[rich_sasha]

I'm guessing BMI is not really a considered obesity metric, but rather a good approximation which any GP can calculate in seconds.

For example, tall people tend to have higher BMI, as if 2 is not the right power to raise height to. But you barely need a 4-op calculator to calculate that. Good luck calculating BMI if the power were 2.18 or whatever gives best fit.

Measuring body fat is not exactly hard but much less straightforward and a touch unpleasant. If you're running a large study, it's much easier to measure BMI than anything else. At population level, I'd imagine correlation to obesity is easily 80-90%.

I guess it is also easier for doctors to say "your BMI is too high" rather than "you're too rounded".