BMI-mortality association: shape independent of smoking status but different for chronic lung disease and lung cancer.

Background: Besides smoking, low or high body mass index (BMI) is associated with chronic lung disease (CLD). It is unclear how CLD is associated with BMI, whether smoking interacts with this association, and how the associations differ from the patterns known for lung cancer. Population and Methods: Our population comprised 35,212 individuals aged 14-99, who participated in population-based surveys conducted in 1977-1993 in Switzerland (mortality follow-up until 2014). We categorized smokers into never, former, light, and heavy; and BMI into underweight, normal weight, overweight, and obese. Hazard ratios (HRs) were obtained with multivariable Cox proportional hazards models. Results: CLD mortality was strongly associated with being underweight. This was mainly due to the effect in men (HR 5.04 [2.63-9.66]) and also prevailed in never smokers (HR 1.81 [1.11-3.00]). Obesity was also associated with CLD mortality (HR men: 1.37 [1.01-1.86], women: 1.39 [0.90-2.17]), but not with lung cancer mortality. In line with lung cancer, for CLD, the BMI-mortality association followed the same shape in all smoking categories, suggesting that this association was largely independent of smoking status. Conclusion: The shape of the BMI-mortality association was inversely linear for lung cancer but followed a U-shape for CLD. Further research should examine the potentially protective effect of obesity on lung cancer occurrence and the possibly hazardous impact of underweight on CLD development.