Association between body mass index and mortality in hospitalised patients with community-acquired pneumonia

The obesity paradox postulates that increased body mass index (BMI) is protective in certain patient populations. We aimed to investigate the association of BMI and different weight classes with outcomes in hospitalised patients with community-acquired pneumonia (CAP). This cohort study is a secondary data analysis of the University of Louisville Pneumonia Study database, a prospective study of hospitalised adult patients with CAP from June, 2014, to May, 2016, in Louisville, KY, USA. BMI as a predictor was assessed both as a continuous and categorical variable. Patients were categorised as weight classes based on World Health Organization definitions: BMI of <18.5 kg·m-2 (underweight), BMI of 18.5 to <25 kg·m-2 (normal weight), BMI of 25.0 to <30 kg·m-2 (overweight), BMI of 30 to <35 kg·m-2 (obesity class I), BMI of 35 to <40 kg·m-2 (obesity class II), and BMI of ≥40 kg·m-2 (obesity class III). Study outcomes, including time to clinical stability, length of stay, clinical failure and mortality, were assessed in hospital, at 30 days, at 6 months and at 1 year. Clinical failure was defined as the need for noninvasive ventilation, invasive ventilation or vasopressors within 1 week of admission. Patient characteristics and crude outcomes were stratified by BMI categories, and generalised additive binomial regression models were performed to analyse the impact of BMI as a continuous variable on study outcomes adjusting for possible confounding variables. 7449 patients were included in the study. Median time to clinical stability was 2 days for every BMI group. There was no association between BMI as a continuous predictor and length of stay <5 days (chi-squared=1.83, estimated degrees of freedom (EDF)=2.74, p=0.608). Clinical failure was highest in the class III obesity group, and higher BMI as a continuous predictor was associated with higher odds of clinical failure. BMI as a continuous predictor was significantly associated with 30-day (chi-squared=39.97, EDF=3.07, p<0.001), 6-month (chi-squared=89.42, EDF=3.44, p<0.001) and 1-year (chi-squared=83.97, EDF=2.89, p<0.001) mortalities. BMI ≤24.14 kg·m-2 was a risk factor whereas BMI ≥26.97 kg·m-2 was protective for mortality at 1-year. The incremental benefit of increasing BMI plateaued at 35 kg·m-2. We found a protective benefit of obesity on mortality in CAP patients. However, we uniquely demonstrate that the association between BMI and mortality is not linear, and no incremental benefit of increasing BMI levels is observed in those with obesity classes II and III.