Why Won't BMI Die?

With Scott Cunneen MD

You’ve probably heard about or calculated your body mass index (BMI). Here's why the ubiquitous measurement is outdated, inaccurate, and problematic. 

Body Mass Index

BMI was first discussed in the 1830s by a Belgian statistician, sociologist, astronomer, and mathematician by the name of Lambert Adolphe Jacques Quetelet. It was known then as the Quetelet Index and was informed by Quetelet’s interest in finding “l’homme moyen,” or the average man. If that sounds biased and dicey, it's because it is.
At the time, actuaries were reporting high death rates. Quetelet’s goal was to study average human physical characteristics, in order to understand why so many people were dying, in case there was a correlation between weight, height, and death. It was designed to look at a pattern of deaths among a large population, as opposed to evaluate any one person's size or health.

Fast forward to 1972. The Quetelet Index became known as the Body Mass Index (BMI) when Ancel Keys, a physiologist (and not a medical professional), stated that it was essential to quantifying health on an individual level. Today, almost two centuries from its inception, everyone from family doctors to insurance companies still refer to BMI as a sort of first-line assessment of health, leading to higher insurance premiums and reductive doctor visits for people outside of the size of the "average man" in the 1830s, among other problems.

If you think it sounds a bit odd that we use an archaic system not developed by a doctor and certainly not taking into account that different people (and sexes) have different body types, you’re right. 

What is BMI anyway?

BMI is currently used by medical professionals as a quick assessment to whether or not a person has a healthy weight, based on an estimation of body fat percentage. It does so by dividing your weight in kilograms by your height in meters squared.

As a result you've probably seen BMI calculators all over the Internet, simply requiring you to fill in your weight and height — and voila, they spit out a so-called picture of your health as represented by a single number. This estimate, however, doesn’t regard your race, age, gender, genetics, amount of body fat versus muscle mass, your lifestyle, or other measurements of health such as your cholesterol or blood sugar.

BMI is said to provide medical professionals with a sense of whether or not a patient is at risk for certain diseases, because obesity is correlated with a greater risk of heart disease, high blood pressure, diabetes, and some cancers, among other comorbidities. While there are well-documented health risks to finding yourself too far on both ends of the weight spectrum, there is no evidence that if you are healthy and are slightly above or below the very narrow range that BMI considers "normal" and "ideal," it has any effect on your overall health.

Is BMI effective? 

The short answer is no. It doesn’t — and cannot — take into account your body composition, including your body’s ratio of lean mass to fat mass.

According to Scott Cunneen MD, FACS, FASMBS, Director of Metabolic and Bariatric Surgery at Cedars-Sinai Medical Center in Los Angeles, “While BMI does give the doctor a very basic guess at a patient’s body fat percentage, it is in no way the entire picture when it comes to his or her overall health or even amount of body fat.”

He continues, “For example, a 35-year-old-guy who hits the gym five days a week and has developed a lot of lean muscle may weigh considerably more than a 65-year-old man of the same height, but the younger man’s higher BMI does not automatically qualify him as unhealthy just because of the number on a chart.” Note that he is also again using the example of a man.

For women, particularly those who are tall and/or athletic and muscular, the "ideal normal" range can seem impossibly low. This is because it is based on the "ideal normal" weight of a woman in the 1830s when women were significantly shorter and almost never exercised or built up muscle mass. It's also why many professional female athletes have BMIs that erroneously put them in the "overweight" category, while women who smoke and don't exercise, but are thin, can be considered "ideal" and "normal."

In other words, athletes who have a lot of muscle are more likely to have a higher weight, which the BMI mistakes as a higher body fat range. In some people, such as older people who lose muscle mass, or thin people with very little muscle mass, BMI can underestimate body fat.

The limitations of BMI go beyond not figuring muscle and physical fitness into the equation, though. It further stigmatizes patients — especially obese patients — who do not fall into a “normal” BMI category, and can keep those who may need medical health the most, including to lose weight, out of their doctors' offices by unfairly increasing their insurance premiums.

BMI doesn't help patients 

When Bee, 32, of Sacramento, California went in to see her doctor this year, she found that her BMI was the start and endpoint of the conversation. “The doctor used weight as an excuse for everything and didn’t look further,” she said.

“They've brushed off many concerns with the excuse of weight. I had what I'm pretty sure was a false negative COVID test this summer and they kept just saying my weight was affecting my asthma (which I didn't even have to any sort of extreme beforehand), even though I'd actually recently lost 20 pounds. It was humiliating, defeating, demeaning, dehumanizing, and quite terrifying while I was so sick to be dismissed so quickly because of my BMI.”

Bee, who has Polycystic Ovarian Syndrome (PCOS), said that her doctor won’t even further investigate the condition as a result of her BMI. “The only recommendations they'll give me for PCOS is for me to lose weight and take birth control.”

According to Sasha Ottey MHA, MT (ASCP), Executive Director of PCOS Challenge at The National Polycystic Ovary Syndrome Association, this weight bias is all too common and highly reductive, considering many patients do have a weight loss plan or eat well and exercise, yet still deal with PCOS and other endocrine conditions.

“Most health information (like BMI) is based on white men in the early 1800s, but ethnicity, body type, lifestyle, and sex differences need to be included in the care of any patients,” Ottey told Endocrine Web.

As Ottey says, one of the main issues with BMI mirrors one of the main issues in medicine in general. It was designed with white males in mind. According to an recent article published in the International Journal of Epidemiology, BMI has some massive gaps.

For one, the article says, it doesn’t apply to children. It doesn’t reflect women or any non-white group of people — and we know that there are important health differences among ethnic groups. And as mentioned above, it doesn’t accurately take into account the elderly and the many phases of life, or how our bodies change naturally over time.

There is also concern that BMI is inherently racist due to it being based on “ideal” white body types  — and that perpetuating its use upholds its racist origins.

Sabrina Strings, an assistant professor at the University of California at Irvine, told The HuffPost, “It is racist, and also sexist, to use mostly white men within your study population and then try to extrapolate that and create norms and expectations for women and people of color.”

BMI can cause more health problems than it stops

As we can see, BMI definitely has its blind spots — and they are dangerous. A study in the International Journal of Obesity found that over 30 percent of people with “normal” BMIs were unhealthy for other reasons.

On the flipside, many of the participants who were labeled “very obese” by BMI were actually metabolically healthy. But, in this case, what does “healthy” mean? “Using the blood pressure, triglyceride, cholesterol, glucose, insulin resistance, and C-reactive protein data, population frequencies/percentages of metabolically healthy versus unhealthy individuals were stratified by BMI,” the study authors concluded.

Janet Tomiyama, the study’s lead author, told the UCLA Newsroom, “Many people see obesity as a death sentence. But the data show there are tens of millions of people who are overweight and obese according to BMI who are perfectly healthy.”

Medical providers may also neglect potential health issues if a patient’s BMI is "normal." For example, people with "normal" or "overweight" BMIs may not be taken seriously if they are struggling with and seeking help for disordered eating.

Another possible issue: If a woman with a "normal" BMI sees a doctor about not being able to get pregnant, she may not be screened for PCOS, the leading cause of infertility — since doctors may incorrectly assume a smaller person wouldn’t have PCOS.

There is yet another major way BMI can cause a health issue: It can trigger disordered eating.

According to the National Eating Disorders Association (NEDA), BMI reporting used in schools can lead to eating disorders. “The National Eating Disorders Association (NEDA) has concerns about BMI report cards, because BMI screenings, such as the ones done in schools, may be triggering for individuals who are struggling with or vulnerable to eating disorders. The fear is that if an individual is already struggling with an eating disorder, being evaluated in this way may serve as a trigger for continued or worsening disordered behavior.”

Alyssa, 25, from the Bronx agrees. “I have a history of disordered eating, so it's really stressful when the main focus of a medical assessment is on losing weight,” she says. “I prefer when doctors focus on information from bloodwork and other tests. I find it a lot more helpful if there are specific lifestyle changes that can be recommended based on whatever the labs indicate, rather than arbitrarily telling me that I need to 'lose weight.'"

What are better ways of improving and measuring overall health?

Physical fitness and lab test results are more applicable-across-the-board markers of health. According to a recent study done on diabetic patients in Diabetes Care, the connection between exercise capacity and mortality was more telling and predictive than BMI as a measure of present and future health.

The US Department of Health and Human Services recommends adults move more and sit less, and get about 150 minutes (or 2.5 hours) of moderate-intensity exercise per week, at minimum. This includes strength training and cardio. They add, “When adults with chronic conditions or disabilities are not able to meet the above key guidelines, they should engage in regular physical activity according to their abilities and should avoid inactivity.” Translation: Regularly move your body at your ability level any way that feels good for you.

Are there more accurate alternatives to BMI?

Many medical professionals are in favor of measuring waist circumference and height, since fat stored in the belly is thought to be more unhealthy than fat stored on other parts of the body, according to a recent report in BMC Sports Science, Medication, and Rehabilitation.

Athletes tend to focus on overall body fat percentage, using hydrostatic weighing to precisely calibrate body fat. There are also less precise home scales that can still measure body fat better than BMI.

Dr. Cunneen says there should be a more in-depth and well-rounded approach to medical assessments. “Body composition scales, found in many doctors’ offices these days, enable the physician to see much more of the story, including the percentage of muscle, fat, bone, and water in the body. All of which combine to comprise a person’s overall weight and vary greatly from person to person.”

He also says other tests are key: “Add to those readings standard diagnostic tests such as EKG, blood pressure, cholesterol, and blood glucose, and you get a much more complete picture of a person’s overall health.”

We know that BMI is more than imperfect, and that being overweight doesn’t always mean a person is unhealthy. That said, Dr. Cunneen stresses the importance of understanding that obesity, in some cases, is in fact associated with health problems, particularly diabetes, hypertension, cardiovascular disease, and stroke, as well as one of the conditions that increases risk factors for severe cases of COVID-19.

In the end, it’s important to look at a patient’s whole picture

“It’s not enough to say, 'Hey, you need to lose some weight,'” Dr. Cunneen says. “There are reasons — some physiological, some psychological, mostly a combination of both — that contribute to obesity, including hormones and the way that our brains are wired that may trick our bodies into the physical 'need' to eat certain foods in amounts that adversely affect weight loss efforts.”

Patients are more than their weight and deserve to be seen and treated as whole and valuable, not just a number on an index. Studies and research have repeatedly confirmed that when medical professionals treat patients who are struggling with obesity with the same level of consideration and objectivity as they would for a person dealing with any other disease, it also helps them achieve medically advised weight loss. So, killing off BMI and adopting a more nuanced, people-first methodology of treating patients is win-win in terms of patients getting more effective treatment and reaching medically advised weight loss goals. May it rest in peace. 

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