Original articleDoes selection bias explain the obesity paradox among individuals with cardiovascular disease?
Section snippets
Graphical representation of the obesity paradox
The directed acyclic graph presented in Figure 1 depicts the hypothesized relations between obesity, CVD, and all-cause mortality. CVD is known as an intermediate (or mediator) variable, as it is on the causal path from obesity to mortality [25], [26]. Let C represent a vector of measured covariates that confound the relationship between obesity and mortality (e.g., age, gender). For the purpose of this article, we will use cardiorespiratory fitness (CRF) as an example of a common cause of CVD
Data source
The obesity-mortality relationship was analyzed using data from the third National Health and Nutrition Examination Survey (NHANES III; 1988–1994), a nationally representative survey of noninstitutionalized civilians in the United States [39]. The study population is recruited through a stratified multistage probability sampling procedure, designed to represent the entire U.S. population [39]. NHANES III participants completed a standardized in-home interview and questionnaire as well as a
Sensitivity analysis
To explore the possibility that the protective effect of obesity on mortality among individuals with CVD is because of unmeasured confounding of the CVD-mortality relationship by CRF (Fig. 1), we conducted sensitivity analyses for the CDE [18], [47]. If Yobese,cvd is independent of Obese|[C, CRF] and Yobese,cvd is independent of CVD|[Obese, C, CRF] for all levels of obesity and CVD, the magnitude of bias in the CDE is given by:where γ is equal to E[mortality|obese,
Acknowledgment
The authors acknowledge Rebecca Anthopolos for her help in creating the graphics for this article.
Hailey Banack was supported by a doctoral research award from the Fonds de la Recherche en Sante du Quebec and a CIHR Institute of Circulatory and Respiratory Health Skills Development Award. Jay Kaufman was supported by the Canada Research Chair program.
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