Why the BMI Sucks

Body mass index (“BMI”) measurements have been used for some time in developing guidelines for obesity and for clinical trials. What is the BMI anyway? And is the BMI actually useful?

The BMI is defined as your weight in kilograms divided by your height in meters squared (kg/m2). This puts you into one of four categories: underweight (B.M.I. less than 18.5), normal weight (18.5 to 24.9), overweight (25.0 to 29.9) or obese (30 or greater).

But as I have said for years, there are severe problems with BMI measurements because it doesn’t take into account body composition. For example, Arnold Schwarzenegger was 6’2” and weighed 235 lbs when he won Mr Olympia giving a BMI of 30.2.

Is this man obese?

I think we can all agree that this man is not “obese”. The BMI can go the other way too. People who have very little muscle mass (this is especially true in the elderly) will have a “normal” BMI but actually be overweight or obese (“skinny fat”). I have seen this in models and suspect it is common in runway models.

As this NY Times article explains, BMI-based guidelines can lead to racial inequities in health care delivery or unnecessary stigmatization of patients who are actually healthy despite having elevated BMIs.

A far better measure of “obesity” is percent body fat (PBF) which can be assessed with specialized scales or via bone density scanning (FYI home scales measuring PBF are widely inaccurate).

In our weight loss programs, we exclusively use PBF as our “obesity” measurement. For health, women should be less than 30% body and men less than 20%. Combining the PBF with standard lab tests can give an excellent assessment of your metabolic health.