The Body Mass Index: Unhelpful at Best, Harmful at Worst

blue tape measuring on clear glass square weighing scale
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Most of us possess at least a bit of familiarity with the famous, or infamous, body mass index. Maybe you learned about it in physical education or health class, or maybe you have seen it on your medical chart after visiting the doctor’s office. Either way, a good majority of people have been acquainted with it before and know that health professionals frequently use the BMI to categorize people into different groups.

After using and calculating the BMI myself in hospitals, mental health centers, and for other nutrition-related tasks, I have come to dislike this metric substantially. People treat the BMI as though it provides a comprehensive picture of a person’s health, but I see it as a flawed calculation that does not carry much genuine merit or practical use.

Although the BMI calculation takes very little time to calculate and explain, I still say it’s a colossal waste of time. Let’s talk more about where the BMI came from, why people, including medical professionals, still use it, and why I deem it faulty.

The History of BMI

Before we get into the pros and cons of BMI and what it can actually tell us (not a lot), let’s take a few moments to investigate where the body mass index even comes from. For those of you that find history boring (how dare you?) please bear with me, as this short summary provides compelling reasons for us to ditch BMI for good. Plus, I see value in knowing where these numbers originate if we insist on calculating them for everyone and their mother.

Adolphe Quetelet, a Belgian mathematician, took a keen interest in statistics and the idea of averages. He collected and analyzed data on all kinds of things, like crime rates, mental illnesses, marriages, and more. Most notably, he had a particular fascination with the idea of an average human body. He was obsessed with bodily measurements, which some attribute to his interest in art and painting.1

Either way, one of his best known projects was l’homme moyen, or “the average man”. The term “average” here doesn’t refer to a ho-hum, boring kind of person. “Average” in this context was more about his perception of the ideal body.

Quetelet collected measurements and data from numerous people from a few different types of populations, and set out to create an equation that would provide some kind of rule of thumb to explain these measurements. He ended up creating the BMI equation we know today, but he didn’t generate it to measure a person’s weight status like many medical institutions do today.

Apparently, Quetelet had no interest in studying or understanding “obesity” or anything health-related. He simply worked to find what one would consider a standard, normal man, statistically speaking, and then study a distribution from there.1 He also investigated the heights and weights of infants and growing children.

Nobody took this equation seriously for medical purposes, or signaled that they intended to use it to diagnose anything. There wasn’t a widespread need to study the relationship between weights and heights about 200 years ago, so that was that.

Interest in body size, as related to height and weight, didn’t really crop up until insurance companies started using these measurements to calculate their premiums. Early adoptions of methods to measure the relationship between weight and illness for insurance purposes were incredibly flawed. There was a severe lack of standardization, so any sort of calculation or measurement taken provided little information.1

In the 1940’s, one insurance company, Metropolitan Life Insurance, released weird “desirable weight” tables for males and females based on height. They consisted of three columns where people could decide if their bodies fit into “small”, “medium”, or “large” frame categories to further narrow down their “ideal” weight. The original tables didn’t account for age, race, or any other meaningful factors.2

Over the decades, Metropolitan Life Insurance’s tables were revamped and refined. Eventually, along came Ancel Keys, the same Keys that ran the Minnesota Starvation Experiment. He decided that Quelete’s equation provided more accurate assessments than the other current measurement offerings, and told the world in one of his papers that it should be called the body mass index.

Since then, the BMI has been used to assign a person’s body to one of the various categories, (“underweight”, “normal”, “obese”) and make judgments about their health. There are a few reasons why it has stuck around for many years, but I’d argue there are many more reasons it’s total BS and shouldn’t be used for any practical purposes.

Before I can roast it to my liking, I will at least cover why we continue to use the BMI.

The Appeals of BMI

People sure love a good rule of thumb. They take comfort in an equation or a formula that spits out an easy-to-digest result, and gives us some kind of concrete number or data point. This, I believe, is the main draw of the BMI.

It takes less than a minute to measure a person’s height and weight, approximately five seconds to calculate, and then another five to find where you lie on a handy chart. It’s a quick screening tool, incredibly inexpensive, and some studies claim the BMI correlates quite well to body fatness.

Many healthcare workers use the BMI as a way to quickly assess the health risks a person may have based on their weight. At the same time, I think many healthcare professionals would come to whatever conclusions the BMI is supposed to help them with just by looking at a patient’s body size. Simply seeing a person in a larger body come through their practice leads many healthcare workers to assume the person in question is at risk for certain diseases, despite whatever healthy habits they may have, like eating lots of fruits and veggies and getting decent amounts of physical activity.

Your physical activity can be strutting down the street with your friend!

Overall, I believe the appeal of the BMI is to try to take a concept that is incredibly nuanced and complicated, like health, and boil it down to one simple metric. It allows healthcare professionals to employ a black-and-white measure, instead of one with more gray area. If you’re in the “normal” category, you’re “healthy”. If you’re in any of the other categories, you’re “not healthy”. It’s like sorting apples from your orchard into buckets of those you can sell, and those you can’t. Black and white, easy peasy.

I know doctors, nurses, and other healthcare professionals do not lack the intelligence to understand that simply being in the “overweight” category does not make someone unhealthy. But that concrete knowledge may not be enough to overcome unconscious biases that come about in regard to people in larger bodies. Having a number, a data point, tell us that a person is considered “overweight” or “obese” may affect how a patient is treated.

Many, including healthcare workers, take that number and position on a chart at face value, letting it speak volumes about a person’s health status. In turn, discussions of weight and BMI dominate appointments with patients who may be there to see a doctor about a completely unrelated topic or condition. As a result, many people in larger bodies decide to avoid seeking healthcare altogether.

The Drawbacks of BMI

The BMI has dozens of flaws, maybe hundreds. But for the sake of this post, I want to focus on four of the main problems I see with its widespread use in clinics and elsewhere.

BMI Doesn’t Differentiate Between Muscle, Fat, and Bone Mass

A person’s BMI is calculated using their weight and height, that’s it. There are no other physical measurements, like waist circumference or bone density to account for differences in physique. As a result, people with higher muscle mass or greater bone density may register as “overweight” or “obese”, even if much of their weight does not come from fat.

This means that athletes see an increased chance of ending up in the “overweight” or “obese” categories of the BMI despite having very little body fat. Michael Phelps and Tom Brady likely register as “obese” using the BMI, for example, even though other measures would show that most of their mass is from dense muscle and not fat.

Another person, athlete or not, may also have a good deal of muscle mass, but it is covered with a bit of fat. That’s perfectly normal. They may not have a six-pack or look like a model, but common sense would tell us this shouldn’t be a problem health wise. This person may also receive the label of “overweight” or “obese”, and that layer of fat may prompt healthcare providers to criticize them about their weight.

Okay, but who does this actually affect besides athletes and particularly muscular people? Turns out, those who are not White may be receiving even less accuracy from a BMI calculation than their White counterparts.

This Makes BMI Even Less Helpful for People Who Aren’t White

The BMI calculation was developed from data Quelete collected exclusively from White people.3

This is problematic because people of different racial groups tend to have slightly different statures, muscle mass, and bone density. Black individuals, for example, generally have a higher bone density and muscle mass than White folks. They may have less of a risk of developing osteoporosis as a result, which is a great thing! But it may also make Black people more likely to be deemed “overweight” or “obese” at the doctor’s office because their calculated BMI differs from the “normal” based on a population of a different race.

In fact, Black folks have the highest prevalence of “obesity” in America, but this is totally based on how they compare to the white people Quelete used to create the equation. On the other hand, Asian populations tend to have lower bone density and muscle mass compared to other ethnicities. As a result, comparing this group to data based solely on White men may also skew their BMI results in the opposite direction as Black people. This is also not helpful in a practical sense.

Who says that what is normal or average for a White man should be used to assess the “health” of people of other ethnicities?

Most of us who took science classes were taught to use a healthy bit of skepticism when reading a study that only uses a homogenous population as the subjects. If a study finds that eating a 1/2 cup of pineapple a day reduced subjects’ risk of getting cancer by 90%, that would certainly be celebrated. But if only White men participated in the study, how do we know these results also translate to Black women or Latino men unless we test those groups as well?

Lots of studies will take time to acknowledge a lack of diversity in their study populations during their conclusions, somewhere near the obligatory “more research is needed” addendum. The best studies are conducted with large and diverse populations that include both sexes, as many ethnicities as possible, and a wide range of socioeconomic statuses, to name a few.

One way to apply this argument and make the BMI more “accurate” would be to redo Quelete’s measurements, but this time on a super diverse and inclusive population set. From there, we could create a new equation that better fits that data and could realistically be used on people of all different ethnicities. Another strategy would be to collect data from specific racial groups and create different BMI equations for each. There are several glaring problems with that idea too.

These “solutions”, in my mind, still wouldn’t produce a metric that actually tells us anything useful. Perhaps these hypothetical equations would yield results more closely aligned to bodies of all or different ethnicities. But they still would not give us a clear picture of how “healthy” a person is solely based on their height and weight, which is how the BMI is incorrectly used all the time.

My favorite solution to fix this problem is to just toss BMI into the garbage, or leave it behind like a toxic ex. Especially since BMI was never intended to be used on individuals, only populations.

The Equation Itself Isn’t Appropriate for Assessing Individuals

Those of us who are in the dietetics field, or other health/fitness-related industries, know the equation by heart. For those of you who aren’t as familiar, it goes like this:

Metric

Weight (kg)/Height (cm)2

Imperial (American)

(Weight (lbs)/Height (in)2) x 703

Notice anything that seems a bit weird about these equations? I never thought much about them, I just calculated them thousands of times throughout high school, and especially college. One day, I realized that the squared part on the bottom of the equation bothered me. Why do we square our height? I thought maybe cubing it would make more sense, because our bodies are 3D objects after all. After some research, I found that Quelete used the square over heights because that allowed the equation to fit his data better.

In the world of science, it’s totally improper to change things around so they fit your data better. I could have gotten kicked out of my chem labs in college if I had changed around equations I was following in order to better fit the pathetic yields I often got. Scientists and researchers don’t typically have patience for engaging in this kind of tom foolery.

At the same time, remember that Quelete wasn’t trying to create an “obesity” index. He was trying to make a method of relating an individual’s height to their weight and a calculation that could be used in population studies. He even specifically stated that the BMI was not useful for individuals, but could help us understand weights and heights in overall populations.

Let’s take a minute to digest this.

Population studies are studies that investigate a group of individuals from the general population that have a shared physical characteristic. Maybe that characteristic is age, sex, weight, or presence/absence of a disease. They often assess whether a drug is effective for certain diseases, or if a particular set of people is more susceptible to particular illnesses.

Population studies, or data, can also give us a snapshot of what a current population may look like. For example, Statista reports that the average household had 1.93 kids in 2020. This doesn’t mean that most households in the data literally have one full kid and then 0.93 of a kid. Instead, this data shows that, on average, families have about 2 kids. This, I hope, is fairly obvious.

Taking data sets from population studies and applying them to individuals makes as much sense as thinking that the average family actually has 1.93 kids. Or that people with 0 or 4 kids are WAY outside the norm, and they deserve a diagnosis of some kind. This is what we do to people who lie outside of the “normal” BMI of 20-25.

The BMI could feasibly be used to assess different populations and see how they differ, on average, compared to other populations instead of individuals. For example, you could see what biological differences exist between soccer moms and cheer moms. Maybe you obtain the heights and weights of everyone in both groups. From there, you could calculate the average BMI for soccer moms and cheer moms and see if they are significantly different.

What you could not do is pluck one soccer mom out of the sample, calculate her BMI, compare it to the average BMI for soccer moms, and deem that she is wildly unhealthy based on this one metric compared to other soccer moms. Comparing one soccer mom’s BMI to one cheer mom’s BMI also wouldn’t tell us anything worthwhile.

When it comes down to it, BMI is based on data sets that measured lots of people’s heights and weights, and it created an index based on an average. So actually, all the BMI does is compare a person’s weight/height2 to the average of the rest of the population. Those between 20 and 25 are closer to average, the rest are a bit further away. That’s it. Is it useful? I don’t think so. Especially since fat people, and their doctors, already know that they weigh more than average.

Even still, as soon as one hits the threshold of “overweight” or “obese” of the BMI, this gets noted on their medical chart, and many doctors automatically start talking about the importance of weight loss. Some charting software even has an alert that pops up to grab a doctor’s attention when a patient’s BMI is over “normal”. This brings me to my next point.

It Slaps Unnecessary Labels on People That Can Trigger Disordered Eating

If your BMI lands at 24.9 (the upper threshold of what is considered “normal”) you had better not gain a single pound, otherwise you might eventually end up in “overweight” and “obese” category. And our society says those two things are among the worst you could ever be.

That’s dramatic for sure, but I have felt this way before. I have had friends feel this way too. They’re worried about being in the upper bracket of what is deemed “normal”, or they feel horrible about themselves when they register as “overweight” or “obese”. Your Fat Friend, a formerly anonymous author, describes the label of “morbid obesity” from the BMI as feeling like a scarlet letter.

She goes on to say:

It has become not only a referendum on my size, but also on my health and subsequently my character. The logic is ruthlessly consistent: anyone my size must have committed a series of unforgivable acts. I must have let myself go. I must be pathological in my need to eat, my greedy desire to stay still. This is a pathology deserving only of disdain, never empathy. Clearly, I have been derelict in my duty to keep myself thin.

Those words hit me like a pile of bricks. We use a virtually meaningless calculation to assign numbers and labels to people’s bodies, and those labels can greatly affect how medical professionals view and treat us, and how we view and treat ourselves. She’s also right about how people look at a person’s body, or their BMI, and make sweeping assumptions about their character.

Many people in larger bodies avoid going to the doctor because they don’t want to get weighed, or hear the same old speech from their doctor about how they really need to lose weight. Doctors, or other healthcare professionals, may launch into a stale lecture about how they need to adjust their diet and exercise, and many don’t bother to learn that some of their fat patients already have healthy habits.

The lack of listening, compounded by half-assed lectures from your doctor can be defeating and annoying. Unfortunately, it can also a trigger downward spiral toward disordered eating for those in recovery, or it can introduce disordered eating behavior in those who have not previously experienced it.

Some might think, “Oh, that’s too bad. But if you just explain to your doctor that you’re in recovery and you don’t want to talk about weight, they’ll respect that.”

I believe that to be true the majority of the time, but sadly, many doctors just cannot get past weight.

In the incredible book Happy Fat (affiliate), Sofie Hagen describes the frustration she felt at the doctor’s office after she specifically explained that she was recovering from an eating disorder, so she didn’t want to get weighed. Sofie even said something along the lines of, “You can tell I’m in the obese category just by looking at me, we don’t need to do this.” Her doctor insisted and said the number on the scale out loud before launching into her own speech about weight loss. Hagen then spent a few days, understandably, wanting to fall back to her disordered eating habits. Luckily, she was able to overcome those thoughts and feelings within a few days.

Even still, a few days of disordered eating after a doctor’s appointment are a few too many, and not every person will be able to bounce back so quickly. Besides risking a relapse into disordered eating, the way doctors view patients with high BMIs may also lead them to ignore other health concerns.

Actual Health Concerns May Get Ignored

Healthcare professionals often use the BMI to “diagnose” a person with obesity. People who receive this diagnosis are treated as diseased and inherently unhealthy, but having fat, or being in a larger body, isn’t a disease. Bodies have always naturally been different, and they always will be. Nobody, not even a healthcare professional, can truly assess another person’s health simply based on their body size or related metrics, like the BMI.

In fact, one huge study from UCLA found that 54 million Americans who are considered “overweight” or “obese” by the BMI’s standards had perfectly healthy metabolic measures. Those other measures included blood pressure, blood sugar, cholesterol levels, and other standard tests. Almost 50% of Americans that were considered “overweight” were metabolically normal, as were nearly 30% of “obese” people. On the other hand, about 30% of individuals who were in the “normal” category showed unhealthy metabolic measures.4

According to this study, using the BMI as a diagnostic or screening tool incorrectly labeled over 74 million Americans as healthy or unhealthy, simply associating a person with a “normal” BMI as “healthy” and “overweight” or “obese” people as “unhealthy”. Again, being fat is not a disease. People who are in larger bodies but do not have an eating disorder or chronic disease are often healthier than thin people who do have these issues or do not engage in healthy habits like eating fruits and vegetables and exercising regularly.

I don’t know about you, but using a measure or calculation that incorrectly labels that many people just doesn’t seem all that worthwhile. Especially since these labels can cause doctors to ignore other health concerns that patients in larger bodies may have. I have read so many accounts of fat patients going in to see their doctor about a broken toe, skin rash, or UTI, only to be given a lecture about their weight. They try to talk about the issue they came in for, but their doctor continues dismissing their concerns and focuses on weight instead.

Me to doctors that keep harping on weight and size when other matters should be attended to

I saw this happen in the hospital I interned at quite often too. Whenever a chart alerted a doctor about a patient’s BMI in the “overweight” or “obese” categories, I was encouraged to go talk to them about their diet and teach them how to lose weight. This was true even if they were there for dehydration or pneumonia.

Dietitians are often associated with weight loss, which is why many patients declined to see me in the first place. They’re in the hospital, feeling ill and not wanting to be there, and suddenly this young dietetic intern comes in and is told to talk about weight and diets. Patients in larger bodies must have heard these things hundreds of times from healthcare professionals, and I bet it is utterly defeating and exhausting, especially when you are feeling unwell enough to be in the hospital.

Everybody deserves to feel heard and understood by their doctor, nurses, and other healthcare team members, and nobody should have to endure weight loss lectures during each visit to receive healthcare. Unfortunately, it can be excruciatingly difficult to speak up and make it known that you don’t want to hear it, and this can ultimately leave you being labeled as “fussy” or “non-compliant”. Sometimes, it just feels easier to let your doctor deliver the lecture so that you can just go home. Even still, this can discourage many people in larger bodies from ever returning to get preventative healthcare.

Doctors dismissing legitimate concerns in place of a person’s weight or size is an incredibly serious issue, and it’s one that doesn’t get enough attention. I see using the BMI to place people in certain categories as just another way to encourage doctors and healthcare professionals to focus on weight more than other issues that might be present.

The label or diagnosis of “overweight” or “obese” can really stick with healthcare professionals and give them tunnel vision, not allowing them to see the bigger picture of health or the complexities of people’s lives. Dropping the BMI will definitely not solve all of these issues, but it may help doctors focus on other, more important matters.

In Conclusion

Okay, I made my case against the BMI being used for medical purposes. When it comes down to it, the BMI can be used as a small part of a health assessment, but I think it is generally taken much too seriously. It can be jarring to see a number on your medical chart that puts you in a certain category, especially when that category is full of stigma and negative connotations for no sound reason.

Maybe the BMI can help a medical professional determine the risk a person has for some diseases, but overall, there seems to be many more cons to using it than pros. As I frequently do, I would encourage healthcare professionals to really think about why we continue using it and what value it actually provides. Does it actually provide tangible benefits? Maybe. Can it also trigger disordered eating, body image woes, and negative stigma? Absolutely.

Any time we implement a policy or rule that affects millions of people every year, we have to be sure it actually provides enough value to offset the potential harm it can cause. I just don’t believe that the “benefits” the BMI can provide actually do more good than harm.

As Always, a Book Recommendation

Health at Every Size is a classic text imperative to one’s understanding of the lie that is the “obesity epidemic”. The notion of the “obesity epidemic” might make an alluring headline, but it can also cause great harm by perpetuating common stigmas about bodies and people who have larger ones. Sadly, even healthcare workers lack immunity to these stigmas.

Lindo Bacon has extensive experience with researching and writing about why size is not a reliable health metric, and Health at Every Size is the first book they wrote on the topic. The phrase “Health at Every Size” may feel confusing and contradictory, but Lindo will show you that body size and health may not be as related as most of us have thought for decades. If you have already read Health at Every Size, also be sure to check out Body Respect and Radical Belonging, Lindo’s other books on similar topics.

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Sources:

  1. Adolphe Quetelet (1796–1874)—the average man and indices of obesity
  2. For Researchers on Obesity: Historical Review of Extra Body Weight Definitions
  3. The BMI Is Racist And Useless. Here’s How To Measure Health Instead
  4. BMI mislabels 54 million Americans as ‘overweight’ or ‘obese,’ study says

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