For the video version of this post, click here. Body mass index. Since the term entered the medical consciousness in 1972, it has served admirably as a proxy measure for body fat, since body fat itself is sort of tough to measure. But it is demonstrably imperfect. Since it relates weight to height, it has no ability to distinguish between fat mass and muscle mass, leading to so called “obesity paradoxes” that are really no such thing. We need something better than BMI.
That something better, according to an article appearing in the Annals of Internal Medicine, may be the ratio of waist to hip circumference.
The study has some dramatic results. Using data from the National Health and Nutrition Examination Survey, researchers examined around 15,000 participants. Each participant had a BMI and a waist-to-hip ratio. It turned out that waist-to-hip ratio was a much better predictor of mortality and cardiovascular disease than BMI.
In fact, once you accounted for waist-to-hip ratio, BMI didn’t predict mortality at all. What this suggests is that all those studies linking BMI to bad outcomes were secretly studies linking central obesity to bad outcomes (because BMI and central obesity are correlated). But when you introduce a better measure of central obesity, the utility of BMI goes out the window. It’s a proxy measure without a home.
It turned out that, among men with normal BMIs, those with a high waist-to-hip ratio had an 87% higher risk of death. Women with normal BMIs and central obesity had a 50% higher risk of death. Perhaps more interesting, men with normal BMIs and central obesity had around twice the risk of death of men who were overweight or obese by BMI but who didn’t have central obesity. Women’s results went in the same direction, though the magnitude wasn’t as great.
So, do we give up on BMIs altogether? Not necessarily. Waist-to-hip ratio does seem to be the superior risk marker, but it’s not as easy to measure. These data were collected by individuals trained to do these measurements the same way, every time - it may not be possible to do that in the doctor’s office and get reliable results. Though maybe we could start employing tailors.
Also, remember that BMI still captures a lot of this data. The finding that individuals with normal BMI but high waist-to-hip ratio have increased mortality is compelling, but only 11% of men and 3% of women fit in this category. In other words, chances are if you have a normal BMI you’re fine. That said, it seems clear now that we need to find something better than BMI, something that helps distinguish between fat mass and muscle in a way that BMI can not. Whether a technological solution, such as bioimpedance analysis, or an anthropometric solution like the one in this study takes the baton, my intuition is that BMI now has a shelf-life.