Is BMI Accurate — and What Does It Really Say About Your Health?

Angela Cabotaje Fact Checked
© Kathryn Swayze / Stocksy United

BMI: Body mass index. Who knew those three letters could hold so much weight? (Pun very much intended.)

What you may think of as a simple, straightforward assessment tool to evaluate your weight and health is actually a lot more complicated than it seems.

“BMI is a screening tool that gives you a starting point, but it doesn’t reflect the true health of that person,” explains Dr. Judy Chen, a surgeon at the Weight Loss Management Center at University of Washington Medical Center-Roosevelt.

That’s because your weight isn’t the only thing that matters when it comes to your health, adds Dr. Sarah Halter, a family medicine physician at UW Neighborhood Factoria Clinic.

“Of course, weight and health have a relationship, but can looking at a number on a scale or at your BMI tell me about who you are as a person, your family history, what you eat and how you move your body?” she says. “No, of course not.”

What is BMI and how is it used?

Developed in 1832 by Belgian statistician Adolphe Quetelet, the body mass index is today considered one of the easiest and most cost-effective ways to evaluate the health of patients.

Doctors consider BMI when determining if a patient is a good candidate for weight-loss procedures like bariatric surgery.

“BMI is just part of it,” Chen notes. “I always think about how I can get surgery done as safely as possible.”

The average person’s experience with BMI, though, is probably during an annual wellness exam. That’s when your doctor measures your height and weight, enters the numbers into a handy BMI calculator and gets your final BMI score. Depending on your score, you’re slotted into one of four ranges: underweight (18.5 or lower), healthy weight (18.5 to 25), overweight (25 to 30) or obese (30 or higher).

And herein lies the problem.

While your weight is related to your health — for example, researchers found your risk of heart disease increases by 28% if you’re overweight — it’s definitely not the only thing that affects your well-being.

“If I reduce your value down to a number, as your BMI does, and use that to tell me if you’re ‘good’ or ‘bad,’ I’m just leaving so much out,” Halter says.

Is BMI accurate?

While doctors appreciate the statistical nature of BMI, the problem with it is that it can focus too much on the numbers. As a result, your BMI doesn’t take into account vital information that can contribute to or detract from your overall health. Think biological and lifestyle factors like the quality of your diet, your genetics, how much you exercise or whether you drink or smoke.

Even in a weight-loss setting, where the main focus is getting to a healthier weight, it’s not only about your BMI score, Chen notes.

“Sure, surgery can pull someone out of a metabolic whirlpool and change their hormone and insulin response, but suddenly having a different anatomy doesn’t change untreated mental health disorders or coping mechanisms,” she says. “There’s a lot that goes into being healthy. For weight-loss surgery, that means a multidisciplinary team evaluates your diet, mental health and whether you meet medical and surgical criteria.”

The other issue with BMI, Halter says, is that it can actually be pretty misleading. What she means is that BMI’s strict categories don’t adjust based on things like your body type, age, race or gender.

“It can overestimate adipose tissue in some patients and underestimate lean muscle in others,” Halter explains.

Take, for example, someone who happens to be fairly muscular. Because muscle weighs more than fat, this buff bod may end up with a BMI that’s classified as overweight when the opposite is probably more accurate.

“You can have a normal BMI but still have visceral fat around your core or be totally sedentary,” Halter says. “You can have a so-called normal weight but still have unhealthy habits. BMI doesn’t distinguish between any of this.”

Is BMI a factor in weight stigma?

It would be easy to dismiss the controversy around BMI by saying that medical professionals use their best judgment when evaluating patients instead of just relying on what the numbers say. But the truth is that doctors are humans with preferences and unconscious biases, too.

“There’s an emotional component that we, as doctors, have to be really aware of,” Halter says. “People might come in for a cough or knee pain, but then their doctor somehow ends up bringing it back to their weight. We need to give more credit to weight stigma and the pressures or fear that patients might feel when they want to come to their doctor.”

If that seems like an overly cautious outlook, it’s actually fairly accurate. Take one study that surveyed more than 2,000 doctors: On average, participants demonstrated a strong preference for thinner people and had a clear implicit and explicit “anti-fat” bias.

“Unfortunately, weight is often tied to discrimination,” Chen says. “There are a lot of challenges because of that, including depression and poor self-image.”

And it’s not just that weight stigma plays into how doctors view patients or how patients end up feeling about themselves — it can also cause real harm when patients avoid seeking routine medical care for fear of being judged.

In a review comparing the findings from 32 published studies, women who were classified as “overweight” or “obese” according to their BMIs were less likely to get regular mammograms, Pap smears or colonoscopies when compared to their peers in the “healthy weight” category.

“There’s a history of trauma that people may feel with their weight,” Chen explains. “So as a result, people try to avoid and protect themselves from having another traumatic experience.”

Is there a future beyond BMI?

It may seem like the only solution here is to ditch BMI all together, but that’s easier said than done. As of yet, there isn’t a comprehensive health screening tool that doctors can reliably use to evaluate patients.

“A comparable method that’s been discussed is measuring abdominal circumference, but it’s not standardized as to where and how you measure on a patient,” Chen says.

Other assessment tools — like machines that can measure your body composition, underwater weighing or scales that can calculate your body fat — aren’t cost effective, aren’t easily accessible or may have similar discrepancies as BMI.

In the end, Halter says, the best takeaway from the BMI controversy is to simply give that number less weight.

“How we as doctors can improve is to take a little bit more time with patients and really talk to them,” she says. “Can we talk about joyful movement? How are you fueling your movement? Am I taking into account that you might be a single mom who is so exhausted that all you can eat is packaged food? How is your mental health? What’s your spiritual health?”

By looking beyond your BMI number and just viewing it as one piece of information in the larger scope of your health, doctors can take more time to investigate all the factors that make you uniquely you.