Numbers first: semaglutide · STEP 8.
This is presented as though it settles something, and I am not convinced it does.
I have two documents that appear to disagree, which is what prompted this.
Which parts of this are informative and which are decoration?
Take the STEP 8 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.
The short version: some lean loss is obligatory, the proportion is modifiable, and most of the early loss is water rather than tissue.
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.
More usefully, 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.
Track absolute lean mass, not lean percentage, or the arithmetic will mislead you.
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Visit GL BiochemStart with what "lean mass" means on your report, because it is fat-free mass including water, glycogen and organ tissue — not muscle protein.
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.
Concretely, 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.
The caveat is that lean mass on a scan is not muscle, and treating the two as identical over-reads the instrument.
Do not read the first fortnight as tissue loss. It is mostly glycogen and its water.
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.
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.
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.
Research-use compounds are not approved for human use, and body-composition planning does not change that.
Same machine, same time of day, same hydration state, or the series is noise.
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.
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.
Comparing scans from different machines is not a comparison. Different manufacturers use different algorithms.
A quarter is typical. Below a fifth is a good outcome. Above a third means slow down.
edited 21 Feb 2026 by Dr_Yusuf_Adeyemi — added a caveat about sampling
Preserving lean mass and gaining it are different objectives with different requirements, and in an energy deficit only the first is realistic for most people.
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.
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.
Protein and progressive resistance training. Those are the two levers; everything else is a detail.
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.