Your BMI Doesn't Matter as Much as You Think. Here's Why

During a doctor's appointment, you're likely to get various measurements taken, such as your blood pressure, lung function and your weight. You may also notice another number on your medical records called the body mass index, or BMI. BMI puts you into a certain weight category based on your height and weight and screens for obesity based on that measurement. The formula for BMI was created in the mid-19th century and became popular in the 1970s. It continues to be a common measurement tool in health care today.

However, it's become controversial over the years because many critics find it inaccurate, exclusionary and outdated. Plenty of patients and experts alike agree that it's time for a change. Here's why your BMI may not matter after all and which health metrics you should pay attention to instead.

What is BMI?

BMI is a tool developed by Adolphe Quetelet in the 19th century, the World Health Organization notes. Quetelet was a mathematician, astrologer and statistician who wanted to apply probability calculus to human bodies, which is how he concluded "the weight increases as the square of the height" when he developed the Quetelet index, as BMI was first known. 

BMI wasn't popularized until the 1970s, when the Quetelet index was used in population-based studies related to weight and was renamed body mass index. According to the WHO, researchers in the '70s thought BMI was a good way to index potential public health issues related to obesity. This was also based on reports from the Seven Countries study, which was the first study to look at how diet influences risk for heart disease. By the early 1990s, BMI was widely used and accepted after the WHO published information based on it. 

Today, medical practitioners use BMI to measure risk level for certain diseases associated with higher weights. The US Centers for Disease Control and Prevention says that BMI is primarily used as a screening tool to group someone into one of four weight categories: underweight, healthy weight, overweight or obese. To measure someone's BMI, you take their weight in kilograms and divide it by the square of their height in meters. You can find this number on the BMI chart to determine the category you fall under. When it comes to screening children and teens, practitioners use a BMI-for-age system that takes age into account, as well as sex.

What's wrong with BMI?

The problem with BMI is that it's used to assess someone's health risk for conditions related to metabolic and other types of diseases, but at best it leaves out a lot of key health indicators, and at worst, it's sexist and racist. 

"BMI is not good to use as a general health guide for several reasons," says Dr. Aaron Hartman of Richmond Integrative & Functional Medicine. "Yes, it can be used as a general concept -- but what's really needed is individualized care for each patient that takes into account sex, stage of life and ethnicity. Saying someone is healthy, or not, based on BMI alone would not be accurate." 

Below, Hartman and other experts share why BMI is outdated and could use a change sooner rather than later. 

It doesn't measure body composition 

Although BMI is linked to weight and body fat, it doesn't really measure someone's body fat, which the CDC echoes: "[BMI] does not diagnose the body fatness or health of an individual." For this reason, Chad Walding, doctor of physical therapy and co-founder of NativePath, says that BMI can potentially miscategorize someone as obese who is not considered clinically obese, and vice versa.

"BMI is not a super accurate way of finding a person's true body fat percentage. Someone could be super lean but very strong and a BMI measurement would say they are obese," says Walding. "You could also have someone whose body weight is within normal limits for their height, but they have very little muscle on their body and a whole lot of body fat. This person may show up as being of 'normal' weight but actually be clinically obese," says Walding. "This limitation is that BMI only takes into account height and weight, so it's often not the entire picture."

The most accurate way to know your body composition is through a DEXA (or DXA) scan. This full body scan provides precise data on your fat, muscle and bone density. This scan can only be done by a certified medical professional.

BMI doesn't measure health

According to Hartman, even though BMI is now used as a screening tool by health care providers, it was used by insurance companies in the 1920s and '30s to determine someone's risk or chances of dying. "When you're looking at general health, you're not looking only at death for insurance policies but you're looking at mortality, as well as morbidity -- which are different gauges of death, longevity and health," Hartman says.

First and foremost, your weight and even your level of body fat say little about your overall health. Ideally, you need much more specific metrics, like blood pressure, cholesterol or blood sugar levels, for example, to get a real picture of someone's physical health. But the health care system has a long history of fat shaming rather than providing supportive care for all people regardless of size, which is unfair and does a disservice to people seeking care. An overemphasis on BMI contributes to this, by focusing medical professionals' attention on weight and body fat, rather than pressing medical issues that require care.

According to Walding, other tools should be used outside of BMI to get a better read on someone's health. "Other measurements that should be taken into consideration when painting a bigger picture of health include blood pressure, HDL and LDL cholesterol numbers, waist size, fasting blood sugar and triglycerides," he says. 

It doesn't account for sex, gender or age

Since BMI was developed in the 19th century, it's easy to see how the tool could now be seen as outdated and out of touch, because it's, well, old. But BMI's problematic history goes even deeper than that. Hartman pointed out a key detail that BMI was developed for a specific, and limited, group: mostly white males. And that's just the beginning of the problem.

"The second issue is that BMI doesn't take gender, sex or age into account. The sex differences in BMI are notable, and BMI changes as a person ages. The BMI of a 50-year-old male and the BMI of a 50-year-old female will most likely not equilibrate," says Hartman.

When it comes to BMI and age, Hartman says it is also easy to overlook some health issues in someone who may have a BMI that is considered healthy. "It's natural for people to gain fat mass and lose muscle mass as they age, but it's possible for their BMI to stay unchanging while they're metabolically getting sick or developing a metabolic syndrome, such as diabetes," he says. 

It's even more inaccurate for certain races

One of the biggest ways that BMI does a disservice to people is in the way it becomes even less accurate if you aren't white. Different populations tend to have different types of body composition, but because BMI was developed from research on mostly white people, it doesn't take any of that into account.

The CDC mentions that BMI is a disproportionate measure for Black and Asian people, compared to white people. It states, "At the same BMI, Blacks have less body fat than do Whites, and Asians have more body fat than do Whites."

For Black people, Hartman notes that a tendency to store fat subcutaneously (under the skin) might complicate BMI. "So an African American male may have a BMI that's considered high -- but that might actually be healthier for that individual due to the fact that their BMI for their height will actually be higher because of where they store their fat," Hartman explains.

And for other groups, such as Asians, BMI again is not a fair measurement due to how they may store fat. "In the Asian population, fat is stored more around organs, which is much more damaging," he says. So someone could technically not be overweight or have a high BMI, but they are storing excess amounts of fat around vital organs. "Science has shown that fat stored around the organs is more damaging than subcutaneous fat. Having fat around your heart, for example, has a much higher risk for heart disease than having fat around your internal organs or even in your liver. BMI is not taking these things into account," Hartman says. 

If you're into numbers, there are other health metrics that are far more insightful than your BMI, including some that you can also measure at home. Try measuring your basal metabolic rate, blood glucose levels or heart-rate variability.