Why BMI Misses Body Fat

BMI is your weight in kilograms divided by your height in meters squared. It was built as a quick way to compare large populations, and it is still the number most offices use to label someone overweight or obese. A BMI of 25 to 29.9 is called overweight, and 30 or above is called obese.

The problem is in the inputs. BMI knows how much you weigh and how tall you are. It does not know what the weight is made of.

Two people, same BMI

Take two hypothetical men who are both 5 foot 10 inches and 215 pounds. Both have a BMI of about 31, which labels both of them obese.

The first lifts four days a week and carries 16% body fat. The second sits most of the day and carries 34% body fat. The first man has about 180 pounds of lean mass. The second has about 142. They share a BMI and very little else about their health risk.

The error runs the other way too, and that direction is easy to miss. A 5 foot 3 inch woman who weighs 120 pounds has a BMI of about 21. If she has very little muscle, she can still carry 36% body fat. Her BMI says normal. Her body composition says otherwise, and that pattern is linked to higher rates of insulin resistance and abnormal lipids.

Why we use body fat percentage

We diagnose obesity by body fat percentage, not BMI. Excess fat is what drives the problems we care about: insulin resistance, fatty liver, high blood pressure, extra load on the joints. Muscle does close to the opposite. It stores glucose, supports the joints and protects you as you age. A measure that counts both the same way will mislabel people in both directions.

Commonly used body fat thresholds for obesity sit around 25% for men and 32% to 35% for women. Where the fat is stored matters as well. Fat in the trunk and around the organs carries more risk than the same amount in the hips and thighs.

A DEXA scan measures body fat percentage and shows where the fat sits, region by region, including an estimate of visceral fat. That gives us a real answer instead of a ratio of height to weight.

BMI still has a place. It is free, it takes seconds, and across a large population it tracks risk reasonably well. In my view it is a screening number, not a diagnosis. It should not be the number that decides whether one person has a disease.

That's why we check body fat directly.

Book a DEXA scan at HYBRD Health ($199)

This article is educational and is not medical advice.