Dr.ObesityMed 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…

I read this differently from Dr.ObesityMed, 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.

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

The figures, for anyone assembling their own picture. Worth knowing that the injection site changes very little. Abdomen, thigh and upper arm are bioequivalent for semaglutide, so a site change is not a plausible explanation for a bad week.

DataDave said:

I think this board over-worries about lean mass.

There is a second half to this that has not been said yet. The mechanism that matters here is not stomach emptying, it is central. GLP-1 receptor agonism in the arcuate nucleus stimulates POMC neurons and suppresses AgRP/NPY signalling, which is why the effect is appetite and food salience rather than physical fullness. Delayed gastric emptying largely tachyphylaxes over the first months; the appetite effect does not.

One thing that is still open after JennaRN’s answer:

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?

OP back with an update, since a thread like this is useless without one.

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.