TrialTracker_MD said:

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

That is correct as far as it goes, and here is where it stops going. 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.

TrialTracker_MD said:

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

I read this differently from TrialTracker_MD, 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.

Answering the narrow version, because the broad one does not have a single answer. 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 protein. Both numbers need a caveat that matters: "lean mass" on a DEXA is not muscle. It includes water, glycogen and the lean fraction of organs, and glycogen alone carries about three grams of water per gram. A large early lean drop is mostly the glycogen and water going, which is why the first scan flatters and the second is the honest one.

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

Dr.GutHealth 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…

This matches mine closely enough to be worth saying so out loud.