Answering the narrow version, because the broad one does not have a single answer. 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.

I would rather be corrected than agreed with, if it comes to it.

TrialNerd_Beth 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…

Agreed on the ranges, with the condition that a single DEXA is not a measurement, it is one draw from a distribution. Hydration status moves the lean compartment by a kilogram or more between scans, so any comparison of scans done at different times of day or different hydration states is noise.

hans_munich said:

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

Same position here, arrived at the long way round. 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.

From the other side of the consultation, briefly.

Body composition tracking via bioimpedance on lean mass (InBody scan every 3 months):

Note: skeletal muscle mass dipped initially then recovered with consistent resistance training + high protein. The visceral fat reduction is where the metabolic magic happens.