BMI Can Tell You Something About Your Health...Just Not What You Think

A few weeks ago as I was putting my shoes on after a Sunday morning barre session, I overheard a pair of fellow class-takers talking about their exercise schedule for the week: barre today, then an at-home strength training session tomorrow. “I’m so close to getting my BMI down,” said one of the women.

BMI, the abbreviation for Body Mass Index, is the calculation used to classify someone as “underweight,” “normal,” “overweight,” or “obese.” These designations are used by doctors and medical professionals to assess health risks, but the calculation alone might not be so accurate.

Here’s the problem: BMI wasn’t calculated with women or people of color in mind—and it also gets thrown out of whack when attempting to categorize athletic people whose muscles pack on weight. “It can set us up for failure in a lot of ways when we and even our doctors are obsessing over numbers and we're not taking other genetic or racial or ethnic factors into play,” says Erikka Taylor, MD, MPH, a physician in North Carolina.

Case in point: rugby player Ilona Maher, who recently posted a video about why the BMI doesn’t work for her. As a pro-athlete, she nearly lands in the obese category—but she’s confident in what her body can do and doesn’t put too much thought into the measurement anymore, despite feeling shame over the years. “There are many athletes that if you were to look at their BMI, they probably would be told that they're obese or overweight,” says Dr. Taylor. “If you weren't looking at them and seeing their activity at the Olympics and just looking at their [BMI], then you would probably give them weight loss strategies and interventions.”

Meet the experts: Erikka Taylor, MD, MPH, is a physician and psychiatrist in Durham, NC. Emilia Thompson, PhD, is an exercise scientist and coach in the United Kingdom. Navya Mysore, MD, is a primary care physician, women’s health expert, and WH advisory board member.

So, chasing after a BMI "goal," like that woman in my workout class, might not be the best way to really measure your health. Here’s what you need to know about the problems with BMI.

What is BMI?

BMI is determined by the following equation: weight in pounds divided by height in inches squared, multiplied by 703. What this means is that BMI tells you your weight relative to your height.

It wasn't originally created to be an indicator of health. In the 1800s, a Belgian statistician created an early version of the BMI as a way to determine measurements for the “average man,” per a 2023 review in Cureus. Later, by the 1970s, statisticians used the measurements of 7,426 “healthy men” to parse out the categories we have now, the review says.

Today, BMI is used by doctors to classify people as underweight, normal weight, overweight, or obese, and it can also influence treatments for patients. If you fall into the overweight or obese category, doctors might recommend weight loss, sometimes without giving enough thought to other health markers or labs, says Dr. Taylor.

One thing that BMI does not take into account is body composition, says Emilia Thompson, PhD, an exercise scientist and fitness coach. Things like bone density, muscle mass, distribution of weight, and fat all contribute to the number on the scale but aren’t incorporated into BMI. For example, BMI might underestimate body fat in older individuals who have lost muscle and/or bone density, and it might overestimate body fat in athletes who have a lot of muscle, per the National Institute of Health.

There’s actually another designation that’s not part of the BMI, “metabolically healthy obese,” that refers to people who are bigger-bodied and also totally healthy, says Thompson. This designation can account for anywhere from six to 60 percent of people considered obese depending on the criteria, according to a 2019 review published in the Journal of Clinical Investigation.

How accurate is BMI?

In 2023, the American Medical Association said that there were “numerous concerns” with the way that BMI has been used to diagnose obesity, encouraging doctors to not use it as a sole measurement of health.

Determining health risks and outcomes on an individual level ideally should incorporate more input than just two factors (height and weight). Genetics are very important, as are lifestyle and social determinants of health like your environment, relationships, and food access, Dr. Taylor says.

That said, there is an association with a higher BMI and an increased risk for things like heart disease, high blood pressure, type 2 diabetes, gallstones, and cancers, according to the National Institute of Health, but each person’s risk is dependent on factors beyond their BMI. And while all-cause mortality was increased independently of other factors in adults categorized underweight or obese, it was not in those classified as overweight, according to a 2023 survey analysis in PLoS One.

It’s important to underscore that BMI wasn’t calculated with a sample population that includes everybody. Women and people of color were left out entirely. “[BMI] is simply a mathematic equation that was never validated amongst diverse populations,” says Dr. Taylor. Black individuals, Latinos, and Native Americans are all more likely to have higher BMIs, according to Dr. Taylor, due to any number of genetic, physical, or social factors.

“Unfortunately, that also means these individuals are likely going to be targeted in terms of ‘You're doing something wrong with your body, and you should change your body,’ not taking into account all those factors,” she says.

But that's not necessarily the case: For example, being at risk for type-2 diabetes has different BMI cut-offs across different ethnicities, according to a 2021 study published in The Lancet.

Women also tend to have more body fat than men, says Navya Mysore, MD, a primary care physician and WH advisory board member. This fat serves a purpose: leptin, a hormone that is released by body fat, plays a role in menstrual health. (This also explains why, when women lose too much weight, they stop getting their periods, she says.) But, the BMI doesn’t take this into account.

It’s best to think of BMI as one datapoint, Dr. Taylor and Thompson agree. Understanding that it might be less helpful for certain individuals is important when you’re determining how much it should matter in that picture.

Beyond BMI

Now that you know a little more about what BMI can—and can’t—tell you, you can get a fuller picture of your health, says Dr. Taylor. With your doctor, go over things like family history, your medical history, genetics, heart rate, body composition, blood pressure, and baseline labs. In addition, you can look at more subjective and holistic measures like energy levels, how your body feels, how you feel waking up, focus, athletic performance, and social health, says Thompson.

You can also advocate for yourself when it comes to focusing on weight in the doctor’s office. “It's okay to ask questions like ‘Are there any other data points that would indicate whether or not I'm at high risk for whatever condition?’” Dr. Taylor says. And, if weight isn’t relevant to your concern, you can request to skip the scale.

More broadly, researchers are developing new ways to assess the risk between size and health conditions, whether that’s by delineating BMI charts for individual populations or coming up with new systems like the body roundness index, which seeks to get more specific about measuring body fat by comparing your waist circumference to your height.

Overall though, your size is just one piece of your health puzzle, so no matter how it’s measured, make sure your doctors are getting to know the entire picture.

Olivia Luppino is an editorial assistant at Women’s Health where she covers health and fitness. She previously wrote for The Cut, POPSUGAR, and Salon and has written about everything from New York Fashion Week to dating app trends to the United States Women’s National Soccer team (a.k.a. her heroes). When she isn’t writing, Olivia is likely catching up on Bravo shows or running late to barre class.