It is well documented that BMI consistently underestimates rates of obesity for large samples of population  Not only that, but with the current state of healthcare in the US, I'm guessing that there are probably significant numbers of overweight or obese Americans who simply don't go to the doctor, and therefore they wouldn't show up in the data. Or am I wrong about that? Would the fat 20somethings and fat 30somethings who still possess some "youthful vitality" and aren't feeling unhealthy enough to want to seek medical treatment (meaning that their medical records/data would not get reported) show up in the data set?