Weaker or Just Lighter? Monitoring Strength and Function, Not Just Lean Mass, During GLP ‐1 Weight Loss

Aims Body‐composition studies of GLP‐1‐based therapy report how much of the weight lost is lean mass, not whether the patient is getting weaker. This review asks how to monitor strength and function during treatment. Materials and Methods Narrative review of October 2026 PubMed searches pairing incretin drugs with strength, performance, walking and measurement‐error terms, plus hand‐checked reference lists. Results In older adults, strength declines about three times faster than lean mass and predicts mortality where lean mass does not. On GLP‐1‐based therapy, randomized trials show longer walks and better reported function, while some cohorts of older adults show falling strength and gait speed; a 2026 review of 35 body‐composition trials found none that measured objective physical function. Recent recommendations advise assessing strength and function, but none identified here judges change against measurement error. Conclusions Functional discordance is a confirmed within‐person decline in strength or physical performance beyond measurement error without an adverse lean‐mass signal (exploratory default: lean loss beyond the scan's least significant change and above 40% of weight lost). For higher‐risk patients (older adults; low strength, frailty or rapid weight loss), the review proposes grip strength, five‐times chair‐stand time and usual gait speed, with a timed stair climb where feasible, before treatment, at 3, 6 and 12 months and around dose changes, with declines confirmed by repeat testing, assistance recorded and stepwise clinical review. The schedule is pragmatic and not validated. If lean‐mass change identifies the patients whose function declines, functional monitoring may add little; if discordance is common, lean mass is an inadequate surrogate.