In such a case, actually, that might make it hard to use the studies we’ve looked at so far to gain information. If the curve is U-shaped, the two ends of the curve may cancel out when averaged together and disguise the effect.
Note that this does not seem to be what has happened at a population level in the US. BMI seems to have increased pretty much at all levels — even the 0.5th percentile has increased from NHANES I (in the early 70′s) to the 2017-March 2020 NHANES, as has the minimum adult BMI. And the difference is not subtle.
For instance, here are the thinnest people in the last NHANES versus the first one:
Note that this does not seem to be what has happened at a population level in the US. BMI seems to have increased pretty much at all levels — even the 0.5th percentile has increased from NHANES I (in the early 70′s) to the 2017-March 2020 NHANES, as has the minimum adult BMI. And the difference is not subtle.
For instance, here are the thinnest people in the last NHANES versus the first one:
The relevant Google Colab cells start here.