Why Doctors Are Still Using The Flawed BMI

Photographed by Erin Yamagata.
The body mass index (BMI) has long been used as a way to judge one's health. It's a simple measure — your weight in kilograms divided by your height in meters squared — that can supposedly tell you how likely you are to develop heart disease, diabetes, and even early death. But, as you may have heard, it's not without its fair share of problems. Although the massive studies done using BMI as a marker of health are statistically sound on a large scale, it's "not the whole story for any individual,” says Mitchell Lazar, MD, PhD, at the University of Pennsylvania. This is a (potentially huge) problem, because BMI is used in doctor's offices everywhere as a way to classify patients as normal weight (read: healthy) versus underweight, overweight, obese, and so on (read: not healthy). So, why do we still use it? And why haven’t we come up with anything better? It's complicated.
A Perfectly Imperfect Measure
What you might not know about BMI is that it's actually more than 200 years old. The formula that became the BMI was originally devised by Adolphe Quetelet in the early 1800s in an effort to quantify the “ideal man.” He wasn’t especially interested in obesity, but long after Quetelet’s death, another researcher — Ancel Keys — really was. In a 1972 study, he found that Quetelet’s calculation was the best of a bad bunch of measures that had been created to approximate patients’ body fat percentage. And as you can probably guess, it stuck. The best thing about the BMI is that it’s easy, explains Janet Tomiyama, PhD, an assistant professor of health psychology at UCLA. “It’s two numbers — weight and height — so anyone can get their [BMI] number without training,” she says. And because it's so quick, cheap, and anyone can do it, it's an obvious choice for researchers on the hunt for data. On top of that, BMI truly is useful as a measure when you're looking at a large set of data. In fact, as a new study in Mayo Clinic Proceedings suggests, on that scale, it may even be more accurate at predicting health problems in a group of people than body fat percentage.

Weight-based stigma can increase stress and affect your body’s ability to deal with pathogens.

Dr. David Frederick
But when you zoom in to look at each individual patient, the story can change dramatically, depending on the person. The most obvious case is that of professional athletes. These perfectly healthy people — heavy with muscle rather than fat — often appear unhealthy by BMI standards. On top of that, BMI — and weight in general — doesn't always match up with other, more accurate measures of health, a recent study authored by Dr. Tomiyama found. Using data from a nationally-representative survey, Dr. Tomiyama and her team matched up participants’ BMI numbers with biomarkers such as cholesterol levels, blood pressure, and insulin resistance. “How many of those folks who are considered unhealthy by BMI standards are actually perfectly healthy based on these markers?” she asks. “We found it was quite a lot.” Quite a lot indeed: Almost 30% of participants considered obese by the BMI were actually totally healthy based on their biomarkers. What’s more, 47% of those considered overweight were healthy, too. “That’s almost a coin toss,” Dr. Tomiyama says. On the flip side, a fair amount (over 30%) of people considered healthy by the BMI actually had concerning results in terms of cholesterol, blood pressure, or insulin resistance. What's disturbing is that the inspiration for Dr. Tomiyama's study was insurance companies' interest in using BMI to assess risk and charge higher premiums to those falling in "unhealthy" categories. As her work shows, that setup may unnecessarily cost those with higher BMIs who are healthy — and let those with normal weights and health problems off the hook, illustrating a potentially harmful side effect of our intense focus on BMI.

Doctors will take a look at your BMI and the advice they give you is to lose weight, but that’s not necessarily helpful.

Dr. David Frederick
Even if your BMI does accurately reflect the makeup of your body, there's another big problem with the measure: Research shows that overweight or obese patients often receive subpar treatment due to weight stigma, which means that reducing the entire picture of your health to one number may actually do physical harm. “Doctors will take a look at your BMI and the advice they give you is to lose weight, but that’s not necessarily helpful,” says David Frederick, PhD, an assistant professor of psychology at Chapman University. “It ignores all the other markers [of health] ... and causes people to go to unhealthy extremes to lose weight. That might be worse than remaining at their current weight.” (And, for the record, shaming patients into losing weight doesn't even work.) “One thing we know is that experiencing stigma (including weight-based stigma) can in-and-of-itself affect health,” says Dr. Frederick, who recently published a study in the International Journal of Obesity looking at the media’s effect on weight prejudice. “It can increase stress and affect your body’s ability to deal with pathogens — that’s the direct negative effect of stigma on the body.”

The Alternatives
There are other measures out there that might be more accurate predictors of health risks, but so far none have caught on the same way BMI has. For example, there are more technical tests that measure body fat percentage or your blood levels of C-reactive protein, which is related to inflammation. But these quickly become too slow and expensive to be feasible alternatives to the super easy BMI.

BMI is not the whole story for any individual.

Dr. Mitchell Lazar
Another option, which may be the closest alternative, is the waist-to-hip ratio, which compares the circumference of your waist to that of your hips. This might be a better gauge because it takes into account not only how much fat you have, but where it is located, Dr. Lazar says. This is an important distinction because you actually have multiple types of body fat. We know that visceral fat, the kind that lurks deep in your abdomen and can widen your waist, is more dangerous, whereas the fat on your hips is subcutaneous or "white" fat, which doesn't seem to be as harmful. In a few studies, Dr. Lazar says, waist-to-hip ratio seems to be better at estimating your actual metabolic health than BMI alone. But Dr. Tomiyama maintains that we don’t need another shortcut to measuring our health. “I think really we already have a better marker — all these markers that we already measure,” she says. “These are much more direct conduits to health.” Blood pressure is an obvious example: “It might be hard to get a blood test for insulin resistance, but blood pressure — that’s so easy,” she says. “It takes 15 seconds, is that really so much more onerous?" Doctors taking other markers into account during the decision-making process — before simply jumping straight to the advice to lose weight — could ultimately improve people's care. Dr. Lazar agrees that incorporating “other inexpensive things that we can add to the mix” is a good approach. “But let’s acknowledge that there might be something a lot better out there,” he says, “and the only way to find that is basic research.” Unfortunately, that also takes time and money. In the end, we may never really be rid of the BMI. So our best approach is to accept it for what it is — a quick and dirty approximation of an incredibly complex and individual thing — and to remember that it is, at best, only the beginning of the story of your health.

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