Which features of smoking determine mortality risk in former cigarette smokers with diabetes?

OBJECTIVE - The effects of quitting smoking on mortality risk in individuals with diabetes is unknown and may differ from the benefits observed in the general population. We therefore determined the mortality risks in ex-smokers with diabetes, compared with subjects who have never smoked, by the number of years since quitting, the number of cigarettes smoked, and the number of years of smoking. RESEARCH DESIGN AND METHODS - An international cohort study of 4,427 individuals with diabetes was studied. Baseline examinations were performed in 1975-1977 when smoking habits were determined by questionnaire. Mortality follow-up continued until 1988. RESULTS - All-cause mortality risks were higher for recent quitters (1-9 years; relative risk [RR], 1.53 [95% CI 1.19-1.97]; P = 0.001) than for those who quit earlier (greater than or equal to 10 years; RR, 1.25 [95% CI 1.03-1.52]; P = 0.02), compared with subjects who have never smoked. These risks were highest in those who had smoked the longest (greater than or equal to 30 years: RR, 1.66 [95% CI 1.22-2.26]; P = 0.001; vs. 1-9 years: RR, 1.17 [95% CI 0.85-1.60]; P = 0.3). Risks were also highest in those who had smoked the most and least number of cigarettes. Adjustment for key confounders, which included a previous history of heart disease, proteinuria, and blood pressure, did not materially affect these relationships. CONCLUSIONS - Quitting smoking does reduce mortality risk in ex-smokers with diabetes, but risks remain high several years after quitting and are highly dependent on thr duration of smoking. Thus, individuals with diabetes who smoke should be encouraged to quit as soon as possible in the course of disease.