What I have: semaglutide · STEP 5.
I can parse the result. I am less sure what it licenses me to conclude.
I have deliberately not looked at anyone else’s interpretation yet.
Which parts of this are informative and which are decoration?
Take the STEP 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.
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.
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.
More usefully, 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.
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.
Protein and progressive resistance training. Those are the two levers; everything else is a detail.
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsMore usefully, 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.
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.
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.
Track absolute lean mass, not lean percentage, or the arithmetic will mislead you.
The relevant confounder is glycogen: each gram of stored glycogen carries about three grams of water, and both register as lean mass.
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.
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 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.
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.
Glycogen's water-binding ratio of roughly three to one is basic physiology and explains most early rapid weight change in any deficit.
A quarter is typical. Below a fifth is a good outcome. Above a third means slow down.
edited 18 Jun 2025 by Dr_Wren_Halliday — added a caveat about sampling
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.
Do not read the first fortnight as tissue loss. It is mostly glycogen and its water.
edited 2 Sept 2025 by RP_C18 — removed a claim I could not source
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.