Why the body mass index can’t determine your health
Why the body mass index can’t determine your health
The Body Mass Index has long been used as a medical screening tool, but it could be causing more harm than good.
Body mass index (BMI) is a ratio of weight in kilograms to height in metres squared, which aims to estimate an individual’s body fat. The results are categorized into ranges labelled “underweight,” “healthy,” “overweight” or “obese,” and with these terms come assumptions about one’s health and habits.
BMI was never intended to be used as a marker of health.
Back in the 1830s, Adolphe Quetelet, a Belgian mathematician, astronomer and statistician, developed the concept of BMI as a way to measure the mathematical mean of a population. His goal was to come up with the average size of a man, and he did this using data he had on French and Scottish subjects. Therefore, the “average” BMI was just that, an average size, which said nothing about those above or below the typical standards being in poorer health. He was clear that BMI was intended as a population-level statistical measure, and was never meant to be used at the individual level to indicate one’s level of health.
Later in the 1900s though, BMI began to be used in doctors’ offices at the individual level as an indicator of body fat and health. This is when the BMI categories came into play:
― less than 18.5 kg/m2—underweight
― 18.5 to 24.9 kg/m2—healthy weight
― 25.0 to 29.9 kg/m2—overweight
― 30 to 34.9 kg/m2—obese class 1
― 35 to 39.9 kg/m2—obese class 2
― 40 kg/m2 and above—obese class 3, also known as severe obesity
However, this simplistic ratio doesn’t do a very good job at measuring what the index claims to measure—body fat. BMI doesn’t differentiate between body fat and fat-free body mass, which includes muscles, bones, organs, tendons, blood and everything else besides fat. Fat is less dense than other body tissues, especially muscle and bone, and therefore it weighs less. So essentially BMI will overestimate body fat in people with a lot of muscle and good strong bones, and as a result, active, muscular people are often misclassified as overweight. Alternatively, it will underestimate body fat in people who have lost muscle and/or bone density, for example, older adults.
BMI also doesn’t take into account the distribution of fat on the body or, in other words, where you carry your weight. Research shows that body fat that accumulates around the midsection or abdomen (the “apple-shaped” body type) is associated with a higher risk of type 2 diabetes, cardiovascular disease and mortality, compared to body fat that accumulates around the hips and butt (the “pear-shaped” body type). This can be calculated using a waist-to-hip ratio and can tell us more about someone’s health risk as it pertains to body fat than BMI.
BMI categories can’t be generalized to everyone the same way.
Genetics, ethnicity and gender all play a massive role in determining someone’s body shape and size, but remember, when Quetelet did his statistical analysis, he used data strictly from white European men, which means the application of BMI categories shouldn’t be extrapolated to include people of other ethnicities and women. Research shows that among black women, a higher BMI cutoff is associated with the health risks of obesity, like hypertension, type 2 diabetes and cardiovascular disease, while this cutoff is lower in Asian populations, meaning a BMI in the “healthy” category can still be associated with these risks. Another example of the limitations of BMI is that the same categories are applied to both men and women, despite the fact that women biologically tend to have more body fat than men.
“Overweight” and “class 1 obesity” BMIs are not associated with higher mortality.
It’s true that higher body fat is linked to higher rates of cardiovascular disease, type 2 diabetes, hypertension and some cancers. A systematic review and meta-analysis shows that a BMI in “class 2 obesity” and “class 3 obesity” categories was associated with higher all-cause mortality compared to a “healthy” BMI. However, what may come as a surprise is the fact that a BMI in the “class 1 obesity” category showed no higher
risk for mortality than those in the “healthy” range. In fact, those in the “overweight” category were at significantly lower mortality risk than people at the “healthy” BMI category.
When it comes to health, it’s more important to focus on good behaviours than BMI. In fact, healthy lifestyle habits are associated with a significant decrease in mortality regardless of someone’s baseline BMI. Research shows that when individuals engage in healthy practices (including eating five or more servings of vegetables and fruit a day, exercising regularly, consuming alcohol in moderation and not smoking) the difference in mortality risk between the “healthy,” “overweight” and “obese” BMI categories was almost nonexistent.
BMI doesn’t differentiate between body fat and fat-free body mass, which includes muscles, bones, organs, blood, tendons, and everything else besides fat.
Why do we still use BMI then?
Basically, BMI is used because it’s easy to calculate and measure. Most doctors don’t have the time or resources to do a more in-depth assessment that would include diet, activity, ethnicity, family history, waist-to-hip ratio and other factors.
How can BMI do harm?
Some argue that it’s a screening tool and nothing more, and downplay the importance of recognizing BMI as harmful and stigmatizing, but let’s consider how having a BMI that labels someone “overweight” could be damaging. Imagine a woman, let’s call her Sheila, who is 45 years old. She’s very active and includes some kind of movement that she loves on most days, including spin classes, yoga and walking her dog in nature. She eats consistent, balanced meals which, the majority of the time, include plenty of vegetables and fruit, lean proteins, whole grains, fish, nuts, seeds and legumes, and in general she has a very positive relationship with food. She doesn’t smoke, and she occasionally drinks a small amount of alcohol. She gets adequate sleep, manages stress through meditation, breathing exercises and journaling, and has a strong social support system. Her biomarkers of health, including average blood sugar, cholesterol, triglycerides and blood pressure, are all in the normal range. Sheila’s weight has been stable for years, and her body type is the same as her mother’s. Based on her height and weight, her doctor tells Sheila her BMI is 29, or high end of the “overweight” category.
All the information we have about Sheila indicates that she’s in excellent health. This is a prime example of how “overweight,” based on the BMI scale, doesn’t equate to “unhealthy,” and attempting to lose weight would be unnecessary and most likely detrimental to Sheila’s health. Ultimately, if Sheila were to embark on a weight-loss program, the consequences could be a decreased metabolism, disordered eating behaviours and a negative relationship with food, not to mention feeling like a failure when she inevitably can’t sustain the energy-restricted diet. Sheila is much better off accepting her body at its “overweight” BMI, and focusing on continuing her healthy lifestyle. Unfortunately, that’s not the advice given to most people who fall in that category. This is important as research shows that a majority of dieters who attempt to lose weight are unsuccessful in maintaining that weight loss over time, and the process of losing and regaining weight may have worse health consequences than simply maintaining a higher BMI. Ultimately, BMI is one outdated and very flawed indicator of health. Focusing instead on sustainable, health-promoting behaviours will go much further in the pursuit of overall well-being and fitness.