On GLP-1 medication, about 25% of weight lost was lean mass in a 160-person tirzepatide sub-study, the same share as on placebo (Look 2025); a review of semaglutide data puts it nearer 40% (Mechanick 2025). At GetLean, daily WhatsApp coaching tracks your weight, eating and exercise. Resistance training and adequate protein are the levers with evidence behind them.
In Singapore, tirzepatide is registered as Mounjaro, and semaglutide injection as Wegovy for weight management and as Ozempic for type 2 diabetes. The figures in this article are given by molecule, as the trials and product labels report them. Whichever medicine is prescribed, GetLean plans the exit from the first consultation. Coming off GLP-1 in Singapore: what happens and how to plan sets out how.
Lean mass: everything in the body that is not fat — mostly muscle, but also bone, organ tissue and water. Trials measure lean mass; muscle is the part of it people care about.
How much of the weight lost is lean mass?
The clearest figure comes from the SURMOUNT-1 body-composition sub-study, which used DXA scans in 160 participants. The paper states it directly: of the body weight lost, approximately 75% was fat mass and 25% was lean mass — for both tirzepatide and placebo (Look 2025). In the tirzepatide arm, fat mass fell 33.9% and lean mass fell 10.9%; in the placebo arm, fat mass fell 8.2% and lean mass 2.6%.
The higher figure people quote comes from semaglutide. A 2025 review, citing the supplementary data of the STEP 1 body-composition sub-study, reports a mean weight reduction of 17.32 kg that included a 6.92 kg mean reduction in total lean mass — and states that 40% of the weight lost was lean mass (Mechanick 2025).
That 40% figure deserves a caveat that it is rarely given. It rests on a 140-person sub-study that was presented as a conference analysis rather than published as a standalone peer-reviewed trial paper (Wilding 2021), and its internal figures have not been fully reconciled. The 25% figure for tirzepatide, by contrast, is a verbatim sentence in a peer-reviewed paper that reports both arms. Both are worth knowing. They are not equally well established.
Does the medication cause the muscle loss?
On the sub-study evidence, no — and this is the part that reframes the whole question.
If GLP-1 medication were doing something specific to muscle tissue, the drug arm and the placebo arm would separate. In SURMOUNT-1 they did not: the lean-mass share of weight lost was about the same in both, and the authors note the proportion was "relatively consistent" across clinically relevant subgroups (Look 2025).
Compare that with ordinary dieting. A systematic review of weight-loss studies found the median proportion of weight lost as fat-free mass was about 14% on standard low-calorie diets and about 23% on very-low-calorie diets, with more severe restriction associated with more fat-free-mass loss (Chaston 2007). Diet-driven weight loss has always cost lean mass. Nobody built a public conversation around it, because diet-driven weight loss on that scale was uncommon.
So the mechanism is not mysterious. Lose a substantial amount of weight and a meaningful share of it comes from lean tissue. What has changed is the scale of the weight loss now being achieved, not the biology of what weight loss is made of. The muscle question became urgent because the results became large.
One more correction worth making, because it runs the other way. Some summaries describe these sub-studies as showing that lean body mass increased. It did not. Absolute lean mass fell. What rose was the proportion of remaining body mass that is lean tissue, because fat fell faster (Look 2025). Those are opposite claims.
Is there a reliable number to plan around?
No, and treating one as reliable is its own mistake.
The frequently quoted rule that about a quarter of weight lost is lean tissue has been examined directly, and the conclusion was that it is an oversimplification which shifts with age, inactivity and whether someone exercises (Heymsfield 2014). It is a rough group-level average, not a constant that applies to an individual.
The pooled dieting data show the same spread from a different angle: 14% on moderate restriction, 23% on severe restriction, 27% in men against 20% in women (Chaston 2007). The number moves with how the weight is lost and who is losing it.
Whether the speed of loss matters independently of its size is genuinely unsettled. Two randomised trials suggest it does: one reported 8.8% fat-free-mass loss on a very-low-calorie protocol against 1.3% on a slower one (Vink 2016), and another found 1.51 kg of lean-mass loss on a rapid protocol against 0.52 kg on a slow one at matched total weight loss (Ashtary-Larky 2017). A third randomised trial, comparing a four-week with an eight-week protocol at matched weight loss, found body-composition changes were similar in both groups (Coutinho 2018). Some evidence indicates rate matters on its own; it is not established.
There is also some evidence that lean-mass loss does not continue indefinitely. In a 106-person single-arm cohort followed for a year on semaglutide, lean mass fell by about 3.0 kg by month seven and then stabilised while fat loss continued, and mean handgrip strength improved by 4.5 kg at twelve months (Alissou 2026). That study had no comparison group, so it cannot show what is attributable to the medication, and grip strength is a measure of strength rather than of muscle mass. It is a useful signal, not a settled answer.
What actually protects muscle?
Resistance training, with enough protein to support it — the two inputs that protecting muscle on GLP-1 works through in full. This is where the evidence is strongest and least contested.
A meta-analysis in people with overweight and obesity found that in the trials where resistance training was combined with caloric restriction, lean mass was maintained — an effect size of roughly −0.3 kg that did not reach statistical significance (Lopez 2022). In other words, the trials that included resistance training did not show the lean-mass loss that trials without it do.
In older adults the effect has been quantified more directly. Pooling six randomised trials in obese older adults, adding resistance training to caloric restriction prevented an estimated 93.5% of the lean-body-mass loss otherwise seen — and did so without reducing fat loss or total weight loss (Sardeli 2018). That figure comes from a small evidence base in a specific age group and should not be read as a universal constant, but the direction is unambiguous.
Protein is the other half. The protein target and how to reach it on Singapore food is covered separately, because hitting it on a suppressed appetite is a practical problem rather than a knowledge problem.
At GetLean, the programme is designed to end, and our philosophy is that the medication is the catalyst and the plan around it decides what you keep. The sub-study data is the reason: the weight is going to come off either way, and what it is made of is the part that is still under your control. The plan for coming off is the GetLean GLP-1 Taper Plan: after hitting your goal weight with the right nutrition and exercise habits built, the dose is stepped down at Dr Quek's monthly reviews towards your lowest effective dose and, where clinically appropriate, off the medication. Individual results vary, and clinical-trial figures describe the populations studied.
A 2026 network meta-analysis of 262 trials put numbers on how the two most effective molecules differ on exactly this measure — one of them removed the most fat mass and the most lean mass. We set the figures out in what the head-to-head shows.
The muscle question does not end when the medication does. What regained weight is actually made of takes up what the weight that returns is made of, and maintaining a loss without medication covers what carries on after the prescription ends.
What happens if you stop Mounjaro (tirzepatide) or Wegovy (semaglutide)?
The appetite effect fades over the weeks the medicine takes to clear, and the eating and training habits built meanwhile are what carry on. The withdrawal trials took it away, by stopping it or by switching people to placebo, and on average much of the weight came back; individual results vary. At GetLean Dr Quek steps the dose down at the monthly reviews rather than stopping it overnight, towards the lowest effective dose and, where clinically appropriate, off the medication, with daily WhatsApp coaching on those habits throughout. Never stop or change a prescribed medicine on your own. Coming off Mounjaro and coming off Wegovy cover the first weeks, months and year.
Common questions
How much of the weight lost on GLP-1 is muscle?
In a tirzepatide sub-study of 160 people, approximately 75% of the weight lost was fat mass and 25% was lean mass (Look 2025). A review citing semaglutide sub-study data reports around 40% (Mechanick 2025). There is no single fixed figure, and the proportion varies with how severe the deficit is, sex and age.
Is 40% of weight lost on GLP-1 really muscle?
That figure comes from a review's reading of a body-composition sub-study presented as a conference analysis, not from a standalone peer-reviewed trial paper (Mechanick 2025). It is worth knowing and worth attributing. The better-evidenced tirzepatide figure is lower, at about 25% (Look 2025).
Does GLP-1 medication cause more muscle loss than dieting?
The sub-study evidence says no. In SURMOUNT-1 the lean-mass share of weight lost was about 25% in both the tirzepatide arm and the placebo arm (Look 2025), and pooled dieting studies report 14–23% depending on how severe the restriction is (Chaston 2007). Weight loss costs lean mass regardless of what drives it.
Does GLP-1 medication build muscle?
No. In the sub-studies, absolute lean mass fell. What increased was the proportion of remaining body mass that is lean tissue, because fat mass fell faster (Look 2025). Those are different claims and are frequently confused.
How do you avoid losing muscle on GLP-1 medication?
Resistance training and adequate protein are the levers with evidence behind them. A meta-analysis found lean mass was statistically unchanged in the trials where resistance training accompanied a calorie deficit (Lopez 2022), and in obese older adults it prevented an estimated 93.5% of the lean-mass loss otherwise seen (Sardeli 2018). Ask a doctor whether resistance training is suitable for you before starting.