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18 September 2006
BMI: A Big Fat Lie?
by Serena Mackesy
Our lazy little brains have a default setting which is, essentially, to believe everything they hear. Skepticism is seriously hard work, and most of us, most of the time, don't have the energy or the willpower to keep on doubting. What we do, instead, is choose a set of beliefs and edit information - which largely means discarding the stuff that's inconvenient - so that it fits with our belief system. That's why systems of propaganda - which, with their basic rules of drip-feeding info that supports a cause while withholding information that might contradict it - are so successful. It's also why it's possible to dismiss, say, horoscopes, as mumbo-jumbo while still believing firmly in, say, life after death.
Reactions to the news from the Mayo Clinic that the Body Mass Index (BMI) isn't the be-all-and-end-all to predicting human health outcomes, have been an entertaining illustration of this lazy wrong-headedness. People in denial about the fact that their weight might well affect their health have taken it as an opportunity to throw the baby out with the bathwater. In a classic example of false logic, there is a school of thought out there which would take this single example of an error in the science of obesity and use it to claim that the whole thing has been concocted as a means of persecution.
There's been a grand tradition of persecution and bullying the big for decades, and it's only got worse in the past few years. Since 1998, to be exact, when the National Institutes of Health lowered the healthy weight BMI from 28 for men and 27 for women to 25, taking an extra 30 million Americans from slightly overweight to obese overnight. It's worth bearing in mind that some of the soaring obesity stats we hear about date from right about that time. And meanwhile, a friend who was shopping for a wedding dress in New York recently found herself turned away from a handful of high-fashion shops because they only went up to size four. That's right: size four. You couldn't fit one of those dresses on one of Dennis Rodman's thighs.
But lets' get back to the BMI. You'd have thought that it was fairly obvious that a system which classifies wrestler-turned-screen-totty The Rock as dangerously obese might be a little out-of-whack. But we've been following the BMI since 1869, so it has to be right, doesn't it? So what if the body frames of, say, Samoans and Thais are entirely different: it's a one-size-fits-all philosophy and if you don't fall into the prescribed range, obviously it's you, and not the scale that's wrong, isn't it?
Actually, the scale is wrong and the BMI is flawed on a number of levels. Furthermore, the 2004 scare-stats put out by the Centers for Disease Control - that 400,000 deaths in the US were caused annually by obesity - were flawed to the point of deliberate lying. They measured how many of the people who died in the given year - died at all; in car crashes, of Lyme's disease, in bar fights, whatever, it didn't matter, they were all lumped in there - were obese, and then lumped all the deaths of obese people in as death from obesity. It's roughly analogous to saying that a smoker who dies in a cholera outbreak died of smoking. They may have been trying to give people a wake-up call but it's shockingly untruthful.
Don't break out the Ben & Jerry's in celebration just yet, however. The Mayo study found (in 40 studies involving some 250,000 people) that people with a low BMI had a higher risk of heart attack than people with a slightly high BMI. This flies in the face of current orthodoxy but here's the crucial thing. The BMI doesn't allow for body frame differences, but more importantly, it doesn't allow for the difference in weight between muscle and fat. And the difference between, say, Vin Diesel and your average couch potato is that Vin Diesel is seriously fit and a square foot of him weighs a good deal more than a square foot of Danny de Vito.
Clearly, the BMI needs to be moved up a smidge, and it shouldn't be used as a bludgeon for those of us who were gifted with a tendency to embonpoint, but looked upon more as a very rough rule of thumb. Because you can be as slim as you like to get into your Vera Wang, but if you starve yourself to get there rather than exercising, you're not going to be reducing your chances of keeling over at your daughter's wedding. You will, of course, be reducing your chances of diabetes, infertility, certain types of cancer, yada yada; but it's looking more and more like these things are risks run more by the morbidly obese than those with a bit extra on the backside.
So, how do you tell whether your shape is, or isn't, healthy? Well, the answer is good old common sense, combined with a bit of mathematical calculation. If you look at fit people, one of the things that isn't hard to see is that they generally - again; there's still good old body type to contend with - have waists. And where fat does most harm is when it's deposited around your midriff, where it lies much more deeply, coats your organs and puts you at far greater risk of diabetes and heart disease. It's muffin-tops, not saggy thighs, that predicate illness. The crucial measurement is the difference between your waist and hips. A woman's healthy waist ratio is 0.8 of their hips; a man's is 0.95. If your math isn't up to it, enter your vital statistics here: http://www.healthstatus.com/calculate/whr. And whatever you do, stop sitting in front of the computer and go out for a nice long walk.
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