Selection bias: A missing factor in the Obesity Paradox debate

The September issue of Obesity featured articles by Tobias and Hu (1) and Flegal and Kalantar-Zadeh (2) that explored the observation that, in clinical populations, such as individuals with heart failure, chronic kidney disease, or diabetes, those with higher BMI often have lower mortality rates than leaner individuals. The articles disagree whether this phenomenon, known as the obesity paradox, is a true causal effect. Flegal and KalantarZadeh assert that the research on the obesity paradox is consistent with greater BMI conferring “modest survival advantages” (2). Tobias and Hu disagree, arguing that the obesity paradox is likely an “artifact of methodological limitations” (1).