Short answer first, then the reasoning. There is also a quality change that the scan cannot see. Weight loss shifts fiber-type distribution toward Type I and improves mitochondrial density and insulin-stimulated glucose uptake per fiber — so the muscle you keep is metabolically better even though there is less of it. That does not make the loss irrelevant, but it explains why function often holds up better than the number suggests.

Dr.PeteFamMed said:

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

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.

SleepFixSam said:

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

Can confirm the pattern SleepFixSam describes. 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.

From the other side of the consultation, briefly.

Sharing my comprehensive lab data relevant to lean mass:

Protocol: Tirzepatide 15mg/wk, resistance training 4x/week, protein 1.6g/kg/day.

The improvements in lean mass are clearly reflected in these numbers. The inflammatory marker reduction (hsCRP) is particularly striking.