Upregulation of Placental Vitamin D Receptor Expression in Gestational Diabetes Is Not Directly Related to Vitamin D Concentration

Vitamin D plays a key role in pregnancy beyond calcium–phosphate regulation, modulating immune responses and glucose metabolism via the vitamin D receptor (VDR). Placental expression may be altered in gestational diabetes mellitus (GDM). This study aimed to assess placental VDR expression in GDM and evaluate its association with maternal vitamin D levels and clinical parameters. VDR expression in the placental tissue of 53 women with GDM and 26 healthy controls was assessed semi-quantitatively by immunohistochemistry. Maternal serum 25-hydroxyvitamin D (25(OH)D) levels, body mass index, weight gain, neonatal outcomes, and other variables were evaluated. Univariate and multivariate linear regression analyses were performed. VDR expression was significantly higher in the GDM group compared to controls (p = 0.0297 for mean, p = 0.0378 for median). No significant differences were observed in serum 25(OH)D concentrations between groups. Stepwise regression revealed that diabetes was the only independent predictor of VDR expression. Within the GDM subgroup, VDR expression was not associated with any clinical parameters, including maternal vitamin D status. Upregulated placental VDR in GDM may represent an adaptive response to metabolic stress. These findings suggest complex regulation of vitamin D signalling in diabetic pregnancies, warranting further investigation.