SarahChen_PharmD said:

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

This is where I part company with the consensus forming above. 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.

rachel_ABQ said:

Two DEXA scans eleven months apart: down 24kg total, of which the scan says 6kg was lean.

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: -0.10 → -0.8 (stable)

No significant bone loss despite 57 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.

BenResearch_OR said:

I think this board over-worries about lean mass.

Muscle fiber type changes during GLP-1-mediated weight loss, relevant to lean mass: preliminary biopsy data suggests that weight loss shifts the skeletal muscle fiber type distribution:

• Decreased Type IIx (glycolytic, fast-twitch) fibers
• Increased Type I (oxidative, slow-twitch) fibers
• Improved mitochondrial density and function
• Enhanced insulin-stimulated glucose uptake per fiber

This "metabolic remodeling" of muscle may contribute to improved insulin sensitivity independent of total lean mass changes. The muscle you keep becomes metabolically better, even if there's less of it[1].

Following on from Dr.BariatricHTX — and this may be the naive question:

Whether resistance training and protein change the ratio or only the absolute amount?

Closing the loop on my own question.

Follow-up: I dropped the cardio to two easy sessions and put the effort into three real lifting sessions. The scale slowed and the scan improved.