Collecting this in one place because it comes up every few weeks and the answer is always assembled from scratch. It is about lean-mass preservation, and it is deliberately narrow — everything I am not confident about is marked as such.
What is actually established
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.
The condition it depends on
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.
The practical version
If you are tracking this properly: same scanner, same time of day, same hydration, at least six months apart, and read grip strength or a lift number alongside it. Function is the outcome; the scan is the proxy.
What I am not sure about
The question I want answered is whether resistance training and protein change the ratio or only the absolute amount. Tell me what I have not thought of.
— mona_PHX · corrections welcome and will be edited into this post with credit