Longitudinal Analysis of Diabetes Mellitus Risk: Smoking Status and Smoking Cessation

Background/Objectives: Smoking cessation is acknowledged for its health benefits. However, it paradoxically increases diabetes mellitus (DM) risk shortly after quitting due to weight gain. This research aimed to investigate how smoking status could affect the development of DM, focusing on how the risk of acquiring diabetes changed over time after quitting smoking, independent of variables such as weight gain. Methods: The data of 386,558 participants of the Kangbuk Samsung Health Study, excluding those with pre-existing DM, were examined. Smoking status and its long-term effects on DM risk were assessed using multivariate Cox proportional hazards models. Lifestyle factors, including weight change, physical activity levels, and alcohol intake, were adjusted as time-varying covariates throughout the follow-up period. Results: Modified hazard ratios (HRs) indicated no notable disparity in DM risk between individuals who previously smoked and those who had never smoked (HR: 1.04, 95% CI: 0.999–1.08, p-value < 0.001). In contrast, current smokers exhibited a significantly increased DM risk (HR: 1.29, 95% CI: 1.24–1.35, p-value < 0.001). Within the first six years post-cessation, former smokers initially faced a higher DM risk than never smokers (0–2 years, HR: 1.22, 95% CI: 1.15–1.31, p-value < 0.001; 3–5 years, HR: 1.11, 95% CI: 1.04–1.20, p-value < 0.001). After 12 years, they realigned with never smokers (12–46 years, HR: 0.92, 95% CI: 0.86–0.98, p-value = 0.002). Current smokers consistently showed a higher DM risk (0–9 years, HR: 1.29, 95% CI: 1.14–1.46, p-value < 0.001). Adjusting for covariates such as weight change and physical activity did not alter these findings. Conclusions: Our results indicated that former smokers initially experienced an elevated risk of DM relative to never smokers. This increased risk aligned with the risk of never smokers after six years, and the risk continued to improve after 12 years compared to never smokers. This contrasted with current smokers, who maintained a heightened risk of DM, even when adjustments were made for weight change, physical activity, and alcohol intake as time-varying covariates.