Impact of smoking and excess body weight on overall and site-specific cancer mortality risk

Background: Smoking and excess body weight are major preventable risk factors for premature death. This study aimed at analyzing their single and combined association with site specific cancer mortality. Methods: Our study population comprised 35,784 men and women of ages 14 to 99 years, who participated in population based health surveys conducted 1977-1993 in Switzerland and were followed up for mortality until 2008. Multivariable Cox proportional hazards models were calculated for different cancer sites, and population attributable fractions were derived. Results: The hazard ratio of dying from cancer (all sites) was 2.32 (95% confidence interval, 2.04-2.63) for heavy smokers (vs. never smokers) and 1.15 (1.01-1.32) for obese [body mass index (BMI) ≥ 30 kg/m²] vs. normal weight individuals. Heavy smoking (≥20 cigarettes/day) was associated with increased mortality due to cancer of the lung, upper aero-digestive tract, pancreas, bladder, liver, and the total of remaining sites. Obesity was associated with higher risk of dying from cancer of the liver and the female genital tract (essentially corpus or cervix uteri and ovary). More than 20% of all cancer deaths in our population were attributable to ever smoking and overweight (BMI ≥ 25 kg/m²). Conclusions: Smoking was a much stronger risk factor for cancer than excess body weight. For lung, liver, and pancreatic cancer, the combination of excess body weight and smoking lead to cumulated higher risks. Impact: Our findings support recommendations for obese persons to quit smoking despite potential postcessation weight gain.