title: Does selection bias explain the obesity paradox among individuals with cardiovascular disease?
abstract: OBJECTIVE
  The objectives of this article are to demonstrate that the obesity paradox may be explained by collider stratification bias and to estimate the biasing effects of unmeasured common causes of cardiovascular disease (CVD) and mortality on the observed obesity-mortality relationship.
  
  
  METHODS
  We use directed acyclic graphs, regression modeling, and sensitivity analyses to explore whether the observed protective effect of obesity among individuals with CVD can be plausibly attributed to selection bias. Data from the third National Health and Examination Survey was used for the analyses.
  
  
  RESULTS
  The adjusted total effect of obesity on mortality was a risk difference (RD) of 0.03 (95% confidence interval [CI]: 0.02, 0.05). However, the controlled direct effect of obesity on mortality among individuals without CVD was RD = 0.03 (95% CI: 0.01, 0.05) and RD = -0.12 (95% CI: -0.20, -0.04) among individuals with CVD. The adjusted total effect estimate demonstrates an increased number of deaths among obese individuals relative to nonobese counterparts, whereas the controlled direct effect shows a paradoxical decrease in morality among obese individuals with CVD.
  
  
  CONCLUSIONS
  Sensitivity analysis demonstrates unmeasured confounding of the mediator-outcome relationship provides a sufficient explanation for the observed protective effect of obesity on mortality among individuals with CVD.
authors: Department of Epidemiology, Biostatistics, and Occupational Health, McGill University, Montreal, QC, Canada. Electronic address: hailey.banack@mail.mcgill.ca.
authors: Hailey R
authors: HR
authors: Banack
authors: Department of Epidemiology, Biostatistics, and Occupational Health, McGill University, Montreal, QC, Canada.
authors: Jay S
authors: JS
authors: Kaufman
mesh_terms: D000328:Adult; D017677:Age Distribution; D015992:Body Mass Index; D002318:Cardiovascular Diseases / Q000175:diagnosis / Q000453:epidemiology* / Q000401:mortality; D002423:Cause of Death; D015897:Comorbidity; D003430:Cross-Sectional Studies; D016208:Databases, Factual; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009749:Nutrition Surveys; D009765:Obesity / Q000175:diagnosis / Q000453:epidemiology* / Q000401:mortality; D015995:Prevalence; D015983:Selection Bias; D012680:Sensitivity and Specificity; D012720:Severity of Illness Index; D017678:Sex Distribution; D016019:Survival Analysis
keywords: Cardiovascular disease; Obesity; Selection bias; Sensitivity analysis