LibrarianMeg said:

The honest range is roughly 20 to 40% of total loss as lean mass without deliberate resistance training, and roughly 10 to 20% with it plus adequate…

I read this differently from LibrarianMeg, 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.

BrianDallas92 said:

I lifted three times a week throughout and still lost lean mass.

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.

InsuranceTom said:

I think this board over-worries about lean mass.

OMG yes. lean mass + resistance training is the combo. Can confirm.

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?

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.