Setup, so nobody has to ask: tirzepatide · SURMOUNT-5.
I would like help reading this properly rather than being told what conclusion to reach.
I have the full report including the method section, so I can quote specifics if that helps.
How should I read this, and where are the traps?
Take the SURMOUNT-5 body-composition substudy, not the main paper, and note how few participants were in it. Composition substudies are typically a small imaged subset of the full randomisation, so the lean-mass fraction carries a confidence interval far wider than the weight figures beside it. Then check the method: DEXA reports lean soft tissue, which includes water, and a large early fluid shift is counted as lean loss whether or not any protein left. The fraction most of these programmes land near is around a quarter of total loss, which is also roughly what caloric restriction alone produces — the interesting question is not the fraction but whether it differs from the deficit-matched control.
Absolute lean mass and lean mass as a percentage move in opposite directions during fat loss, which causes endless confusion.
Resistance training two to four times weekly with progressive load is the intervention with the strongest evidence for reducing the lean fraction of loss. Cardiovascular exercise does not substitute for it on this endpoint.
Bone mineral density is measured on the same scan and falls slowly with weight loss; it is worth tracking on the same series rather than as a separate exercise.
Protein intakes in the 1.6 to 2.2 g/kg range are supported by meta-analyses of resistance-training studies for lean-mass retention in energy deficit.
Do not read the first fortnight as tissue loss. It is mostly glycogen and its water.
edited 18 Jan 2025 by lucia_marchetti — updated for the 2026 guidance change
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsAnswering this needs the rate of loss, because faster loss reliably costs a larger lean fraction.
Glycogen depletion in the first fortnight releases the water bound to it — roughly three grams of water per gram of glycogen — which shows up as several kilograms of "lean mass" lost before any tissue has gone anywhere.
A deficit of five hundred to seven hundred and fifty kilocalories per day is the range within which lean preservation is generally achievable. Larger deficits work faster and cost more lean tissue per kilogram lost.
Glycogen's water-binding ratio of roughly three to one is basic physiology and explains most early rapid weight change in any deficit.
Track absolute lean mass, not lean percentage, or the arithmetic will mislead you.
edited 18 Jan 2025 by tabular_nums — tightened the wording; no substantive change
Start with what "lean mass" means on your report, because it is fat-free mass including water, glycogen and organ tissue — not muscle protein.
Because appetite is suppressed in this class, protein intake tends to fall in absolute terms even when it rises as a percentage of a smaller intake. That is the specific mechanism by which lean loss gets worse here.
Put another way, a daily protein intake in the range of 1.6 to 2.4 grams per kilogram of reference body weight is the range the resistance-training literature supports for lean-mass preservation in a deficit. At the top of that range the marginal return is small.
Trials in this class that measured body composition report lean fractions broadly consistent with other weight-loss interventions of similar magnitude.
A quarter is typical. Below a fifth is a good outcome. Above a third means slow down.
Answer first: lean mass falls during any substantial weight loss, typically as twenty to thirty per cent of the total, and the levers that change that fraction are protein intake and resistance training rather than anything pharmacological.
The conventional figure is that lean tissue accounts for roughly a quarter of total weight lost in unstructured weight loss, falling towards ten to fifteen per cent with adequate protein and progressive resistance training, and rising above a third with very rapid loss and no training stimulus.
Nothing here is medical or dietetic advice, and anyone with kidney disease has a protein question that needs a clinician.
Protein and progressive resistance training. Those are the two levers; everything else is a detail.
The honest answer is that a lean fraction around a quarter is normal, that below a fifth is good, and that above forty per cent suggests the protocol needs attention.
Scan-to-scan precision on lean mass is around one to two per cent for a good DXA under standardised conditions, so a change of half a kilogram is inside the noise.
The twenty-five per cent figure for lean loss in unstructured weight reduction is a long-standing result from body-composition studies across many interventions, including surgery.
The caveat is that lean mass on a scan is not muscle, and treating the two as identical over-reads the instrument.
Same machine, same time of day, same hydration state, or the series is noise.
Ask PeptideStack is a static archive. Posting is closed, but the norms are worth stating: answer the question that was asked, show your working, cite the trial or the certificate, and say plainly where the evidence runs out.