Smoking affects the oral glucose tolerance test profile and the relationship between glucose and HbA1c in gestational diabetes mellitus

Aims Current smokers in the general population have a lower 2 h plasma glucose after an oral glucose tolerance test (OGTT) and a higher HbA1c than non‐smokers, but the relationships between OGTT/HbA1c and smoking status have not been addressed in pregnancy. We analysed glycaemic measurements in women with gestational diabetes mellitus in relation to smoking status. Methods We performed a review of the prospectively collected database of the diabetes and pregnancy clinic. We included women with gestational diabetes mellitus and a singleton pregnancy who delivered between 1986 and 2006. Bivariate and multivariate analyses were used to evaluate patient characteristics in relation to smoking status. Results A total of 2361 women met the inclusion criteria: 556 (23.5%) were active smokers, 266 (11.3%) quit during pregnancy and 1539 (65.2%) were non‐smokers. Most baseline characteristics were similar across groups. Diagnostic OGTT was performed at a gestational age of [median (25th, 75th centiles)] 29 weeks (26, 33). Women who smoked at the beginning of pregnancy had a higher 1‐h plasma glucose than non‐smokers [11.8 (11, 12.7), 11.6 (11, 12.6) and 11.5 (10.8, 12.5) mmol/l, for active smokers, those who quit during pregnancy and non‐smokers, respectively, P < 0.001] and a lower 3‐h plasma glucose [7.3 (5.9, 8.4), 7.6 (6.4, 8.7) and 8.0 (6.8, 9.0) mmol/l, respectively, P < 0.001]. HbA1c was higher in women who smoked at the beginning of pregnancy. Multiple regression analysis confirmed the independent association of smoking status with HbA1c and OGTT plasma glucose. Conclusions In women with gestational diabetes mellitus who smoke at the beginning of pregnancy, the shape of the OGTT is consistent with accelerated glucose absorption, and HbA1c is higher than expected for glycaemic values. What's new? Women with gestational diabetes mellitus who smoke at the beginning of pregnancy had an oral glucose tolerance test consistent with accelerated glucose absorption and a disproportionately higher HbA1c. In these women, 1 h plasma glucose and HbA1c overestimate glucose exposure.