Body mass index – how did a simple mathematical formula become a contentious marker of health? Today we’re tracing its origins, the pitfalls of its misuse, implications for individuals and public health, and potential future application.
M: I’m Professor Megan
S: and I’m Professor Susan, and we’re
Both: Your Nutrition Profs!
M: We are registered dietitians and college professors who have taught more than 10,000 students about health and nutrition. We have answered a LOT of questions about nutrition over the years –
S: Some questions we get asked every year and some are rarely asked but very interesting.
M: We’re here to share our answers to these common (and uncommon) nutrition questions with you.
S: So bring your curiosity and let’s get started.
Both: Welcome to our class!
M: Hello everyone, we hope you’re doing well. Thanks for joining us. Susan, what question are we tackling today?
S: Well this is a question I get asked periodically and it is “What’s the deal with BMI?” I mean what is it? Is it accurate? What does it actually measure? Should we be using it?.... I mean, what is the deal with BMI? So today we’re going to do our best to answer it.
M: Yes, when discussing nutrition assessment or even general health assessment in class and when discussing body composition analysis, we always mention body mass index or BMI. Most students, and I think most people in general are familiar with at least the term BMI.
S: Yeah, I think that’s true. And when you see a healthcare provider, what’s the first thing they do at the start of every visit? They take your body weight and they might measure your height if they don’t already have it on file. And those two numbers are the basis of your BMI.
M: BMI is used by healthcare providers and even insurance companies to diagnose overweight and obesity but here’s the question… should it be?
S: Hmmm… And when you hear statistics about the percentages of Americans that are overweight or who suffer from obesity, those numbers are based on BMI data. For example, we often hear that such-and-such city is the most obese…that’s BMI.
M: So where did BMI come from?
S: That is such a good question. Where did BMI come from? Well, nearly 200 years ago, in 1832, 23 year old Belgian mathematician, astronomer, and statistician Adolphe Quetelet introduced the concept that was originally called the Quetelet Index.
M: That’s an original name…
S: I’d like to have an index named after me… the Kazen index. He tracked men’s heights and weights at various ages through the lifespan and he was trying to determine what the “average man” was. Now this gets a little technical and confusing, but he theorized that in human growth weight increases at the square of height, if you exclude growth spurts like in childhood and puberty.
M: But this was to define “average”, it was not meant as a tool for individual health assessment. And It’s important to note that his subjects were all white Belgian men.
S: Well, that would make a difference if all your subjects are the same group.
M: So fast forward to the mid 20th century, where Louis Dublin, a statistician and VP of Metropolitan Life Insurance Company noted a correlation between weight of policyholders and morbidity. So morbidity is a sciencey word for having a disease or condition. He noted that more claims were coming from their “obese” policyholders.
S: So Dublin created tables of what he called “normal” weight based on average weight for height, taking small, medium, and large frames into account. The average weight in each category was identified as the ideal weight. And percentages of this weight were used to identify risk.
M: So if you weighed 70 to 100% more than the ideal weight for your frame category, you were labeled as morbidly obese and likely had to pay much higher insurance premiums.
S: But again, this data was based on a population that could afford insurance in the years directly following the Great Depression…
M: …so rich, white men.
S: Yup.
M: Yet, these tables soon became the standard for calculating premiums amongst most insurance companies.
S: But it was in 1972, that the term Body Mass index was introduced by a researcher named Ancel Keys. His landmark study, published in 1958, found that using Quetelet’s height to weight formula “matched” body fat measurements in more than 7,400 men in five different countries.
M: And that is when BMI began to be used in healthcare. Today it’s still used to diagnose overweight and obesity, as a criterion for life insurance policy premiums, and even eligibility for medical procedures.
S: So what exactly is BMI? Well as we said, it’s a ratio of a person’s height to their weight. It’s calculated by taking your weight in kilograms and dividing it by your height in meters squared. There are four categories that are used to classify BMI. The categories are underweight, normal weight, overweight, and obese.
M: The underweight category may indicate that an individual has insufficient body weight for their height which might mean malnutrition or underlying health issues.
S: But not– it doesn’t automatically mean this.
M: Correct.
S: The “normal” weight category may indicate that a person has a healthy weight compared to their height. But I really don’t like that it’s called “normal”.
M: No, it’s terrible.
S: But even the term “healthy” is not much better because one’s height and weight alone cannot identify healthfulness, right? So perhaps we could go with “recommended”?
M: Yeah, I agree that “recommended” is much better than “normal”.
S: Definitely. Well the overweight category can indicate excessive body weight relative to height.
M: And the obese category is actually divided into three subclasses, class 1, class 2 and class 3 which is also known as extreme or clinically severe obesity. You may have heard this last class called morbid obesity but that phrase is no longer used.
S: I agree with that too!
M: Yeah, good. When I ask students in class what BMI is or ask them to define it, the most common response I get is that it is a measure of body fatness or body composition.
S: Yeah I get that too and that’s a really common belief, but guess what? BMI is NOT a measure of body composition. They are two different things.
M: Body composition is defined as the ratio of fat to fat free mass (sometimes called “lean” mass) in the body. Fat free mass is anything that’s not fat so it includes your body water, organs, bone, and muscles.
S: And body composition is expressed as a percent body fat. Like I’m 22% fat or I’m 17% fat or I’m 38% fat.
M: And height and weight are just total numbers. The number of pounds you weigh doesn’t tell you how much of your total weight is muscle or bone or water…or fat.
S: So why use height and weight or BMI if it doesn’t tell you much about your specific health? Well, it’s easy. It’s simple to obtain a person’s height and weight.
M: Right. I mean, most people already know about how tall they are and lots of people have scales in their homes to measure weight, but measuring actual body composition is more complicated…and extremely costly.
S: Right. You can do things like a dual energy x-ray absorptiometry using a DEXA machine, or air-displacement plethysmography measured using what’s called a BodPod. But both of these methods require specialized equipment, trained personnel, and quite a bit of cash.
M: Excellent and good job pronouncing with those sciencey words!
S: Thank you!
M: But BMI is simple, inexpensive, and noninvasive.
S: So what does categorizing a person based on height and weight actually mean?
M: Well research does suggest that the higher your BMI, the greater the risk of developing type 2 diabetes, osteoarthritis, liver disease, high blood pressure, sleep apnea, and even several types of cancer (like breast cancer and colon cancer).
S: And since height and weight are common components of nearly all medical records, these records can provide BMI data on literally millions of people. Then you can compare the types of medical conditions with their BMI and you can find these types of correlations.
M: So that makes BMI useful in epidemiological or population based study.
S: So what's the problem? Well… It does work great when trying to make inferences about large groups of people and to provide some general guidance for risk factors, but it doesn’t give much specific information about an individual’s actual risk of developing a disease.
M: Exactly and BMI is not applicable to everyone. During pregnancy or periods of fluid retention due to some chronic diseases like congestive heart failure or chronic kidney disease, BMI is just not going to be accurate.
S: It may be inaccurate in athletic populations because their total weight is greater due to increased lean tissue or muscle mass. I mean, let’s use Patrick Mahomes, as an example. You know Patrick, quarterback for the Superbowl champions the Kansas City Chiefs , he’s 6’2” and he weighs 225 lb. If we calculate his BMI it’s just over 30 which is the lower end of the obese category… And Patrick Mahomes is not obese, he’s probably one of the fittest people on the planet. And his body composition, and his risk for disease, is much different than a couch potato who would be the same height and weight and BMI.
M: And the elderly often have inaccurate BMIs. With older age, adults tend to lose lean tissue while increasing fat tissue which, of course, cannot be accurately represented by BMI.
S: And some studies even suggest that the lower muscle mass is more of a health risk in the elderly than the higher fat mass. SO that’s why weight training for older adults is so important!
M: And remember, BMI is currently being used widely to diagnose overweight and obesity, in addition to these issues – it’s NOT a measure of body composition.
S: And you know not a lot of people understand the difference. So because of that, BMI has become a sort of catchall for body fat percentage, nutritional status, and health risk.
M: Yeah, but…research suggests that while total body fat is important, the distribution of that fat may be a better marker of metabolic disease risk, so risk for diseases like type 2 diabetes or cardiovascular disease. Fat distribution is about where you store fat on your body.
S: You may have heard people say that “it’s better to be shaped like a pear than an apple”. That’s all about fat distribution.
M: Having an apple shape might mean you have more fat around your abdomen. This is called visceral fat. It accumulates around the abdominal organs resulting in a larger waist circumference.
S: And visceral fat or being apple shaped is more strongly associated with increased chronic disease risk than fat around your hips and thighs that would make you more pear shaped.
M: So if you measure around your waist just above your belly button, that’s your waist circumference. Waist circumference has been proposed as a possible addition or alternative to BMI. It’s simple to measure, it’s straightforward, and it’s cheap.
S: And you can add in hip circumference, a measure around the largest portion of your rear end. And if you divide your waist number by your hip number you obtain waist-to-hip ratio which provides additional information about body fat distribution.
M: You know, there's also evidence that overreliance on BMI can negatively impact disordered eating diagnoses. A study of more than 14,000 young adults found a higher rate of disordered eating behaviors in those classified as overweight or obese compared to those in the normal or underweight categories. Yet they were half as likely to receive a clinical diagnosis.
S: So what you’re saying is, if they had disordered eating patterns, but they were in the overweight or obese categories, they were not likely to be diagnosed?
M: Yes.
S: Yikes!
M: And another issue, as we mentioned earlier, BMI was developed in white adult populations, mostly men and mostly those with money… so not representative of typical Americans…
S: for sure. I mean, research suggests that BMI overestimates disease risk in Black populations and underestimates risk in Asian populations. So using it for things like health assessments or insurance rates in these groups has been associated with perpetuating “racist exclusions” and is harmful.
M: In fact, this past summer (2023), the American Medical Association issued a statement that BMI should not be used as a single measure. Which makes sense….we don’t use a single blood glucose finding to diagnose diabetes…
S: Right.
M: We don’t use a single blood pressure reading to diagnose hypertension.
S: Right. The AMA recommends that due to the “significant limitations associated with the widespread use of BMI in clinical settings” it should be used in conjunction with other valid measures. Things we’ve already mentioned like waist circumference, hip circumference and hip-to-waist ratio.
M: Another option is the Body Adiposity Index (or BAI), so this is similar to BMI but it’s the ratio of height to hip circumference. Or another one, Relative Fat Mass (RFM) which is the ratio of height to waist circumference.
S: There is also A Body Shape Index (or ABSI) and this calculation takes into account waist circumference, height, and weight.
M: So what’s the bottom line on BMI? Should we stop using it altogether? Not necessarily. The key is to recognize its significant limitations and consider it as only a possible starting point for individual application.
S: BMI has been found to be correlated with fat mass for the general public, and white adult men in particular, but it’s less accurate when applied to individuals.
M: So if BMI continues to be used, categories need to be reevaluated to make them applicable to other ethnic and sex groups beyond non-Hispanic white men.
S: The AMA recognizes that it can be useful when looking at large groups and populations, but it loses predictability when applied to a single individual. You should use additional measures to get a better idea of a person’s risk for a specific disease.
M: There are so many options that are easily measured to provide additional guidance for people.
S: So what’s the deal with BMI? It’s just one single factor that may indicate possible risk of chronic disease, but it does not measure body composition and it should not be looked at as a stand alone marker or risk factor.
M: Such a good question. That’s a lot to unpack with BMI.
S: Yeah, a lot more than I thought.
M: Yeah.
S: Well thanks for joining us. Join us next time when we answer the question: What is lab grown meat?
Both: Class dismissed.
S: We hope you enjoyed this episode. You can find the show notes and a list of sources on our website, yournutritionprofs.com.
M: Your homework is to follow us at your nutrition profs on Instagram and to listen to our next episode. You can listen on Amazon Prime, Apple Podcasts, Spotify, YouTube, or anywhere podcasts are found. We’d appreciate it if you’d “like” us, write a review, subscribe, and invite your family and friends to join us too.
S: If you have a nutrition or health question you’d like answered, let us know! We may even do a show about it! Send an email to [email protected] or click on the “Contact Us” page on our website.
M: Thanks to Brian Pittman for creating our artwork. You can find him on instagram @BrianPittman77
Both: See you next time!