Vitamin D Status and Gestational Diabetes: Effect of Smoking Status during Pregnancy.

BACKGROUND: Vitamin D status, as measured by serum 25-hydroxyvitamin D (25(OH)D), has been shown in some studies to be inversely associated with gestational diabetes risk. Recently, it has been suggested that maternal smoking status may modify this relationship. We explored the association between 25(OH)D concentration and gestational diabetes and determined if there was an interaction between smoking and 25(OH)D.

METHODS: A nested case-control study was conducted in Halifax, Nova Scotia and Quebec City, Quebec. Women were recruited before 20 weeks gestation and 25(OH)D was measured. Cases were women who developed gestational diabetes and controls were frequency matched to cases on study site, gestational age at blood draw, and season and year of blood draw. Logistic regression models estimated adjusted odds ratios (aOR) and 95% confidence intervals (CI). Models were tested for multiplicative and additive interaction, which was estimated by relative excess risk due to interaction (RERI).

RESULTS: The study included 395 gestational diabetes cases and 1925 controls. Women who smoked during pregnancy and had 25(OH)D concentrations <30 nmol/L had an aOR = 3.73 [95% CI 1.95, 7.14] compared to non-smokers with 25(OH)D concentrations ≥50 nmol/L. Additive interaction was detected between smoking status and 25(OH)D [RERI = 2.44, 95% CI 0.03, 4.85].

CONCLUSION: Our study supports the inverse association of vitamin D status with gestational diabetes risk, particularly among women who smoke during pregnancy. More research is needed to confirm this finding and, if confirmed, to determine the mechanism by which the combined effect of smoking and low vitamin D status increases the risk of developing gestational diabetes.