What it means
Two people with the same height and weight can have very different body composition. One may carry more muscle and the other more fat, and their health risks can differ even though their BMI is the same.
The CDC notes that BMI does not measure body fat directly, although it tracks reasonably well with other measures in large groups. Tools that estimate composition include DEXA scans, bioimpedance scales, skinfold calipers and tape-measure formulas. Each has error, and results can shift with hydration and technique.
During weight loss, both fat and lean mass usually go down. The goal for most people is to lose mostly fat while keeping as much muscle as possible, which is where protein intake and resistance training come in.
Waist size, strength on a few key lifts and how clothes fit are simple, low-cost ways to watch composition change between formal measurements.
Why it matters
On a GLP-1, the scale alone cannot show whether you are keeping muscle, so composition is the better progress check.