So what’s happening over at Slate these days? Let’s take a look:
For years, critics of the body mass index have griped that it fails to distinguish between lean and fatty mass. (Muscular people are often misclassifed as overweight or obese.) The measure is mum, too, about the distribution of body fat, which makes a big difference when it comes to health risks. And the BMI cutoffs for “underweight,” “normal,” “overweight,” and “obese” have an undeserved air of mathematical authority. So how did we end up with such a lousy statistic?
Oh man, not this again. Yes, it’s true: there are a few of us with such Adonis-like physiques that our BMI is high even though we’re not overweight. But not many, and you know who you are anyway. For most of us, let’s face facts: if you have a high BMI it’s because you’ve been eating a few too many Snickers bars.
What’s more, it’s no mystery why BMI has become so widely used: it might not be perfect, but it’s a pretty good rough-and-ready measure of obesity and it’s really, really easy to measure. Mine is about 28. And anyway, all these articles moaning about how bad BMI is never give us anything better to use.
Except — wait! Hallelujah! This one does:
Our continuing reliance on BMI is especially grating given there’s a very reasonable alternative. It turns out that the circumference around a person’s waist provides a much more accurate reading of his or her abdominal fat and risk for disease than BMI. And wrapping a tape measure around your gut is no more expensive than hopping on a scale and standing in front of a ruler.
OK, so what’s the formula? WC squared divided by neck size? Or what? Is Slate seriously going to make us click those links and wade through a couple of epidemiological studies instead of just telling us? Jeebus. But fine. I’ll go look. From the second link, here it is:
Men and women who have waist circumferences greater than 40 inches (102 cm) and 35 inches (88 cm), respectively, are considered to be at increased risk for cardiometabolic disease….Waist circumference measurements should be made around a patient’s bare midriff, after the patient exhales while standing without shoes, both feet touching, and arms hanging freely. The measuring tape should be made of a material that is not easily stretched, such as fiberglass.
That’s it? No formula? Just one number? That’s pretty nice — though I don’t really like this one much. My BMI tells me I’m a little heavier than I should be, but not that much heavier. Hooray! My WC, on the other hand, clocks in at 42 inches, clearly higher than it should be. Boo!
But as it turns out, this is a point in favor of WC since I’ve always felt that BMI is too kind to me. My gut is considerably more jello-like than it should be, and my WC measurement makes that clearer than my BMI does.
Still, don’t take this too seriously. The study in the first link above shows that WC is a better measure of various kinds of fatty tissue than BMI, but not that much better. And the second study says that although WC provides “incremental value” in predicting diabetes, CHD, and mortality rate above and beyond that provided by BMI, it’s not clear if it provides enough incremental value to be worth it: “Based on NHANES III data, 99.9% of men and 98.4% of women would have received the same treatment recommendations proposed by the NHLBI Expert Panel by evaluating BMI and other cardiovascular risk factors, without an assessment of WC.”
So go ahead and measure your waist. It’s fast and easy, and if you don’t cheat it’s a fairly decent predictor of body fat. But for 98% of us, if you know your BMI already you’re probably not going to learn anything you don’t already know.
(Now, whether you should care is another question entirely. I’ll leave that for another day. But regardless of your weight, don’t forget to exercise! Everyone agrees that a sedentary lifestyle is bad for you.)