Heavy smoking raises risk for type 2 diabetes mellitus in obese men; but, light smoking reduces the risk in lean men: a follow-up study in Japan.

Purpose: To investigate interactive effects of cigarette smoking and body mass index (BMI, measured in kilograms per square meter) on the risk for type 2 diabetes mellitus (DM).
Methods: We conducted a follow-up study in 16,829 apparently healthy men 30 to 59 years of age. At baseline, 23.1%, 24.6%, 24.0%, and 28.4% of the men were 'never' smokers, ex-smokers, light smokers (<or=20 cigarettes/d), and heavy smokers (>20 cigarettes/d), respectively. BMI was expediently categorized into quartiles (Q1: <21.3; Q2: 21.3-22.9; Q3: 23.0-24.7; and Q4: >24.7 kg/m2). Incident DM was identified by a fasting serum glucose level of 7.00 mmol/L or higher or the criterion 'under treatment for DM.' The risk ratio for incident DM, hazard ratio (HR) and the 95% confidence interval (CI), according to smoking and BMI, were estimated by using Cox proportional hazard models. Age, drinking, exercise, and education were computed as confounders.
Results: During mean follow-up of 7.4 years, 869 men developed DM. In Q4 of BMI, heavy smoking raised the risk against 'never smoking' (multivariate-adjusted HR (95%CI): 1.37 [1.05-1.80]), but neither light smoking nor ex-smoking did. In Q1 of BMI, light and heavy smoking reduced the risk (multivariate-adjusted HR (95% CI): 0.45 (0.23-0.88) and 0.74 (0.41-1.33), respectively).
Conclusions: Smoking and BMI interactively influence risk for DM and the interaction is considerably complex. Heavy smoking moderately increases the risk for DM in obese men. But, light smoking reduces the risk in lean men.