Choosing definitions: BMI vs. DXA (or, Why BMI Is Not a Lie)

There is a great post over on Kaiser Fung’s blog on the BMI/DXA debate. I suggest you go read it.

What it is about is this — there’s a claim to be made that BMI doesn’t measure fat accurately, and a number of people are saying BMI should be replaced by this other measure (DXA) which measures fat accurately.

But why do we care about fat to begin with? Well, it turns out we care about it because high BMI is associated with all sorts of negative outcomes. We have decades of decent research showing this.

BMI is the predictor here, not fat. And it’s true that there are probably some ways to increase BMI that don’t decrease health, or decrease BMI in ways that puts your health at risk. If you’re a person that’s figured out a way to cheat the BMI metric, through starvation dieting or the like, if you managed to get down your BMI by letting your muscles deteriorate, then maybe DXA will keep you honest. Or maybe you should just alter your method of achieving lower BMI.

Outside of personal decision — once we look at this from a population perspective — DXA makes even less sense. If the BMI association holds across the population as a whole, and the association is strong, then that is what you want to target from a public policy perspective*. The edge cases are already accounted for in that calculation. There are people that smoke all their life and don’t get cancer for genetic and environmental reasons we do not yet comprehend, but reducing smoking in the population as a whole has saved hundreds of thousands of lives. Do we need a better predictor than smoking — or do we just need to reduce smoking?

There are cases, of course, where changes in metrics have made sense. Cholesterol measurements now look at the ratio of LDL to HDL. But the reason they do that is not because “that’s what cholesterol really is”. The reason they do that is because the ratio has a been demonstrated as having a much stronger relationship to heart health than LDL alone or some other metric.

This isn’t the case with BMI vs. DXA. Again, we have pretty good evidence on the BMI front of a strong relation:

That’s a pretty damning plot that has withstood decades of research. It’s a robust finding in a world of fads.

In the DXA corner we have nothing. We don’t know that it predicts mortality one iota better than BMI.

In 10 or 20 years, if DXA can significantly improve on that prediction, stand the test of time, and come out on top, then it might be worth a look. Until then, it’s just silliness.

* There’s actually a pretty decent argument being made right now that BMI is not as manipulable a risk as something like smoking, and that the focus should be on long term interventions that stick, like those for cardiovascular fitness, instead of those that tend not to stick, like weight loss. And it can be argued that CVF predicts mortality even better than obesity. I’m interested in all of this — but it doesn’t change the fact the measure you need to make these arguments with is probably BMI.

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