Body Mass Index (BMI) is a way to group individuals into categories based on their height and weight. It’s a simple formula: Kg/m2. Divide your weight in kilograms over your height in meters squared and you’re left with a number. This number places you into a category that you’ve probably seen and heard before from doctors, peers, teachers, and maybe Wii Fit.
But that begs the question, where did this formula come from and what is it based on? How were the categories developed? How did it come to pass that we use BMI so heavily in our healthcare system? These questions and more are important to discuss so we can all make a decision about where BMI belongs when we have conversations about health, weight, body size, and nutrition – if it belongs in these conversations at all.
BMI chart (source: nhlbi.NIH.gov)
Adolphe Quetelet is the physicist credited with the idea that weight increases with height squared. This measurement was known as the Quetelet Index until 1972 when the name was changed to body mass index (aka BMI). Quetelet dedicated his career to the application of statistics to society, which earned him the title of a pioneer in social sciences1. Quetelet is often credited with describing the “average man” through his use of the Quetelet Index in his research, however, his career took place in Europe during the nineteenth century and centered primarily on white men. Though he was a social scientist, his attempts to describe the average man through BMI has left a significant impact on public health2.
Ancel Keys is the physiologist that picked up BMI where Quetelet left off. Keys is a renowned physicist who is credited with describing the relationship between cholesterol and heart disease, created K-rations for the military, and studied starvation and the impact it has on the body in the Minnesota Starvation Experiment during WWII3. Body mass index was one of the anthropometrics used by Keys in his Seven Countries Study to categorize participants based on body size. This is one of the reasons that BMI became so popular in our modern healthcare system.
Today, BMI is a part of our daily lives. It’s taught in most basic health and nutrition courses, there are BMI charts in gym bathrooms, some insurance policies use BMI to determine rates, and it’s even in some fitness focused video games. BMI has become one of the quickest ways to categorize body size – and for some, it’s how they determine health status. Most of us know our BMI, and if you’ve ever been on a diet, tracked your macros, or taken any interest into changing what you eat in pursuit of weight loss, you may have found yourself somewhat obsessing over that number.
Even though it’s used everywhere, there are valid criticisms of BMI that call into question how useful the number is. The first and most often recognized issue is that BMI doesn’t take into account muscle mass, fat mass, frame size, and water weight. Muscle tissue weighs more than adipose (fat) tissue, so someone who is active and has more muscle on their body may fall into the “overweight” or “obese” category, even if they are at a healthy weight for their body7. The second issue is that BMI was developed using white European men. Remember, Quetelet developed BMI by doing research in nineteenth-century Europe, meaning the population he used to develop the formula and categories are from just one area of the world and all from one race and gender. Body size and body composition varies between genders, with women having a naturally higher body fat percentage than men, which makes using BMI as a tool to categorize everyone problematic. The third issue is that BMI is often used as a justification for weight-based discrimination. As previously mentioned, BMI can be used to categorize people based on body size, which can be used as a justification for weight based descrimination4. Another example of this is the practice of sending home “BMI report cards” with public school students. These cards are designed to raise parents’ awareness of their child’s health status, but have been criticized for the potential harm they can cause in terms of weight discrimination and bullying between students6.
So, where does BMI belong. That answer depends on who you ask. Health providers have used BMI as a basic assessment of body size, but this doesn’t mean that BMI is inherently a marker of health or that it should be used as such. Health professionals who use BMI should be mindful that, though it does give an idea of a patient’s size, it doesn’t give insight into overall health and is by no means a comprehensive measurement. For someone not in healthcare, BMI doesn’t mean much as a standalone number. Before putting any meaning on BMI, consider your relationship with food, your body, and how you feel. Those pieces of the health “puzzle” will hold more meaning than BMI.
|Provides a quick and simple method of assessing body size.||Does not take into account body fat, muscle mass, or water weight.|
|Provides a standardized way to assess risk for certain health conditions.||Used frequently in non-healthcare settings to assess overall health.|
|Provides standardized measurements across the board.||Can easily overestimate risk for some (ex. athletes, people with larger frame size, etc.)|
1. Eknoyan G. Adolphe Quetelet (1796 1874) the average man and indices of obesity. Nephrology Dialysis Transplantation. 2007;23(1):47-51. doi:10.1093/ndt/gfm517
2. Faerstein E, Winkelstein W. Adolphe Quetelet. Epidemiology. 2012;23(5):762-763. doi:10.1097/ede.0b013e318261c86f
3. Oransky I. Ancel Keys. The Lancet. 2004;364(9452):2174. doi:10.1016/s0140-6736(04)17578-8
4. Rasmussen N. Downsizing obesity: On Ancel Keys, the origins of BMI, and the neglect of excess weight as a health hazard in the United States from the 1950s to 1970s. Journal of the History of the Behavioral Sciences. 2019;55(4):299-318. doi:10.1002/jhbs.21991
5. Roser M, Appel C, Ritchie H. Human Height. Our World in Data. https://ourworldindata.org/human-height. Published October 8, 2013.
6. Thompson HR, Madsen KA. The Report Card on BMI Report Cards. Current Obesity Reports. 2017;6(2):163-167. doi:10.1007/s13679-017-0259-6
7. Body Mass Index Advantages and Disadvantages. LIVESTRONG.COM. https://www.livestrong.com/article/40808-body-mass-index-advantages/.