Thank you for highlighting how we attach undue importance to a single number, BMI, to capture whether or not a person's weight is contributing to other health issues.

One thing that you did not mention regarding the skewing of statistics is that the underweight population is over-represented by people who have illnesses (cancer, chronic lung or kidney problems) that cause them to be underweight. We don't know what overall impact being underweight itself may have on health because so many members of this group are sickly to begin with, and are increasing overall mortality rates for this group.

Interestingly, in my dealings with doctors, I have run into a mirror image problem of fat-shaming. I've been a marathon runner and still have six pack abs at 59, but my blood pressure and cholesterol have been elevated at EVERY SINGLE MEASUREMENT for the last 35 years, and the doctors say - "Oh, you're already exercising and eating well and not smoking, so you don't have to worry about those numbers." I've tried in vain to tell them that we should be particularly concerned that I have such aberrant lab measurements despite "doing the right things".