Lean Body Mass

Lean body mass, sometimes called fat free mass, is your total weight minus your fat: muscle, bone, organs, connective tissue and water. If you weigh 80 kg at 20 percent body fat, your lean body mass is 64 kg.

It is the figure worth watching during a period of weight loss, because the aim is nearly always to lose fat while keeping as much lean mass as possible. A diet that takes five kilograms off the scale while taking two of them from muscle has gone badly, and nothing but a body composition measurement will tell you.

It is also a better input than body weight for two common calculations. Protein requirements scale with lean mass rather than total weight, since fat tissue has almost no protein requirement, and the energy your body uses at rest tracks lean mass closely enough that equations using it beat equations using weight for anyone very lean or carrying a lot of fat.

What it is not

It is not the same as muscle mass. Lean body mass includes your bones, organs and the water in them, so it is considerably larger than your skeletal muscle, and a change in it can be a change in hydration rather than anything you built or lost.

Where you see it in FitnessSyncer

Any connected scale that reports fat free mass sends it through to your body composition data, charted on your Dashboard. Our protein target calculator explains how to use it in place of your weight.