Time for large randomised trials of vitamin D for women with gestational diabetes mellitus to improve perinatal health outcomes.

Gestational diabetes mellitus (GDM) affects 7-14% of pregnancies in the USA and its incidence is rising. Several small trials have demonstrated improved insulin sensitivity and glucose tolerance with vitamin D supplementation among women with GDM. In this issue of Diabetologia (DOI: 10.1007/s00125-014-3293-x ), Asemi and colleagues report that co-supplementation of vitamin D and calcium among women with diet-controlled GDM improves several biomarkers of metabolic status, including insulin, glucose and cholesterol. Whether such improvements in biomarkers would translate into clinically meaningful reductions in the harmful sequelae of GDM remains unknown. The promising results from Asemi and colleagues justify investment in large randomised, double-blind, placebo-controlled trials of vitamin D (with or without calcium) for women with GDM. Such trials should be powered for perinatal outcomes such as Caesarean section for macrosomia, postnatal hypoglycaemia and respiratory distress; ideally, the trials would include long-term follow-up of mothers and infants for later metabolic consequences of GDM.