydroxyvitamin D (25OHD) levels and: 1) the risk of developing GDM; 2) insulin resistance/sensitivity, beta cell function and compensation indices in a large population-based prospective cohort of pregnant women. Participants (n = 655) were seen at first (6–13 weeks) and second (24–28 weeks) trimesters for blood samples. At first trimester, 25OHD levels were measured. At second trimester, glucose and insulin were measured 3 times during the oral glucose tolerance test to estimate insulin resistance (HOMA-IR), beta cell function (HOMA-B), insulin sensitivity (Matsuda index), insulin secretion (AUC_ins/gluc) and beta cell compensation (ISSI-2). Based on IADPSG criteria, 54 participants (8.2 %) developed GDM. Lower first trimester 25OHD levels were associated with higher risk of developing GDM even after adjustment for vitamin D confounding factors and GDM risk factors (OR = 1.48 per decrease of one SD in 25OHD levels; P = 0.04). Lower first trimester 25OHD levels were associated with higher HOMA-IR (r = − 0.08; P = 0.03), lower Matsuda index (r = 0.13; P = 0.001) and lower ISSI-2 (r = 0.08; P = 0.04). After adjustment for confounders, we found no significant association with HOMA-B and AUC_ins/gluc. Our results suggest that low levels of 25OHD at first trimester are (1) an independent risk factor for developing GDM and (2) associated with insulin resistance at second trimester.