Muscle mass

Lean mass loss on GLP-1: what the evidence actually shows

Body-composition substudies of the STEP and SURMOUNT trials and related real-world cohorts: an estimated 25-40% of total weight lost on GLP-1 therapy is lean tissue. What the numbers mean and what is known about mitigations.

By Priya Krishnan, MD, ABOM; Hannah Ekberg, MS, RD, CDCES · Published February 7, 2026 · Last updated April 21, 2026 · Medically reviewed by Jonathan Park, MD, FACE

The most-cited body-composition concern with GLP-1 receptor agonist therapy is lean-mass loss. This article describes what the published trial body-composition substudies actually report, what the numbers mean clinically, and what is known about mitigations. The framing is deliberately hedged because the published evidence base on GLP-1 body composition is still evolving as larger and longer-duration analyses are published.

What the trials report

Body-composition substudies of the major GLP-1 trials use DXA (dual-energy X-ray absorptiometry), MRI, or bioelectrical impedance to estimate fat mass and lean mass change at intervals during the trial. Across the published substudies of STEP-1 (semaglutide), STEP-3, SURMOUNT-1 (tirzepatide), SURMOUNT-3, SCALE (liraglutide), and related real-world cohorts, the proportion of total weight loss attributable to lean tissue commonly falls in the approximately 25-40% range, with substantial variability:

The percentage varies across studies because the measurement modality (DXA vs MRI vs BIA) produces different estimates.
The percentage varies across doses and durations: longer durations and higher absolute weight loss are associated with somewhat larger absolute lean-mass loss, with the proportional relationship roughly preserved.
The percentage varies across patient populations: older adults and patients with lower baseline lean mass tend toward higher proportional lean-mass loss.

A representative figure from STEP-1 substudy analyses is approximately 39% of total weight loss as lean tissue at the 2.4 mg semaglutide dose; SURMOUNT-1 substudy reports cluster in the 25-35% range at the highest tirzepatide dose. Different reports give different numbers depending on the specific subcohort and analysis.

What the proportion means clinically

A 25-40% lean-mass-loss proportion is broadly consistent with what is observed in non-pharmacologic rapid weight loss (very-low-calorie diets, bariatric surgery without resistance training). The proportion is not unique to GLP-1 medications; what is new is the scale at which it is now occurring across millions of new patients.

The clinical relevance depends on the patient’s baseline lean mass and functional status:

A young adult with high baseline lean mass losing 30% of a 15% total weight loss as lean mass may end the year still well within healthy lean-mass range.
An older adult with sarcopenic obesity at baseline losing the same proportion may move into clinically significant sarcopenia territory, with implications for strength, function, and quality of life.

This is part of the reason that clinical follow-up of GLP-1 patients increasingly emphasizes body composition rather than weight alone.

Known mitigations

Two modifiable inputs are consistently cited in the published literature for preserving lean mass during rapid weight loss:

Adequate protein intake. See the protein-target framework article. The starting range commonly cited is approximately 1.2-1.6 g protein per kg of lean body mass per day for adults under hypocaloric conditions with normal renal function (1.4-1.8 for older adults).
Resistance training. See resistance training while on GLP-1. The clinical recommendation is approximately 2-3 sessions per week of progressive resistance exercise covering major muscle groups.

Aerobic exercise is beneficial for cardiometabolic outcomes but is not a substitute for resistance training when the goal is lean-mass preservation. Patients who do only aerobic exercise on GLP-1 typically show body-composition trajectories closer to the no-exercise group than to the resistance-training group on the lean-mass dimension.

Some emerging interventions (e.g., the bimagrumab antibody, in trials for muscle preservation) are not yet available outside trial settings and are not part of standard clinical guidance.

Where the evidence is thin

Several questions are not yet fully answered by the published literature:

The lean-mass-loss-vs-fat-loss ratio at very long durations (multi-year continuous GLP-1 therapy).
Whether the proportion of lean-mass loss is meaningfully different between GLP-1 and GIP/GLP-1 dual-agonist therapy at equivalent weight-loss magnitudes.
The functional consequences (strength, function, quality of life) of GLP-1-related lean-mass loss across the broader treated population, not just trial cohorts.
Optimal dosing of resistance-training intervention specifically in GLP-1 patients (vs general resistance-training literature).

This is an active area of clinical research. Recommendations should be expected to be refined.

Implications for the patient

For an individual patient on GLP-1 therapy, the evidence supports:

Awareness that some lean-mass loss is expected as part of the total weight loss.
Attention to protein adequacy as one of the two consistently-cited modifiable inputs.
Engagement in resistance training as the other consistently-cited modifiable input.
Periodic body-composition measurement (DXA where available, BIA otherwise) during the active weight-loss phase, in coordination with the prescribing clinician and dietitian.
A long-term perspective: lean-mass changes happen over months, not days, and are best evaluated in that timeframe.

Implications for the clinician

For the clinician managing GLP-1 patients, several points emerge:

Body composition is a meaningful follow-up metric in addition to weight.
Protein intake adequacy is worth assessing routinely, particularly in older patients and patients with lower baseline lean mass.
Resistance-training engagement is worth specifically asking about and supporting.
Patients with pre-existing sarcopenic obesity warrant a more cautious and intensive lean-mass-preservation approach.

References

Wilding JPH, Batterham RL, Calanna S, et al. Once-weekly semaglutide in adults with overweight or obesity. New England Journal of Medicine. 2021;384(11):989-1002.
Wadden TA, Bailey TS, Billings LK, et al. Effect of subcutaneous semaglutide vs placebo as an adjunct to intensive behavioral therapy on body weight in adults with overweight or obesity. JAMA. 2021;325(14):1403-1413.
Jastreboff AM, Aronne LJ, Ahmad NN, et al. Tirzepatide once weekly for the treatment of obesity. New England Journal of Medicine. 2022;387(3):205-216.
Wadden TA, Chao AM, Machineni S, et al. Tirzepatide after intensive lifestyle intervention in adults with overweight or obesity. Nature Medicine. 2023;29(11):2909-2918.
Holmstrup ME, Fairman CM, Calanna S, et al. Body composition during pharmacologic weight loss with GLP-1 receptor agonists: implications for protein adequacy and resistance training. Obesity Reviews. 2025;26(4):e13721.
Cava E, Yeat NC, Mittendorfer B. Preserving healthy muscle during weight loss. Advances in Nutrition. 2017;8(3):511-519.
Conte C, Hall KD, Klein S. Is weight loss-induced muscle mass loss clinically relevant? JAMA. 2024;332(1):9-10.

Medically reviewed by Jonathan Park, MD, FACE on April 21, 2026.