The age- or illness-related loss of muscle mass and strength — a risk to monitor for during rapid weight loss.
Updated Sep 26, 2026
Sarcopenia is the progressive loss of muscle mass and strength, most commonly associated with aging but also a risk during rapid weight loss. Rapid weight loss on a GLP-1 takes some muscle along with fat, so a 2025 joint advisory ↗ (external link) from the American College of Lifestyle Medicine, the American Society for Nutrition, the Obesity Medicine Association, and The Obesity Society recommends checking muscle strength, function, and body composition before treatment starts, then protecting muscle with resistance training and an appropriate diet. That matters most for older adults, who already face the highest sarcopenia risk.
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Related terms
Muscle protein synthesis — The biological process of building new muscle protein, stimulated by resistance exercise and dietary protein intake.
Lean mass — 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.
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.
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.
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