PharmacoVig_BOS said:

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

I read this differently from PharmacoVig_BOS, on substance rather than tone. I think this board over-worries about lean mass. Some lean loss is obligatory — you need less muscle to move a smaller body, and preserving mass you no longer need is not a health outcome. The question is whether strength and function held, and mine did.

hyun_seoul said:

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

My endo's protocol for lean mass plateau: if weight loss stalls for 6+ weeks at current dose, we go through this checklist before titrating up:

1. Food diary review — is intake creeping up?
2. Protein check — minimum 100g/day
3. Activity audit — steps + resistance training
4. Sleep quality — poor sleep kills GLP-1 efficacy
5. Stress/cortisol management

Only after addressing all of these do we increase the dose. Often fixing one lifestyle factor restarts progress without needing more medication.

mike_nyc said:

I think this board over-worries about lean mass.

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].

Following on from RunnerRach — and this may be the naive question:

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?

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.