OBJECTIVE: To investigate the association between vitamin D deficiency and risk 
of gestational diabetes mellitus (GDM) in pregnant Chinese women.
METHODS: A nested case-control study was conducted. Clinical and biochemical 
data were analyzed for 200 subjects with GDM and 200 subjects with normal 
glucose tolerance (NGT).
RESULTS: The median (interquartile range) serum 25-hydroxyvitamin D (25OHD) 
levels were 22.39 (17.67, 29.38) and 25.86 (19.09, 34.88) nmol/L in the GDM and 
NGT groups, respectively. Rates of 25OHD deficiency or insufficiency were 
significantly higher in the GDM group than in the NGT group. Subjects with 25OHD 
levels <25 nmol/L had a 1.8-fold higher risk of GDM compared with subjects with 
higher vitamin D levels. In the GDM group, serum 25OHD was independently 
associated with HbA1c and insulin resistance after adjusting for confounding 
factors. In the NGT group, serum 25OHD was independently associated with fasting 
plasma glucose and systolic blood pressure after adjusting for maternal age and 
other confounding factors.
CONCLUSION: 25OHD insufficiency is very common in Chinese women. Low 25OHD 
status may be associated with insulin resistance and act as a risk factor for 
GDM.