The obesity paradox is the idea that people who are overweight live longer than normal-weight people. While later studies have found this claim invalid, the notion stayed in public discourse ever since.
There are many explanations for this odd observation. One of them goes with the parameter of measurement itself – the survival rate after getting cardiovascular disease. Studies found that obese people may get the disease much earlier in life and therefore survive a longer proportion of life with it.
Another one is collider stratification bias, which happens when two variables, e.g., risk factor and outcome, influence a third, namely, the likelihood of being sampled. It works in the following way:
Obese individuals may have developed CAD because they are obese or because of another stronger condition, e.g., smoking or genetics. In other words, CAD, the collider, is caused by 1) obesity and 2) the (more severe) condition (smoking). In this simple two-cause model, a stratification of variables means among individuals with CAD, obese individuals are less likely to be smokers, and non-obese individuals are more likely to be smokers. Subsequently, obesity may appear protective against mortality (outcome) because its presence indicates the absence of a more harmful risk factor – smoking.
References
The ‘obesity paradox’ may not be a paradox at all: International Journal of Obesity
Obesity is bad regardless of the obesity paradox for hypertension and heart disease: J Clin Hypertens
Association of Body Mass Index With Lifetime Risk of Cardiovascular Disease and Compression of Morbidity: JAMA Cardiology
