Answering the narrow version, because the broad one does not have a single answer. 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.

Dr.ObesityLA said:

There is also a quality change that the scan cannot see.

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.

Ask again with the specifics and you will get a better answer than this one.

zoe_NC said:

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

This matches mine closely enough to be worth saying so. The intervention that works is boring and well evidenced: resistance training two to three times a week at a genuine effort, protein at 1.4 to 2.0 g/kg of reference body weight, and not making the deficit larger than the training can support. Cardio does not protect lean mass and in a deep deficit competes with recovery.

Adding the clinical framing, because it changes how the question reads.

DEXA scan update relevant to lean mass:

Scanned at months 0, 6, and 12. Here are the body comp numbers:

Lean mass preserved — actually GAINED 1-2 lbs thanks to progressive overload + 1.5g/kg protein. The lean mass discussion needs to include body comp data, not just scale weight.