Why Does My Doctor Keep Using BMI When I Can See My Abs?
Have you ever walked out of a doctor's appointment feeling more confused than when you walked in? I did. Last month. I was there for my annual physical — the one I actually keep because I spent four years running body-composition testing and I know what happens when you skip them. My vitals were fine. Blood pressure: 112/74. Resting heart rate: 58. I deadlift 1.4 times my body weight on a bad day.
Then my doctor — who is lovely, truly, and means well — looked at my chart and said, "Your BMI is 26.2. You're in the overweight category. We should watch that."
I looked down at my stomach. I could see the outline of my obliques. Not a six-pack — I'm not that person — but visible muscle definition. I have 19% body fat according to the DEXA scan I did three weeks ago at the studio where I used to work. Nineteen percent. For a woman my age, that's athletic. That's the category where bone density is high and metabolic risk is low.
But my BMI said overweight. Because I'm 5'6" and I weigh 162 pounds. And the BMI formula — weight in kilograms divided by height in meters squared — doesn't know that 35% of that weight is muscle. It doesn't know that my waist-to-height ratio is 0.46, which puts me in the "low risk" category. It doesn't know anything except two numbers that were never designed to assess individual health.
I didn't argue with her. I used to argue. I used to pull up studies on my phone and wave them around like evidence in a courtroom. But I've learned that doctors are operating under time constraints and insurance requirements and guidelines written in 1998. The old shortcut, as I call it. BMI is easy to calculate. It takes ten seconds. Body composition testing takes equipment and training and money. In a fifteen-minute appointment, what do you think wins?
But here's what really bothers me. It's not my doctor. It's the patients who don't know what I know. The ones who get that "overweight" label and start crash dieting. The ones who lose muscle along with fat because they think the number on the scale is the whole story. The ones who come to my old studio crying because their doctor told them they were unhealthy and they believed it.
I had a client — let's call her Sarah — who was exactly this story. She was a 34-year-old CrossFitter in Portland. Deadlifted 200 pounds. Ran half marathons. Her BMI was 27.1. Her doctor put her on a "weight management plan" that involved eating 1,200 calories a day. She lost 15 pounds in two months. She also lost muscle, energy, and her period. She came to me after her DEXA scan showed she had dropped from 22% body fat to 21% — but lost 8 pounds of lean mass. She was weaker. She was colder. She was miserable.
"Why didn't anyone measure my body fat?" she asked me. I didn't have a good answer. Because insurance doesn't cover it? Because most doctors don't have the equipment? Because BMI is the old shortcut and old shortcuts die hard?
The research on BMI's failures is not new. A 2016 study in the International Journal of Obesity found that BMI misclassified 54 million Americans as unhealthy. Fifty-four million. People who were actually metabolically healthy by blood markers, waist circumference, and body fat percentage. But their BMI was over 25, so they got the label. They got the lecture. They got the anxiety.
And on the flip side — the side that keeps me up at night — there are people with "normal" BMIs who are actually at risk. Normal weight obesity. It's a real thing. A BMI of 22 with 35% body fat. Skinny on the outside, metabolically compromised on the inside. Those people don't get flagged. They don't get screened. They walk around thinking they're fine because the old shortcut told them so.
I don't blame doctors. I really don't. The system is broken, not the individuals in it. But I do blame the continued reliance on a metric that was invented by a Belgian mathematician in the 1830s to study population trends, not individual health. Lambert Adolphe Jacques Quetelet never intended for his index to tell a woman in Portland whether she should lose weight. He was looking at averages across large groups. And somehow, 200 years later, we're using it to decide insurance premiums and medical treatment plans.
What do I tell people now? I tell them to get a second opinion. Not from another doctor — though that can help — but from a body composition test. A DEXA scan if you can afford it ($75-150 in Portland). A Bod Pod if there's one nearby. Even the Navy method with a tape measure is better than BMI alone. At least it accounts for your waist. At least it knows you're a three-dimensional human and not a ratio on a chart.
I also tell them to track their own numbers. Strength. Energy. Sleep. Mood. How their clothes fit. The mirror. These are data points too. They're subjective, yes. But so is BMI, despite its mathematical costume. The difference is that your energy levels actually know something about your health.
My doctor and I are fine now. I brought her my DEXA results. She added a note to my chart: "Patient is athletic build. BMI not representative." It took an extra appointment. It took me advocating for myself in a way that a lot of people don't have the knowledge or confidence to do. That shouldn't be necessary. But until the system changes, it is.
Your body is more than a number. Your health is more than a ratio. And if anyone tells you otherwise — even if they have a white coat — ask them what your body fat percentage is. Watch them blink. Then go get it measured yourself.
— Emily, from a studio with a DEXA machine and a very strong opinion about the old shortcut
