hannah_MT said:

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…

Muscle fiber type changes during GLP-1-mediated weight loss, relevant to lean mass: preliminary biopsy data suggests that weight loss shifts the skeletal muscle fiber type distribution:

• Decreased Type IIx (glycolytic, fast-twitch) fibers
• Increased Type I (oxidative, slow-twitch) fibers
• Improved mitochondrial density and function
• Enhanced insulin-stimulated glucose uptake per fiber

This "metabolic remodeling" of muscle may contribute to improved insulin sensitivity independent of total lean mass changes. The muscle you keep becomes metabolically better, even if there's less of it[1].

A narrower follow-up, since the general answer is now clear:

What fraction of loss being lean mass is actually normal, because the numbers quoted here range from 10% to 40% and cannot all be right?

jason_paloalto said:

Muscle fiber type changes during GLP-1-mediated weight loss, relevant to lean mass: preliminary biopsy data suggests that weight loss shifts the…

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.

Closing the loop on my own question.

Second scan under matched conditions came back with the lean number nearly a kilogram higher than the first, so a chunk of what I panicked about was hydration. Strength numbers were up throughout.

PharmD_Rodriguez said:

Regarding lean mass: I accidentally injected my dose intramuscularly instead of subcutaneously last week (went in at wrong angle, too deep).

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.

If somebody has the primary source to hand I would rather cite it than paraphrase it.