That “important” number may not be such an accurate marker of health after all.
If you’ve been anywhere near a doctor’s office or exercise app — or stumbled across any of the weight-loss content suffocating the internet — you’ve definitely come across the term “BMI”. How do you calculate your BMI? Just pop your weight and height into one of the many available online BMI calculators, and you’ll get yours. Cool, but what does your BMI mean?
What is BMI, really?
The BMI — Body Mass Index — is a person’s body mass divided by the square of their body height (kg/m²). A BMI between 18.5 and 24.9 is considered “healthy” by the CDC, while a BMI over 25 is “overweight,” and a BMI over 30 is “obese.” Doctors typically use these numbers as one of their key metrics to assess patients’ health, and to gauge what type of advice to dole out (eg: “lose weight”). Sounds reasonable, right? But there’s a catch…
The BMI wasn’t created by doctors — or intended to measure health
You’d assume that a tool used by physicians all over the world had been designed by someone involved in medicine or healthcare, right? Think again. The BMI was created over two centuries ago by a Belgian astronomer and mathematician named Lambert Adolphe Jacques Quetelet, and originally called Quetelet’s Index. It was never intended to assess individuals’ health — or health at all — but rather to measure populations as a quick hack to assist in allocating resources.
Weight only became thought of as an essential indicator of health in the early 20th century, when insurance companies (which weren’t staffed by medical doctors) began factoring in peoples’ notoriously unreliable home measurements to determine what to charge potential policyholders.
Oh, and it’s really exclusionary
The BMI was created at a time when racism was particularly rife within the scientific community. Characteristics associated with white Western Europeans were thought to represent l’homme moyen — the “ideal” average man. Most of the people Quetelet studied when creating what became known as the BMI were therefore white men, making it even less relevant to anyone who isn’t one (ie: most people).
On the decidedly non-medical basis of l’homme moyen, people with larger bodies — eastern and southern Europeans, as well as Africans — were considered to be inferior, while women of all backgrounds were left almost entirely out of the picture. With thinness and whiteness so closely aligned in the academic psyche, it “naturally” followed that thinness must be good, and non-thinness must be bad. Only that’s not medically true.
For example, research has shown that between one-third and three-quarters of people classified as obese according to their BMI are metabolically healthy. Meanwhile, unfit “skinny” people are more likely to develop diabetes than fit “fat” people. Which is just one of the reasons why it’s amazing that doctors rely so much on BMI, given that…
BMI tells you nothing about your body’s composition
“BMI doesn’t take into account individual differences,” Kimberly Gomer, MS, RD, LDN, Director of Nutrition at Pritikin Longevity Center, told KCM. “Mainly, it doesn’t differentiate between muscle mass and fat. So a very fit athlete may look overweight on the BMI table, and a very inactive, low muscle, or older person may appear to have a perfect BMI, when in reality, they’re far from healthy.”
Even the boundaries for what counts as “overweight” according to the BMI are super arbitrary. In 1998, the National Institutes of Health lowered the threshold for “overweight” from a BMI of 27.8 to 25, meaning that millions of Americans “became” overweight overnight.
Also, BMI feeds into discrimination within the medical community
Thanks to a heavy emphasis on weight reinforced by use of the BMI, an overwhelming number of “overweight” people report discriminatory treatment in doctors’ surgeries. This can range from physicians ignoring a patient’s input in favor of lectures about weight loss, to missing dangerous conditions like cancer.
“There is no question that many health professionals assume that someone is healthy if they are lean and not healthy if they are overweight,” says Gomer. “The stigma of being overweight is real, and almost every overweight person has felt that at some time or another when seeing a doctor or other health professional.”
This stigma can lead to people with BMIs that land in the “overweight” and “obese” ranges avoiding going to the doctor. In doing so, they can miss out on crucial medical care, or risk being misdiagnosed if they do make an appointment. They may be charged artificially high insurance premiums by companies that rely on the BMI in favor of more reliable measures of health — which isn’t a great motivator for getting yourself checked out. Racial discrimination can come into play again, since Black people tend to have less visceral fat given the same BMI than white people — but the BMI doesn’t take this into account, so they’re penalized just the same. Health risks for Asian people on the other hand tend to be underestimated by the BMI scale, which may lead to an underdiagnosis of some conditions.
The consistent discrimination faced by all “overweight” people even increase physical stress markers like cortisol and blood pressure, raising the likelihood of grim health outcomes. And all this pressure to lower BMI through dieting is likely futile, because…
Diets don’t work
“We know that many professionals are giving their patients or clients unsustainable advice to lose weight [based on their BMI], with little support,” says Gomer.
Not only that, but as anyone familiar with The Biggest Loser will tell you, attempts to lower your BMI through dieting may seriously harm your metabolism, making it harder to keep the pounds off in future. A 2016 study showed that years after shrinking dramatically on the show, only one out of 14 contestants hadn’t regained the weight. Even after returning to their larger size, most were also burning fewer calories post-dieting.
That’s an extreme example, but there’s solid research to show that even more “normal” dieting can slow your metabolism through metabolic adaptation, meaning that your body uses less energy to complete everyday tasks once you reach a “normal” BMI than someone with the same BMI who hasn’t just lost weight. In fact, dieting is one of the most reliable predictors of future weight gain.
So forget chasing that “normal” BMI, and try building positive habits
The good news is, studies are increasingly showing that behaviors are much better predictors of health than how high someone’s BMI is. So a person who’d be deemed medically “obese” on the BMI scale, who sleeps well, does regular exercise, eats a variety of nutritious foods, and doesn’t smoke is likely far healthier than a “normal” weight person who, say, drinks heavily and lives off the occasional apple or snack cake.
The way to truly improve your health? Ditch the scale, eat as many whole foods as possible, kick your late-night social media habit, and try to get some steps in whenever you can. And don’t sweat your BMI.
(P.S. Hungry to learn more about your health? Here’s everything you need to know about your thyroid, plus a helpful guide to cutting down the booze, for the sober-curious!)
