Body composition

Body composition breaks total mass into its parts. The simplest model splits it in two — fat mass and fat-free mass — while more detailed models separate muscle, bone mineral, organ tissue and water. It answers what scale weight cannot: what you are made of, rather than how much of it there is.

It matters because two people at the same weight and height can be in completely different health situations. Composition tracks with metabolic risk, physical function and how you look far better than body mass alone, which is why the same weight can be good news or bad depending on what changed.

In practice measuring it accurately is harder than it sounds. DEXA is the usual practical reference, bioimpedance scales are heavily affected by hydration, and calliper and tape estimates carry real error. Whichever method you use, consistency of conditions matters more than the absolute number, and the direction of travel over months is the useful signal.