Taking the question as asked, rather than the general version of it. 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.

COA_Karl 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…

Right, and protein targets should be set on a reference weight rather than current weight. Setting 1.6 g/kg on a starting weight of 130kg produces a target nobody hits on a suppressed appetite.

Dr.CardioMD 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.

Bone density monitoring on lean mass: DEXA bone density at baseline and month 12.

Lumbar spine T-score: -0.10 → -0.7 (stable to slightly improved)
Femoral neck T-score: -0.14 → -0.8 (stable)

No significant bone loss despite 67 lbs of weight loss. Key factors: adequate calcium (1200mg/day), vitamin D (5000 IU/day), resistance training 3x/week, and adequate protein. Weight-bearing exercise is critical for maintaining bone density during rapid weight loss.