Key facts
- Total weight minus fat mass.
- Includes muscle, bone, organs, and water.
- Preserving it is the goal during fat loss.
- Used to calculate FFMI.
Lean body mass (LBM) is everything in your body that isn't fat. While most of the focus is on muscle, LBM also includes your bones, organs, and the water in your body. It's calculated by subtracting your fat mass from your total weight.
Tracking lean mass matters because the goal of a good diet is to change fat, not muscle. During fat loss, you want the weight you lose to come from fat while preserving lean mass — achieved with adequate protein and resistance training. Lean mass and body fat together are also what feed into composition metrics like FFMI.
Frequently asked questions
How do I preserve lean body mass while dieting?
Keep protein high (around 1.6–2.2 g per kg), keep lifting weights, and avoid overly aggressive deficits. This signals your body to hold onto muscle while losing fat.
Is lean body mass the same as muscle mass?
Not exactly. Lean mass includes muscle but also bone, organs, and water. Muscle is the largest changeable part, which is why the terms are often used loosely.
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