BMI is used because it’s really easy to calculate. If you are muscular, it is not particularly helpful. How many people honestly wonder if they are fat or muscular? Honestly.
If you lose muscle mass and gain fat, it improves your BMI. If you add muscle and lose fat, your BMI will get worse. Not a good metric in and of itself.
Body composition is more useful, but a lot more difficult to measure.
Right, the BMI is a simple metric for tracking body composition trends in populations over time. When used in that way—as a statistical tool—it works just fine. When it’s applied to individuals it just causes confusion.
As a lot of medical measures are these days, its a back of the envelope screener. If you are a big person, muscular and wide...they can do caliper tests or even still do the immersion/displacement test.
But, for the most part the key readings are done in total, and a profile is done.
My doc goes though my weight, blood pressure, blood enzymes, blood glucose, and cholesterol and comes up with some sort of index.
My weight and blood pressure haven’t changed much in 40 years—and my blood pressure was always low—so low it hindered me from jobs like flying because they were afraid I would pass out.
It was a curse as a youth—but now at 60 I feel blessed.