But wait, am I obese? I don't think so... I cycle 8 miles each day, do manual labor, and eat (relatively) healthily. I haven't checked my resting heart rate recently, but I'm guessing that it's between 60 and 70. I have a 36" waist, and 48" shoulders. I can benchpress 200 pounds, easily leg press 500 pounds, and do calf extensions at 250 pounds per side. How is this obese?
Recently, Devlin's Angle did a piece on BMI - the history, and stupidity of it. On the one hand, the usage of the BMI to indicate a single person's relative health is a good example of misuing a [simplistic] formula meant to determine population level characteristics, not individual ones. On another hand, this is also a story of how numbers and scientific wrappings seem to hold social significance. On yet another hand (this is turning into a Vishnu-statue of "other hands"), the BMI is a major tool in looking at trends in obesity. Finally, on the remaining hand (of Vishnu), it's completely meaningless, mathematically speaking.
Here's the equation:
BMI = weight in pounds/(height in inches^2) x 703
Devlin's Angle goes on to explain where the 703 comes from, and ponders the question of why the height is squared...
Devlin's Angle outlines two reasons why the BMI is useless as a individual measure: it's a population-based measure, and populations are made up of sedentary individuals (not atheletes) and it was derived at an early time in the statistic-ization of sociology. On the first point, the formula assumes that all "extra" weight on an individual is from fat. On the second point, the formula is derived to measure the trend of the majority of the population.
I would like to outline two other reasons why the BMI sucks for people like me: height and body proportion. Devlin's Angle points out that the BMI was derived in the early 1900s in Belgium. Looking at Wikipedia's entry on human height, one finds that in the mid-nineteenth century the average height in the Netherlands and France (they don't list Belgium) was164 cm and 165 cm, respectively. Looking at these measures "today", one sees a "slight" difference: 182 cm (169.7 cm) and 177 cm (164.6 cm), respectively (female heights in parenthesis). I assume that female heights weren't included in the mid-nineteenth century measurements, but we see that after 150 years, Dutch females are taller (males much taller) and French females as tall (males much taller) than their ancestors. What is so important about this? Well, remember that the equation for BMI was based on the average person. The average person being (among males) as much as 12-18 cm taller than the people measured to derive the BMI.
Let's try and make two individuals who matches the criteria of different BMI groupings; one from the mid-nineteenth century, and one from today. Since BMI is only a relationship between height and weight, this shouldn't be difficult to figure out. Therefore, a mid-nineteenth century Dutchman of average height (roughly 65 inches), would have the following BMI table:
Underweight (below 18.5): below 110 lbs.
Ideal (18.5 to 24.9): 110 lbs. to 148 lbs.
Overweight (25.0 to 29.9): 148 lbs. to 178 lbs.
Obese (30.0 and above): 178 lbs. and above.
We can imagine the "average build" 5'5" person and think, "Okay, that works." However, now let's look at what the BMI chart would mean for the average height Duchman of 6'0":
Underweight (below 18.5): below 135 lbs.
Ideal (18.5 to 24.9): 135 lbs. to 183 lbs.
Overweight (25.0 to 29.9): 183 lbs. to 219 lbs.
Obese (30.0 and above): 219 lbs. and above.
That man would have to be one skinny person. Imagine a person who is 6'0" and 135 lbs. Jim Carrey - someone we might think of as tall and skinny - is according to this site - 6'1" and 180 lbs. That's on the upper end of "ideal". President Barack Obama is - according to this site - 6'1.5" and 180 lbs. (Which, strangely, gives him a slightly higher BMI of 23.4 compared to John Kerry's 22.5, even though I would say that Obama's more atheltic now than Kerry was in 2004).
So, height is a determinant. Therefore, BMI was a good measure of estimating the height/weight relationship of early 1900s Belgians, not necessarily early 21st century Americans.
Also, there is body proportion. Having a relatively long torso, I have been blessed with not having to worry about too little leg room on aircrafts (yet), but I am annoyed at how low the backs of seats cut me (usually well below the shoulder, even on "tall" chairs). There are trends on body proportions, both in terms of proportions of height (i.e., long torso vs. long legs) and width (e.g., hip-to-waist ratios). Neither of these are included in the BMI. True: certain regions have slightly different cut-off points for BMI measurements, such as in SE Asia where there is a relatively consistent height body proportion, however, in the United States - where there is such a wide range of different height and width body proportions that BMI loses its meaning even faster. But why?
Well, take me for example. I have a long torso. What does that mean, though. Well, a single inch of height that is comprised of torso has more mass than an inch of height comprised of leg; there are more organs, more girth, and more water in the torso than in the leg. Therefore, if someone has a relatively short torso compared to me (i.e., they are my height, but have much longer legs than me), having a 240 lbs. somewhat athetic build would mean that individual would have legs even more massive than mine (or a torso that was gigantic). In other words, body proportions will have a direct bearing on the amount of mass you are carrying around (and therefore, your weight), irregardless of how much muscle or fat you have.
Related to the issue of body proportion is the issue of amputation. If you amputated a leg, you could lower your total body weight by 20-40 pounds (depending, obviously, on the weight of your legs). If I cut off legs, then my BMI would drop to about 25 (still "overweight", but not "obese"). However, if you had both your legs amputated (or weren't born with both legs), then you could shorten your height by 27" (plus or minus) and your weight by 40-80 lbs. However, due to the formula, my BMI would actually increase to 44 if that happened to me.
What to do? Well, I have a novel idea of doing a BMI survey of a cohort of incoming students, combined with a number of other measures that will give an indication of body proportion, muscle mass, and fitness. Then, look at doing statistics on the whole set to see what sort of relationship one would get for "modern" BMI cut-offs, and determine if there is any easier way of determining a general figure of fitness that can be used by an individual (as opposed to being a proxy for an entire population, mostly made up of sedentary individuals).
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