The obesity paradox paradox (crossposted at Crooked Timber)

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.

Am I missing something?

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

26 thoughts on “The obesity paradox paradox (crossposted at Crooked Timber)

  1. The big problem with the BMI is that it works 2 dimensionally whereas the body is in three dimensions, which essentially means as you get taller you must get thinner to stay within the BMI range. For example someone 1.5m tall with a weight of 56kg has a BMI of 25, now if you were to scale that person up to 2m leaving all the same proportions they would weigh in at 133kg (they’re 1.3 times taller, 1.3 times wider and 1.3 times thicker, or 2.37 times the volume)which puts them into the obese category at a BMI of 33.3. If they wanted to maintain a BMI of 25 they’d have to lose 33kg.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s