The independent and combined effects of smoking and chronic obstructive pulmonary disease on body mass index trajectories.

Low body mass index (BMI) is a common feature of severe chronic obstructive pulmonary disease (COPD) but in the general population, cigarette smoking is also associated with low body weight. Many people with COPD remain smokers after diagnosis, and it is unclear whether low BMI is because of the disease itself or its most common risk factor. We aim to assess the independent and combined effects of smoking and COPD on BMI trajectories. 27,651 patients without COPD and 25,990 with COPD from The Health Improvement Network (2005-2019) were grouped into: never-smokers, former smokers, sustained quitters, intermittent smokers, and continuous smokers (ten total COPD-smoking status groups). BMI trajectories over 10-year time horizon were modeled by these status groups using multivariable mixed-effect models adjusted for age (in continuous years), sex, Townsend score (a measure of material deprivation), alcohol consumption (yes/no), exacerbation history (yes/no, only for COPD patients) and any history of asthma, cancer, chronic kidney disease, diabetes, or cardiovascular disease (yes/no). Individuals with COPD who smoked at baseline (intermittent, sustained quitter, or continuous smokers) had a lower initial BMI (27.1 kg/m² [26.9-27.3]; 26.6 [26.4-26.9]; 26.2 [26.0-26.4], respectively) than non-COPD controls in the same smoking categories (28.0 [26.6-28.2]; 27.6 [27.2-27.9]; 26.7 [26.4-26.9]). Current smokers had lower initial BMIs than never and former smokers, regardless of COPD status. In individuals with COPD, compared to former smokers, continuous smokers lost weight faster (-0.071 kg/m²/year [-0.097 to -0.045]; p < 0.001), while quitters gained weight (0.266 [0.233 to 0.298]; p < 0.001). Non-COPD controls showed similar but less pronounced patterns when continuous smokers and quitters (-0.059 [-0.090 to -0.028] and 0.213 [0.173 to 0.254], respectively; both p < 0.001) were compared to former smokers. Those with a baseline BMI of < 30 also showed a decrease in longitudinal BMI, especially among COPD patients. COPD patients had lower baseline BMI than controls, but BMI trajectories were similar between groups, with continuous smokers losing weight faster and quitters gaining weight. These findings suggest that smoking behaviour significantly influences weight loss in COPD, emphasizing its importance in clinical evaluations and nutritional support consultations.