Body Mass Ineptitude
A number of states have already implemented body mass index, or BMI, testing in schools. When Michelle Obama launched her “Let’s Move!” campaign, she proudly noted that they were partnering with the American Academy of Pediatrics to work on BMI testing. Use of the BMI has become so commonplace that I don’t think most people know its history or question its validity. But we should.
A little background: the BMI used to be called the Quetelet Index, after the Belgian statistician who designed it – all the way back in 1832! He was interested in measuring average or relative weights of populations, and it turns out he was pretty good at it. As researchers and doctors began to draw correlations (key word here) between excessive body weight and health problems in the early twentieth century, insurance companies also developed an interest in average and desirable weights. Through much of the century, then, multiple indexes have sprung up claiming to best generalize what “healthy” weight actually is, without much agreement among professionals about which was superior.
In 1972, Dr. Ancel Keys, the same physician who conducted the Minnesota Starvation Experiment, analyzed all of these indexes of relative weight and concluded that the Quetelet Index was the most proficient at examining populations, renaming it the Body Mass Index in the process. So, if you wanted to know the relative weight of a school, a state, or a country, the BMI is a useful tool because, Dr. Keys explains, it’s results showed the least sensitivity to changes in height.
However, neither Keys nor Quetelet himself ever regarded the BMI as a tool which could give you valuable health information about individuals. Of course, on the lower end of the spectrum, the BMI has the potential to provide some meaningful information about your health. After all, only so much weight can be lost before it becomes unhealthy, and low BMI scores can help add perspective to that. But for determining if someone is “overweight”, it’s simply not very good.
To estimate actual body fat percentage of an individual, Keys endorsed something called hydrostatic weighing , which involves getting in a pool and measuring the amount of water displaced by the body. You could have two people of the same height AND weight, but with very different muscle and fat composition. The BMI wouldn’t notice because it doesn’t account for those things, but hydrostatic weighing does. Unfortunately, one of the reasons the BMI is so popular is that it’s so easy to calculate, while hydrostatic weighing is a more involved process (and certainly can’t be done online!)
You may be wondering, then: if the BMI is only good for measuring groups of people, not individuals, then why do we use it? The Center for Disease Control (CDC) ordered a report in 2007 about BMI testing in schools and identified two types of testing: surveillance and screening. Surveillance programs use the BMI for what it was designed to do – reporting on an entire student population, often anonymously, and have therefore been fairly uncontroversial. Screening programs, however, usually involve calculating individual students BMI and then reporting them to parents. Students who have a BMI over 25 receive some kind of warning that they are “at risk,” since that’s considered the range for “overweight.”
But did you know that before June 1998, the “overweight” range began at 28 and higher? That changed after a vote in Congress guided by the National Institute on Health, so that on an individual basis, millions of people who were in the “normal” range were suddenly “overweight.”
Furthermore, recently published research (read a great summary here) has confirmed what we’ve always known – that health is less about weight and more about physical activity and lifestyle. Following over 40,000 people for almost 25 years, the study assessed actual obesity rates through (you guess it!) hydrostatic weighing and then evaluated cardiovascular and respiratory health. They found that people who were considered obese but had good metabolic health were at no greater risk of mortality than their metabolically healthy but normal-weight peers. This isn’t the first study with these kinds of findings, but because they didn’t fit the common association of thinness and health, they don’t often have the impact they should.
Until the health industry as awhole changes to recognize that health can’t easily be determined through weight, we’ll continue to see things like individual BMI testing and scares over body size. The good news is, we already know health can come in all different sizes, so don’t believe everything you hear.
Matt Wetsel is an eating disorder survivor turned activist, based in Richmond, VA. He’s a regular volunteer lobbyist with the Eating Disorders Coalition and currently serves on the EDC Junior Board. You can keep up with him at http://arenomore.wordpress.com or on Twitter.