The obesity paradox paradox

by John Quiggin on June 19, 2013

I see lots of stories made up of handwringing over the “obesity paradox”, normally presented as saying that even though obesity is a risk factor for all kinds of diseases, obese people appear to have lower mortality than others. A typical finding is the one reported here

being overweight or slightly obese was linked to about a 6 percent lower risk of dying, compared to people considered “normal weight. Being severely obese, however, was still tied to an almost 30 percent higher risk of death.

People are tying themselves in knots over this, but it doesn’t seem to me that there is any paradox to be explained. The obvious reading of the data is that the Body Mass Index.[^1] ranges used for the various categories (20-25 Normal, 25-30 Overweight etc) were set a bit too low when they were originally estimated, or rather, guessed. From my quick look at the data, if you bumped the ranges up by a couple of points, the paradox would disappear. People at the bottom of the current normal range, who tend to have high mortality, would be classed as underweight, while those currently classed as slightly overweight would be reclassified as normal, and so on.

Am I missing something?

[^1]: This point is logically separate from the general problems of the BMI, regarding muscle mass and so on.