Lean Body Mass
Total body weight minus fat mass, comprising muscle, bone, organs, and water. Rapid weight loss can deplete lean body mass alongside fat. GLP-1 receptor agonists may cause proportionally more lean mass loss than exercise-based weight loss, which is an active area of investigation.
Technical Context
Lean body mass (LBM = total body weight minus fat mass) includes muscle, bone, organs, and water. Sarcopenia (age-related muscle loss) and sarcopenic obesity (low muscle mass with excess fat) are conditions where further lean mass loss from weight management therapy is particularly concerning. In semaglutide STEP trials, lean mass loss represented approximately 35-40% of total weight lost. Tirzepatide SURMOUNT data showed similar proportions. For context, bariatric surgery typically causes 20-35% lean mass loss as proportion of total. Resistance exercise during GLP-1 RA therapy can attenuate lean mass loss — the STEP 3 trial (semaglutide + intensive behavioural therapy including exercise) showed preserved lean mass compared to drug alone. The development of myostatin inhibitors and activin receptor antagonists as adjuncts to GLP-1 RAs is an active research area aimed at specifically preserving muscle during pharmacological weight loss.