| Devlin's Angle | ||
Since there was a discussion about BMI, I am posting this rather long article from the WSJ, that I saved on my computer many years ago.
May 2009
Do You Believe in Fairies, Unicorns, or the BMI?
Take a look at the guy in the photo. According to the Centers for Disease Control and Prevention, he is overweight. They base this classification on a number called the body mass index, or BMI. Also overweight, according to this CDC endorsed metric, are athletes and movie stars Kobe Bryant, George Clooney, Matt Damon, Johnny Depp, Brad Pitt, Will Smith, and Denzel Washington. Tom Cruise scored even worse, being classified as downright obese, as was Arnold Shwarzenegger when he was a world champion body-builder. With definitions like that, no wonder Americans think of themselves as having an overweightness epidemic. (Using the CDC's BMI measure, 66 percent of adults in the United States are considered overweight or obese.)
Yes, it's that time of year again, when I go for my annual physical. I know the routine. My body mass index regularly comes out at around 25.1, putting me just into the "overweight category," and the doctor sends me a fact sheet telling me I need to lose weight, exercise more, and watch my diet. Notwithstanding that fact that the person he has just examined has a waist of 32 inches, rides a bicycle in the California mountains between 120 and 160 miles a week, competes regularly in competitive bicycle events up to 120 miles, does regular upper-body work, has a resting pulse of 59 beats per minute, blood pressure generally below 120/80, healthy cholesterol levels, and eats so much broccoli I would not be surprised to wake up one morning to find it sprouting out of my ears. (Yes, that's me in the - recent - photo. No, I'm not a "fitness junkie". And I am certainly not a professional athlete. I'm just a fairly ordinary guy who was lucky to be born with good genes and who likes being outdoors on my bike when the weather is nice, and I have a competitive streak that makes me want to race every now and then. A not atypical Californian academic, in fact.)
Why do we have this annual BMI charade? Why would otherwise well-educated medical professionals ignore the evidence of their own eyes? Because the BMI is one of those all-powerful magic entities: a number. And not just any number, but one that is generated by a mathematical formula. So it has to be taken seriously, right?
Sadly, despite that fact that completion of a calculus course is a necessary prerequisite for entry into medical school, the medical profession often seems no less susceptible than the general population to a misplaced faith in anything that looks mathematical, and at times displays unbelievable naivety when it comes to numbers.
(Actually, my own physician is smarter than that. I chose him because he is every bit as compulsive an outdoorsy, activities person as I am, and he seems to know that the BMI routine we go through is meaningless, though the system apparently requires that he play along and send me the "You need to lose weight and exercise more" letter, despite our having spent a substantial part of the consultation discussing our respective outdoors activities.)
So what is the BMI? A quick web search on "BMI" or "body mass index" will return hundreds of sites, many of which offer calculators to determine your BMI. All you do is feed in your height and your weight, and out comes that magic number. Many of the sites also give you a helpful guide so you can interpret the results. For instance, the CDC website gives these ranges:
below 18.5 = Underweight
18.5 to 24.9 = Ideal
25.0 to 29.9 = Overweight
30.0 and above = Obese
(Tom Cruise, with a height of 5'7" and weight of 201 lbs, has a body mass index of 31.5, while the younger Schwarzenegger, at just over six feet tall and about 235 pounds, had a BMI over 31. The figures I quote for athletes and movie stars are from data available on the web, and I believe they are accurate, or were when the information was entered.)
Some sites even tell you how this mystical number is calculated:
BMI = weight in pounds/(height in inches x height in inches) x 703
Hmmm. No mention of waist-size here? Or rump? That's odd. Isn't the amount of body fat you carry related to the size belt you need to wear or how baggy is the seat of the jeans the belt holds up?
And what about the stuff inside the body? One thing all those "overweight" and "obese" athletes and movie stars have in common is that they have very little fat and a lot of muscle, and possibly also stronger, healthier bones. Now, a quick web-search reveals that mean density figures for these three body component materials are: fat 0.9 gm/ml, muscle 1.06 gm/ml, and bone 1.85. In other words, the less fat you have, and the more your body weight is made up of muscle and bone, the greater the numerator in that formula, and the higher your BMI. In other words, if you are a fit, healthy individual with little body fat but strong bones and lots of muscle, the CDC (and other medical authorities) will classify you as overweight. Note the absurdity of the whole approach. If I actually did take my physician's BMI-triggered, form-letter advice and exercise more, I would put on even more muscle and lose even more of what little body fat I have, and my BMI would increase! With a medical profession like that, who needs high cholesterol as an enemy?
Admittedly, those same authorities also say that a male waistline of 40 inches and a female waistline of 35 inches are where "overweight" begins. But this of course is totally inconsistent with their claim that the BMI is a reliable indicator of excess body fat. In contrast, it is consistent with my observation that it is the density of the stuff inside the body that is key, not the body weight. If you ignore that wide variation in densities, then of course you will end up classifying people with 32 inch waists as overweight. Yet this blatant inconsistency does not seem to cause anyone to pause and ask if there is not something just a little odd going on here. Isn't it time to inject some science into this part of medical practice?
Time to take a look at that BMI formula and ask where it came from. I've already noted that it ignores waistline, rump-size, and the different densities of fat, muscle, and bone. Next question: Why does it mysteriously square the height? What possible scientific reason could there be to square someone's height for heaven's sake? (Multiplying height by girth at least has some rationale, as it would give an indication of total body volume, but it would put girth into the denominator in the formula, which is not what you want.) But height squared? Beats me.
Then there is that mysterious number 703. Most websites simply state it as if it were some physical constant. A few make the helpful remark that it is a "conversion factor." But I could not find a single source that explains what exactly it is converting. It did not take long to figure it out, however. The origins of the BMI, of which more later, goes back to a Belgian mathematician. The original formula would thus have been in metric units, say
BMI = weight in kilograms/(height in meters x height in meters)
To give an equivalent formula in lbs and inches, you need to solve the following equation for C
1lb/(1in x 1in) x C = 0.4536kg/(0.0254m x 0.0254m)
which gives C = 703 (to the nearest whole number).
