Cross-sectional study on the relationship between body mass index and smoking, and longitudinal changes in body mass index in relation to change in smoking status: The Tromsø Study

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Abstract
Aims: To evaluate the effects of smoking and other lifestyle factors on body mass index (BMI), and changes in BMI in relation to changes in smoking status. Methods: A cross-sectional study was performed on 10,920 males (3937 smokers) and 12,090 females (4343 smokers) who participated in the fourth Tromsø Study (performed in 1994—95). A longitudinal study was performed on 2364 males (732 smokers in 1994—95) and 2738 females (942 smokers in 1994—95) who participated in both the fourth and the fifth Tromsø studies (performed in 2001). Results: In the cross-sectional study, current smokers of both genders had a lower BMI (25.0±3.4 vs. 25.5±3.2 kg/m2 in males, and 23.9±3.9 vs. 25.3±4.6 kg/m 2 in females, p<0.01), a lower degree of physical activity, and a higher consumption of coffee and alcohol than never-smokers. We found a U-shaped relationship between number of cigarettes smoked per day and BMI, with the lowest BMI in those smoking 6— 10 cigarettes per day. Heavy smokers and never-smokers had similar BMI. In the longitudinal study, continuing smokers had a smaller increase in BMI than those who gave up smoking. In those who gave up smoking, there was a significant, positive relationship between number of cigarettes smoked in 1994—95 and increase in BMI. Conclusions: There is a U-shaped relationship between number of cigarettes smoked per day and BMI. Smoking cessation is associated with an increase in weight as compared to those who continue smoking.