To examine the association of cigarette smoking with the risk of type 2 diabetes and to find out whether the association is modified by obesity and physical activity.
A prospective study comprising 41,372 men and women aged 25--64 years without a history of diabetes, coronary heart disease or stroke at baseline. Data on incident cases of diabetes were ascertained through the nationwide Drug Register and the Hospital Discharge Register. During the mean follow-up of 21 years 2770 subjects were diagnosed with type 2 diabetes. The Cox proportional hazards model was used to estimate the effect of smoking and other factors on the risk of type 2 diabetes.
Smoking had a graded association with the risk type 2 diabetes, and it remained significant after controlling for age and major risk factors. The multifactorial-adjusted (age, study year, education, body mass index (BMI), systolic blood pressure, physical activity and coffee and alcohol drinking) hazard ratio was 1.22 [95% confidence interval (CI) 1.04--1.43] amongst men smoking less than 20 cigarettes per day and 1.57 (95% CI 1.34--1.84) amongst men smoking 20 cigarettes per day or more. In women the corresponding hazard ratios were 1.46 (95% CI 1.21--1.76) and 1.87 (95% CI 1.36--2.59) respectively. Smoking increased the risk of type 2 diabetes at all levels of BMI and physical activity.
Smoking is a risk factor for type 2 diabetes independently of BMI and physical activity. Prevention of smoking should be encouraged as a part of efforts to reduce the risk of type 2 diabetes, and it will result in other health benefits, too.