Body weight from everything other than fat: mostly muscle, plus organs and water, and in the broad sense bone. On a DXA report, lean mass usually leaves bone out.
Updated Sep 26, 2026
Lean mass (or lean body mass) is, broadly, total body weight minus fat mass: muscle, organs, water, and bone. The exact meaning depends on how it's measured. On a DXA report, "lean mass" usually means lean soft tissue only, with bone mineral listed as a separate number (Minetto et al., Frontiers in Rehabilitation Sciences, 2021 ↗ (external link)). The figure that adds bone back in is fat-free mass.
On GLP-1 medications, a meaningful share of total weight lost (often cited around 25-40%) can come from lean mass rather than fat, which is why preserving muscle through protein intake and resistance training is a common focus of care. See our guides on strength training and protein, fat, and carbohydrates.
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Related terms
Fat-free mass — Everything in the body that isn't fat — muscle, organs, bone, and water — measured as total body weight minus all fat mass.
Body recomposition — Losing fat while keeping or building muscle, so the body's fat-to-muscle ratio improves even if total weight changes little.
Muscle protein synthesis — The biological process of building new muscle protein, stimulated by resistance exercise and dietary protein intake.
Sarcopenia — The age- or illness-related loss of muscle mass and strength — a risk to monitor for during rapid weight loss.
Osteoarthritis — The most common form of arthritis, in which the cartilage and other tissues of a joint gradually break down, causing pain and stiffness; excess weight is a major risk factor, especially for the knees.
Related guides
Why strength training matters on a GLP-1 — Some of the weight you lose on a GLP-1 is lean tissue, much of it muscle: roughly 25-40% of the total in recent trials. Strength training, paired with enough protein, is the best-supported way to protect that muscle, and there's a good case for starting early in treatment.