The association between smoking cessation and glycaemic control in patients with type 2 diabetes: a THIN database cohort study

Background Smoking increases the risk of developing type 2 diabetes. However, several population studies also show a higher risk in people 3-5 years after smoking cessation than in continuing smokers. After 10-12 years the risk equates to that of never-smokers. Small cohort studies suggest diabetes control deteriorates temporarily during the first year after quitting. We examined whether or not quitting smoking was associated with altered diabetes control in a population study, for how long this association persisted, and whether or not this association was mediated by weight change. Methods We did a retrospective cohort study (Jan 1, 2005, to Dec 31, 2010) of adult smokers with type 2 diabetes using The Health Improvement Network (THIN), a large UK primary care database. We developed adjusted multilevel regression models to investigate the association between a quit event, smoking abstinence duration, change in HbA(1c), and the mediating effect of weight change. Findings 10 692 adult smokers with type 2 diabetes were included. 3131 (29%) quit smoking and remained abstinent for at least 1 year. After adjustment for potential confounders, HbA(1c) increased by 0.21% (95% CI 0.17-0.25; p<0.001; [2.34 mmol/mol (95% CI 1.91-2.77)]) within the first year after quitting. HbA(1c) decreased as abstinence continued and became comparable to that of continual smokers after 3 years. This increase in HbA(1c) was not mediated by weight change. Interpretation In type 2 diabetes, smoking cessation is associated with deterioration in glycaemic control that lasts for 3 years and is unrelated to weight gain. At a population level, this temporary rise could increase microvascular complications.