What fraction of loss on semaglutide is lean mass according to STEP 1?

Stated plainly: semaglutide · STEP 1.

I have read the primary source rather than the summary, which has left me with more questions.

I understand the headline. I do not understand the footnotes, and the footnotes look important.

What can I legitimately conclude from this figure?

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.

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.

To be exact about it, 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.

Trials in this class that measured body composition report lean fractions broadly consistent with other weight-loss interventions of similar magnitude.

Same machine, same time of day, same hydration state, or the series is noise.

Answering this needs the rate of loss, because faster loss reliably costs a larger lean fraction.

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.

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 10 Jun 2025 by Dr_Yusuf_Adeyemi — added the method parameters

The relevant confounder is glycogen: each gram of stored glycogen carries about three grams of water, and both register as lean mass.

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.

The caveat is that lean mass on a scan is not muscle, and treating the two as identical over-reads the instrument.

Protein and progressive resistance training. Those are the two levers; everything else is a detail.