It’s the moment many of us dread.
The doctor, after glancing at a number on your chart, suggests you lose some weight. Then a nurse hands you a photocopied list of healthy foods — as if it never occurred to you to eat fruits and vegetables.
No one asked about your diet or fitness routine. They don’t know if you can run a 10K, do a pullup, or lead a yoga class. All they did was look at your body mass index (BMI) and pass judgment.
The truth: BMI doesn’t define your health — it’s one small part of a larger health story. Now’s the time to change that narrative.
How did scientists come up with BMI?
The concept of BMI dates back to the 19th century, when a Belgian statistician named Adolphe Quetelet set out to find the population’s average size and shape. (And by “population,” he meant White men of military age.) He believed that the closer a person was to those averages, the closer they were to perfection.
His “Quetelet Index” of weight divided by height was picked up and simplified by American nutrition scientist Ancel Keys in the early 1970s. Coincidentally, that’s around the time American body weights began to climb — a trend that would soon be described as an obesity epidemic.
When did BMI become an accepted health marker?
Doctors have known about the link between body weight and health for as long as there have been doctors.
But it wasn’t until the 20th century that life insurance companies began using height-to-weight tables to codify risk, trying to flag applicants whose policies were likely to pay out early.
The U.S. government adopted BMI in 1980, with the goal of establishing cutoff points for a “healthy” or “normal” weight vs. “underweight,” “overweight,” or “obese.” (You can find the current cutoff points here.)
How did BMI become so ubiquitous in health care?
Blame researchers — they made the number stick.
Governments across the globe have compiled decades’ worth of BMI data, used to track changes in their populations. Those publicly available statistics allow researchers to correlate BMI with health outcomes.
For example, we know that higher BMIs are consistently linked with a higher risk of heart disease, type 2 diabetes, cancer, arthritis, and many other health conditions.
But the problems with BMI as a health metric remain.
Why is BMI controversial?
BMI was never meant to assess individual health, mainly because it only represents one thing: a person’s weight relative to their height.
It can’t tell you how much of that weight is fat vs. lean mass (which includes muscle, bone, water, and everything else that isn’t fat).
It also can’t tell you how an individual’s fat is distributed. We know belly fat (the classic “apple” shape) is more associated with health risks like heart disease and type 2 diabetes, compared to fat stored elsewhere, like the hips and thighs.
Another problem: A “normal” BMI is anything but. The average U.S. adult has a BMI of 30 — just above the cutoff for obesity, a health condition that more than 40% of Americans have.
Are there better ways to assess individual health risks?
Yes, absolutely.
A simple measure of waist circumference can tell you a lot. If a woman’s waist size is less than 35 inches, measured at the belly button, she probably has a healthy body fat percentage, which is defined as 35% or less.
Other measurements, like waist-to-hip or waist-to-height ratio, can give a reasonably accurate snapshot of your body composition. Most accurate is a DEXA scan, an imaging test that also measures bone density.
But even more important than body fat are metrics like resting heart rate, blood pressure, and fasting blood tests that measure things like cholesterol, triglycerides, and blood sugar.
A doctor can also ask how you feel. Is body weight causing problems, like joint pain or sleep apnea? Does it limit your daily activity, or cause social anxiety?
If BMI has so many issues, why do doctors still use it?
Because it’s easy, reliable, and 100% reproducible. Someone who’s 5-foot-4 and 160 pounds has a BMI of 27.5 — right in the middle of the “overweight” category — no matter who’s doing the calculations.
BMI is also a zero-touch metric, avoiding the potential awkwardness and human error of measuring a patient’s waist or hips.
It’s also used by medical professionals to diagnose obesity, which is required by health insurance companies that cover GLP-1 medications prescribed for weight loss. BMI is baked into the system and difficult to extricate.
And as imperfect as it is, BMI can offer some insight into a person’s health.
A BMI of 30 or above is strongly correlated with a high body fat percentage, and a recent study found that 61% of people with obesity have at least one obesity-related health condition.
But as we mentioned above, it’s only one part of the story.
What does BMI miss?
The latter study found that half of people with a “normal” BMI (between 18.5 and 24.9) also have at least one obesity-related condition.
Another study found that 38% of women in that BMI range had a body fat percentage of 35 or above.
If a health care provider assumes a patient’s BMI is a reliable proxy for their overall health, they could easily miss an elevated risk of heart disease or diabetes. A person with a smaller frame and high body fat might also eat poorly and not exercise much. But a doctor would be much less likely to send them home with a list of healthy foods.
How should you assess your own health, regardless of BMI?
Look at your diet. Is it mostly foods you might see on doctors’ printouts: fruits, vegetables, whole grains, lean proteins, healthy fats?
Look at your physical activity. Do you regularly walk, run, ride, or swim? Do you work out in a gym, or take group fitness classes?
Look at what your body can do. How’s your strength? Can you lift and carry whatever (or whomever) needs to be lifted and carried? How’s your endurance? Can you keep up with your spouse? Your children? Your co-workers?
All those things — nutrition, exercise, strength, aerobic fitness — are associated with a longer, healthier life.
If you feel you’re doing OK in all those areas, your BMI probably isn’t holding you back.
