Health is about so much more than height and weight.
As the conversation around diet culture continues to evolve, people are starting to question the accuracy of body mass index (BMI). It’s not entirely certain that BMI is the main predictor of metabolic syndrome, which refers to heart and metabolic problems that increase the risk of disease.
So, What is BMI?
You can calculate your BMI by taking your weight in kilograms and dividing it by your height squared in meters. In other words, kg/m2 = BMI. (You can use an online converter for pounds and inches.)
The result puts you into four categories:
- Obese (>30)
- Overweight (25.0 to 29.9)
- “Normal” weight (18.5 to 24.9)
- Underweight (<18.5)
However, only 25 percent of people in the United States fit into the “normal” category, so is it actually representative of the U.S. population?
Well, here’s where it came from:
- First, a Belgian scientist named Adolphe Quetelet reviewed weight and disease statistics in “average” men (FYI, that means white, European men). He concluded that you can calculate a healthy relative weight by dividing the weight (in kilograms) over height squared (in meters). This chart was dubbed the Quetelet Index.
- Then, American physiologist Ancel Keys noticed how life insurance companies in the 1970s were using an index that didn’t factor in the difference in body fat composition as people age. He recirculated the Quetelet Index with the name Body Mass Index as a replacement.
What BMI Misses In the Full Picture of Health
Fat distribution has different purposes in men and women. Fat stores on women may help with reproduction, for example. Plus, there are variations in fat distribution across different races. However, these differences aren’t factored into BMI. As a result, women and Black patients are more likely to fall in the “overweight” or “obese” categories, even when they may be otherwise healthy.
In general, each health measurement can only tell you a piece of someone’s overall health. People with the same BMI can display different levels of illness and health risk. It’s possible to have an “overweight” or “obese” BMI, and yet have healthy insulin production, blood pressure and sugars, and waist circumference, and vice versa.
Similarly, BMI also misses some individuals in the “normal” or “underweight” categories who actually have other risk factors for illnesses. Visually “thin” people can have dangerous amounts of visceral fat surrounding their organs. They may also have high blood pressure and high cholesterol. (Learn more here about how you can’t tell someone’s health by looking at them.)
When it comes to weight as a predictor of illness, waist circumference might be more accurate than BMI. That’s because weight and visceral fat held in the belly are thought to be related to future metabolic disturbances. Visceral fat (fat surrounding the internal organs) is linked to increased inflammation, so it may be harmful to the body’s organs over time.
Why We Need to Look More Than Skin Deep
Aspiring to reach a certain arbitrary BMI may affect mental health. This is particularly concerning in a society with increased disordered eating and body dysmorphia (distress with your appearance). In many cases, obsession with BMI may lead to low-calorie or unsustainable fad dieting, rather than a total shift in wellness. Studies have shown that “yo-yo dieting” may actually increase the risk of disease over the long term.
Some doctors are now turning to other ways to measure health in patients. For example, they can check certain blood markers and physical capabilities (like being able to climb stairs without feeling out of breath). This may help prioritize building healthier habits around food, exercise, and sleep—instead of only worrying about the number on the scale.
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