Methodological issues in observational studies of obesity and mortality

Obesity has important health hazards, and the epidemic seems to be growing in developed countries. There is scientific evidence for higher risk of earlier death among the obese. However, most evidence of the effects of obesity on mortality comes from observational studies. The aim of this manuscript is to review some of the most important issues in designing, analyzing and interpreting analytic studies of the effects of obesity on mortality. Key issues are clarity in the definition of the effect under study, confounding, measurement error and a phenomenon sometimes termed reverse causality in which obesity causes some diseases, but some of the diseases also affect adiposity and mortality