McCormack Speaks

I Am Not a Cardboard Cutout! The Bias of Body Mass Index (BMI)


by Thomas Nee, McCormack Graduate School public administration student

image of measuring your waist lineAn insidious movement is discriminating against people like me. So far, only my life insurance rates are materially affected but this movement is so pervasive and accepted that it’s only a matter of time before it affects my relationship with my doctor, my health insurance provider, and my government. These groups increasingly rely on Body Mass Index (BMI) to categorize me. This simple formula combines height and weight into a single index. It is quickly applied to large populations and simplifies research across space and time. I am not writing about the common observation that BMI does not measure fat vs. muscle but about something I cannot change: my height.

BMI = m / h2

Where m is mass in kilograms and h is height in meters (multiply by a conversion factor of 703 if mass is in pounds and height is in inches). The formula has a fundamental flaw: the exponent 2.  It is applicable only if we are two-dimensional beings!  It allows us to grow in height and width but not depth. I am not a flat, cardboard cutout.

The consequences to researchers are numerous. BMI is higher for taller individuals than shorter ones with the same body fat ratio. Children’s BMIs increase as they grow, even while maintaining the same body fat ratio. Young children with normal body weight have artificially low BMI. The average man is taller than the average woman. Average height in the United States is higher than it was 100 years ago and higher than in other countries to which we are compared today. BMI is biased against taller people.

I am former offensive lineman, 6’2.5” tall, and have always carried a little extra weight. I know I am overweight but I don’t think I should be categorized as obese. At 57, I am still spry enough to jog a little (when I can find time away from my course workload to warm up and baby my Achilles tendons).  I have a 6’6” friend in excellent shape who is categorized as overweight at his “Skeletor” weight of 225 lbs.

Professor of Numerical Analysis Nick Trefethen, University of Oxford, called the BMI formula “a bizarre measure” and wrote, “millions of short people think they are thinner than they are, and millions of tall people think they are fatter.”  Changing the exponent to 3 seems logical but humans don’t scale exactly in all three dimensions. Trefethen suggested an exponent of 2.5. A statistical study of US population data showed the true exponent should be about 2.6 

Changing the BMI exponent to 2.6 and changing the conversion factor is trivial with today’s spreadsheets and other computer tools.  The only debate is selecting a base height where the old and new formulas yield the same BMI. Some committee should decide what is the average adult height in the world and use that. The only barrier is inertia; people are used to the flawed formula. An exponent of 2.6 would lead to improved research comparing populations across time, geography, and demographics, and end BMI discrimination against taller people. I challenge academia to make this change on its own but BMI is so entrenched a change is more likely to come from Congress.

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