This one has a reasonably settled answer, so here it is. 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.

Correct me if the detail matters more than I have assumed.

labquiet_amy 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.

Dr.NutriCornell 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.

Back pain as a lean mass side effect: not from the medication itself, but from rapid body composition changes. Losing 64 lbs shifts your center of gravity and your core muscles need time to adjust.

Physical therapy has been essential. Core strengthening exercises (planks, dead bugs, bird dogs) daily for 10 minutes have eliminated my lower back pain. Your body is literally relearning how to carry itself.