Body composition

Body composition is what your weight is made of rather than how much of it there is: fat mass, muscle, bone, organs and water. Two people at identical height and weight can be composed quite differently, which is the whole reason the concept exists.

It matters because the health and performance implications of weight depend almost entirely on composition. More muscle supports strength, metabolic health and independence in later life; excess fat, particularly around the organs, tracks with insulin resistance and cardiovascular risk. The scale adds them together and reports one number.

The simplest useful split is fat mass and fat free mass, with body fat percentage expressing the ratio. Fat free mass includes bone, organs and a great deal of water, which is why it is not a synonym for muscle and why it moves with hydration.

Measuring it is where expectations need managing. DEXA is the practical reference and still varies between machines and visits. Bioimpedance scales are strongly affected by hydration and can swing several percentage points across a day. Skinfolds are good in trained hands. Circumference equations are cheap and rough.

Because of that spread, one method used consistently beats switching between them, and the direction of travel over months is worth far more than any single reading.

Changing it usually means one of three projects: losing fat while keeping muscle, gaining muscle while limiting fat, or the slower simultaneous version. Each needs adequate protein and resistance training; only the energy balance differs.

In practice, pair a composition estimate with a waist measurement and gym performance, since between them they capture what a scale cannot. Our body fat, lean body mass and FFMI calculators cover the estimates.