This one has a reasonably settled answer, so here it is. The honest range is roughly 20 to 40% of total loss as lean mass without deliberate resistance training, and roughly 10 to 20% with it plus adequate protein. Both numbers need a caveat that matters: "lean mass" on a DEXA is not muscle. It includes water, glycogen and the lean fraction of organs, and glycogen alone carries about three grams of water per gram. A large early lean drop is mostly the glycogen and water going, which is why the first scan flatters and the second is the honest one.

RetaRick_CA said:

The honest range is roughly 20 to 40% of total loss as lean mass without deliberate resistance training, and roughly 10 to 20% with it plus adequate…

Right, and protein targets should be set on a reference weight rather than current weight. Setting 1.6 g/kg on a starting weight of 130kg produces a target nobody hits on a suppressed appetite.

BethLabQueen said:

Two DEXA scans eleven months apart: down 24kg total, of which the scan says 6kg was lean.

Same position here, arrived at the long way round. There is also a quality change that the scan cannot see. Weight loss shifts fiber-type distribution toward Type I and improves mitochondrial density and insulin-stimulated glucose uptake per fiber — so the muscle you keep is metabolically better even though there is less of it. That does not make the loss irrelevant, but it explains why function often holds up better than the number suggests.

From the other side of the consultation, briefly.

Regarding lean mass: I accidentally injected my dose intramuscularly instead of subcutaneously last week (went in at wrong angle, too deep). Called my pharmacist — they said it's not dangerous but may absorb faster, potentially causing more GI effects.

Sure enough, had worse nausea that day. Lesson learned: pinch the skin properly and use the right needle length for your body composition.