Metabolic Health
30% weight loss. Nobody published how much of it was muscle.
In May 2026, Eli Lilly reported the largest weight-loss result ever recorded in a phase 3 obesity trial. Retatrutide, a triple agonist, took participants down an average of 28.3% of body weight over 80 weeks — 70.3 lbs. In the subgroup that continued to 104 weeks, the figure was 30.3%, or 85 lbs. Forty-five per cent of people on the top dose lost more than 30% of their body weight, which is bariatric-surgery territory achieved with a weekly injection.
The press release runs to several pages. It reports weight, BMI, waist circumference, blood pressure, lipids and knee pain.
It does not report body composition. Not once.
If you are a senior leader currently on a GLP-1, considering one, or being asked about one by your board, that omission is the only number that matters. Because weight is not the outcome you are buying. Capability is.
What the scale does not tell you
A pound of fat and a pound of skeletal muscle both read as a pound. They do opposite things to an executive’s capacity.
Where body composition has been measured, the picture is consistent. In the SURMOUNT-1 DXA substudy, tirzepatide participants lost 15.9 kg of fat mass and 5.6 kg of lean soft tissue over 72 weeks — meaning roughly 26% of everything they lost was lean tissue. Real-world body-composition data puts the fat-free fraction at 25–33% of total weight lost.
This is arithmetic, not evidence
If retatrutide’s body composition profile resembles tirzepatide’s — and that is an assumption, not a finding, because the data does not exist yet — then the 85 lbs lost by the 104-week cohort would include on the order of 22 lbs of lean tissue.
Twenty-two pounds. That is not a rounding error on a body scan. In a fifty-four-year-old CFO, that is grip strength, stair-climbing capacity, glucose disposal, bone loading, and the metabolic floor that determines whether the weight comes back the moment the prescription stops.
There is a further wrinkle. A 670,422-patient real-world analysis published in April 2026 found that tirzepatide produced greater lean body mass loss than semaglutide at every measurement point — an excess of 1.1%, 1.5%, 1.3% and 2.0% at 3, 6, 9 and 12 months. The more potent the agent, the more attention lean mass appears to need. Retatrutide is the most potent agent yet tested.
Why executives are the worst-case group
The trial populations were selected for obesity. Senior leaders on these drugs are frequently a different animal: 40s and 50s, already sarcopenic relative to their twenties, training inconsistently because of travel, and eating in restaurants and airport lounges. Three things stack against you specifically.
Protein intake collapses exactly when it matters most
These drugs work by suppressing appetite. Published guidance for preserving lean mass during pharmacological weight loss sits around 1.2–1.5g of protein per kg of body weight per day. Measured intake among actual GLP-1 users averages 77.3g/day — for a 95 kg executive, about two-thirds of the lower bound of that range and just over half the upper bound. You cannot defend muscle tissue you are not feeding.
Age has already started the clock
Adults lose lean mass from the fourth decade onward without intervention. A drug-driven deficit lands on top of an existing downward trend, not on a flat baseline.
Nobody is measuring
Weight is measured weekly because it is free. Body composition is measured rarely because it requires equipment and a decision. What gets measured is what gets defended, so fat loss gets defended and lean mass does not.
The protocol that changes the ratio
None of the above is an argument against these medications. They are the most effective pharmacological tools for weight loss that have ever existed, and for many people the risk calculus is straightforwardly favourable. The argument is that the drug determines how much you lose and your protocol determines what you lose. Only one of those is in your hands.
1.Protein first, above the clinical floor
My clients run 1.8 to 2.2g per kg, not 1.2 to 1.5. The published range is written for general populations who are not simultaneously trying to hold onto executive-grade output. Appetite suppression makes this a logistics problem rather than a willpower problem, which is why the work goes into ordering frameworks, travel protocols and protein-dense defaults rather than meal prep.
2.Three compound resistance sessions a week
Resistance training is the only stimulus that instructs the body to retain lean tissue during an energy deficit. During GLP-1 therapy it moves from beneficial to load-bearing. Every protocol is built to run in a hotel gym.
3.Measure composition, not weight
Baseline body composition before or early in therapy, then re-measured on a schedule. Without it you are flying blind on the only variable that matters, and you will not know the ratio has gone wrong until it shows up as fatigue and a stalled recomposition.
4.Cortisol and sleep architecture
Rapid weight loss is itself a physiological stressor. Sleep architecture and cortisol timing influence how much of the deficit is paid for out of fat versus lean tissue.
5.Train for the outcome, not the number
VO2 max remains the single strongest predictor of all-cause mortality. A lighter, weaker, less aerobically capable executive has bought a smaller number and sold the thing the number was supposed to represent.
What I do and do not do
I am a performance coach, not a physician. I do not prescribe these medications, supply them, or advise you on whether to start or stop one. That conversation belongs with your doctor, and I will say so on the call.
What I do is the other half of the work, the half that is currently unowned: the protein, training, sleep and measurement architecture that determines whether your weight loss is a body recomposition or a managed decline. A significant and growing share of my clients are on a GLP-1 when they come to me, and the programme is built to run alongside it.
Work out your own numbers first
The GLP-1 Lean Mass Projection tool applies the published body-composition ratios to your weight, target loss and current protein and training load, and shows you the projected split both with and without a preservation protocol. Free, instant, no signup.
Run the projectionOr bring it to a call
A free 30-minute discovery call. If you are already on a GLP-1, bring your starting weight, your current weight, and your best estimate of daily protein. That is enough to tell you whether your ratio is defensible.
Book a discovery callCommon questions
Does retatrutide cause muscle loss?
No body composition data has been published for retatrutide as of September 2026. The TRIUMPH-1 phase 3 results reported weight, BMI and waist circumference, but no lean mass measurement. In tirzepatide's SURMOUNT-1 DXA substudy, approximately 26% of total weight lost was lean soft tissue. Whether retatrutide behaves similarly is currently unknown.
How much protein should I eat on a GLP-1?
Published guidance for preserving lean mass during pharmacological weight loss is around 1.2 to 1.5g per kg of body weight per day. Measured intake among GLP-1 users averages 77.3g per day, well below that. In my coaching practice the target is 1.8 to 2.2g per kg. Discuss any substantial dietary change with your doctor, particularly if you have kidney disease.
Is resistance training enough on its own?
No. Resistance training without adequate protein gives the body the signal to retain muscle but not the substrate to do it. The two are a single intervention, not two optional ones.
Which is worse for lean mass, semaglutide or tirzepatide?
A 670,422-patient real-world analysis published in April 2026 found greater lean body mass loss with tirzepatide than semaglutide, with an excess of 1.1% to 2.0% depending on the timepoint. This is observational data from routine care, not a head-to-head randomised trial.
Do you prescribe GLP-1 medications?
No. I am a Certified Personal Trainer and executive performance coach, not a physician. I do not prescribe, supply or advise on starting or stopping any medication. I coach the nutrition, training, sleep and measurement side, which is what determines the composition of the weight you lose.
Related
Sources
- Eli Lilly, "Lilly’s triple agonist, retatrutide, delivered powerful weight loss in pivotal Phase 3 obesity trial", 21 May 2026 (TRIUMPH-1, NCT05929066, n=2,339).
- Look et al., "Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study", Diabetes, Obesity and Metabolism, 2025.
- "Greater lean-body-mass decline with tirzepatide than semaglutide in routine care", medRxiv preprint, 13 April 2026 (n=670,422; 7,965 with paired body-composition measurements).
- "Lean Mass and Musculoskeletal Preservation in GLP-1-Based Obesity Treatment", Metabolites, 27 May 2026.