"Bogus Mass Index"

Back when I was in elementary school I remember seeing charts in gym class called “height/weight” charts. They were easy to use: you looked up how tall you were and then you could see how much you were supposed to weigh. Weigh more than that and you were too fat. (Things were pretty black and white in elementary school back then.) I recall very clearly one year when the medical community finally spoke out against their use. They were overly simplistic, they said, and failed to account for such obvious items as the fact that muscle and bone had higher densities and weighed more than fat. Those arguments rely on scientifically sound facts and conclusions and the use of the old height/weight charts faded into the dustbin of history.

Until, that is, some bright bulb somewhere repackaged it for the 21st century. Today they call it the “Body Mass Index” or BMI. It’s pretty simple to use when you have the charts: you look up how tall you are and cross-reference your weight to get which of 4 categories you fall into – underweight (ba-har-har-har), ideal, overweight, and obese. Despite the fact that it’s the bloody CDC pushing the use of this garbage, it has exactly the same issues as the old height/weight charts.

I always knew the BMI should really stand for “bogus mass index” but I lacked the point-by-point items to address just how bogus it was. Until now, that is, thanks to (can’t believe I’m saying this) the National Public Radio (NPR). Keith Devlin writes an article there titled, “Top 10 Reasons Why The BMI Is Bogus.” Just a sample from the list:

1. The person who dreamed up the BMI said explicitly that it could not and should not be used to indicate the level of fatness in an individual.

The BMI was introduced in the early 19th century by a Belgian named Lambert Adolphe Jacques Quetelet. He was a mathematician, not a physician. He produced the formula to give a quick and easy way to measure the degree of obesity of the general population to assist the government in allocating resources. In other words, it is a 200-year-old hack.

4. It gets the logic wrong.

The CDC says on its Web site that “the BMI is a reliable indicator of body fatness for people.” This is a fundamental error of logic. For example, if I tell you my birthday present is a bicycle, you can conclude that my present has wheels. That’s correct logic. But it does not work the other way round. If I tell you my birthday present has wheels, you cannot conclude I got a bicycle. I could have received a car. Because of how Quetelet came up with it, if a person is fat or obese, he or she will have a high BMI. But as with my birthday present, it doesn’t work the other way round. A high BMI does not mean an individual is even overweight, let alone obese. It could mean the person is fit and healthy, with very little fat.

The logic item I’ve listed above is a problem with the application of the BMI to individuals not necessarily with the BMI itself. Of course, the problem with the BMI is that it’s scientifically, physiologically wrong as I referenced above. What makes this less of an example of effectively harmless quackery and more of a real problem is the fact that insurance companies and even some private firms are basing decisions on extending services and job offers on this idiocy. I’ll admit that I’m no where close to ideal weight but the fact of the matter is that methods used to actually measure my body fat have shown a significantly lower level of fat than the BMI is implying. My body type is just heavier, my bones and muscle mass weighing more than the equivalent volume of body fat. A person over 6 feet tall and ripped like Arnold Schwarzenegger in his prime comes up with the same BMI as I do and the only 6-pack you’re going to see on me is the the one with the diet soda label I’m carrying out to the car from the local Wegmans.

The only reason the BMI is seeing use is that it’s simpler to use than methods that show the actual percentage of body fat on an individual and it’s cheaper, besides. We aren’t going to get anywhere by slapping an “obese” label on people, particularly when that label is being applied to people whose fitness levels are well above the norm. We should be advocating the use of methods than can show truly individual measurements and stop using the method we’ve already tossed out once and whose creator warned us should not be used in this fashion.

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One comment

  1. Can’t do that. For the same reason you can’t have a true democracy on a scale sufficient for the current USA. And that problem is “scale”.

    True democracy, where every citizen has a vote in all collective decisions, works well if the group is small, the resources limited and the decisions are made to affect local problems. When the scale increases on *any* of these three, true democracy starts to bog down. Decision-making becomes paralyzed when a certain group size threshold is exceeded, when the problems become too large for the individual to adequately assess, or when the resource base is too large. (That’s right: too large. The collective group literally cannot coordinate spending, and certain problems will get buried in useless additional aid, while something else slips through the cracks.)

    Same problem exists with the use of individual measures to determine obesity levels *in populations*. Too many variables, too much data, too much conflicting data. So you either choose representative samples and call them “the reality” (that’s the method we chose for governance: representation), or you go with something that while it isn’t completely accurate for each individual, it allows you to make sweeping statements that permit the decision making needed to create policy. And there’s the BMI.

    No, it’s not accurate. Laughable, in fact; our BMI numbers indicate that we would need to be 154 pounds to be the maximum “normal” weight. For me to get there, I’d need to be extremely active, and I build muscle easily. By the time I reduced my body fat to an acceptable percentage, my lean body mass would exceed 154. I would literally need to lose muscle to meet the BMI. And that’s ridiculous.

    Personally, I think that in a world of imperfect solutions, the BMI has its place, or something like it. I seriously object to its use as an insurance tool, however. And I have had premiums increased as a result of my weight. Undoubtedly, they used the BMI calculation to decide how much extra to charge.

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