Yes. Significant weight loss from any method — diet, surgery, or peptide-based drugs — includes some lean mass, and body-composition substudies of the GLP-1 trials confirm it: roughly a quarter of the weight lost was lean mass, with the rest coming from fat. Notably, that ratio was about the same in the placebo groups, suggesting the drugs shift how much weight is lost more than what it is made of.
Scale weight vs body composition
A scale reports one number, but that number is the sum of very different tissues: fat mass, muscle, bone, water, and organ tissue. "Lean mass" in research papers means everything that is not fat — so it includes muscle but also water and other tissue, which is why lean-mass changes are not a pure measure of muscle.
Two people can lose the same 15 kg with different compositions, and the difference matters: skeletal muscle supports strength, physical function, and metabolic health. This is why trials of weight-loss drugs increasingly include dual-energy X-ray absorptiometry (DXA) substudies that split weight change into fat and lean components rather than reporting scale weight alone.
What the trial substudies report
The most detailed published data come from a DXA substudy of SURMOUNT-1, the phase 3 trial of tirzepatide. Among 160 participants scanned at baseline and week 72, tirzepatide reduced body weight by 21.3%, fat mass by 33.9%, and lean mass by 10.9% (vs 5.3%, 8.2%, and 2.6% with placebo). Of the total weight lost, about 75% was fat and about 25% was lean mass — and that ratio was essentially the same in the placebo group, and held across subgroups by sex, age, and amount of weight lost (Look et al., Diabetes, Obesity and Metabolism 2025).
Two readings of the same numbers are both fair:
- Reassuring: the fat-to-lean ratio of the loss resembled ordinary (lifestyle-driven) weight loss; the drug did not preferentially strip muscle.
- Cautionary: because total weight loss is so much larger on these drugs, the absolute amount of lean mass lost is larger too. Losing 25% of a 21% body weight reduction is far more lean tissue than losing 25% of a 5% reduction.
For context on scale: in the pivotal trial of semaglutide 2.4 mg (STEP 1, n=1,961), mean weight loss was 14.9% of body weight at week 68 versus 2.4% with placebo (Wilding et al., NEJM 2021) — losses large enough that even a one-quarter lean fraction represents kilograms of lean tissue.
Why researchers are paying attention
The lean-mass question has moved from footnote to active research topic. A 2024 comment in The Lancet Diabetes & Endocrinology, pointedly titled "Muscle matters," argues that skeletal muscle deserves specific attention during medically induced weight loss (Prado et al., 2024). The underlying physiology is standard: skeletal muscle is a major site of insulin-stimulated glucose uptake and a reserve that supports recovery from illness, so preserving it during large weight changes is a reasonable priority — particularly for older adults, who start with less muscle and regain it more slowly.
Open questions the field has not settled include whether lean-mass loss on these drugs translates into measurable strength or function loss (mass and function do not always move together), and what happens to body composition during the weight regain that commonly follows discontinuation.
Strategies discussed in the literature
The approaches studied or proposed for protecting muscle during pharmacologic weight loss are the same ones studied for dieting generally:
- Resistance training — the most consistent stimulus for retaining muscle during an energy deficit, and a standard co-intervention in body-composition research.
- Adequate protein intake — trials of weight loss in older adults in particular have examined higher protein intakes as a lean-mass-sparing measure.
- Slower rates of loss — the titration schedules built into the drug labels naturally spread weight loss over months rather than weeks.
- Monitoring composition, not just weight — DXA or similar measures are how the research itself distinguishes fat loss from lean loss.
These are descriptions of how the question is being studied, not a program to follow; how any of it applies to an individual is a clinical conversation.