Alcohol, Smoking, and Their Synergy as Risk Factors for Incident Type 2 Diabetes

Introduction: Smoking has been independently related to an increased risk of Type 2 diabetes, whereas the role of alcohol remains controversial. The joint impact of tobacco and alcohol use on Type 2 diabetes risk is understudied. This study investigated individual and combined effects of smoking and alcohol on Type 2 diabetes risk. Methods: Data from 110,076 participants in the NutriNet-Santé cohort (2009–2023), who were free from Type 2 diabetes at baseline and with alcohol and smoking data, were analyzed. Multivariable Cox regression models assessed the association of alcohol consumption (<2 vs ≥2 portions/day, <10 vs ≥10 portions/week, grams/day of ethanol) and smoking (never versus former/current smoking) with Type 2 diabetes risk. Combined effects of heavy smoking (≥20 cigarettes/day) and heavy drinking (>8 and >15 portions/week for women and men, respectively) were also evaluated. Results: Over 7.5 years of follow-up (820,470 person-years), 1,175 Type 2 diabetes cases were identified. Alcohol consumption, including heavy intake, was not significantly associated with Type 2 diabetes risk. People who formerly or currently smoke had a 36% higher risk of Type 2 diabetes than people who have never smoked (hazard ratio=1.36; 95% CI=1.20, 1.53). Those who smoked heavily had over twice the risk of those who smoked lightly or moderately (hazard ratio=2.10; 95% CI=1.46, 3.02). Combined exposure to smoking and heavy alcohol use did not significantly increase Type 2 diabetes risk (hazard ratio=1.11; 95% CI=0.95, 1.29). Conclusions: These findings support smoking as an independent risk factor for Type 2 diabetes and show that alcohol consumption did not confer protection. The combined effect of alcohol and tobacco use on Type 2 diabetes risk and the mechanisms behind this relationship should be further explored. Trial registration: This trial is registered at NCT03335644 at ClinicalTrials.gov.