WHAT THE SCANS SHOW
The clearest data comes from two trials with DEXA scans (a body-composition measurement). In the semaglutide trial behind the Wegovy approval, the people who got the peptide lost lean mass and fat mass in different proportions: lean dropped by about a tenth, fat dropped by about a fifth, and visceral fat dropped by more than a quarter.
The lean-to-fat ratio actually improved over the trial. In the tirzepatide research behind Mounjaro and Zepbound, the composition of weight loss came out close to 75% fat and 25% lean. The body is not preferentially losing muscle on these peptides. It is losing more fat than lean.
WHAT MOVES THE NUMBER
The peptide produces a calorie deficit by reducing appetite. What gets lost inside that deficit depends on protein intake and how the body is used.
What this means
The popular framing ("the peptide destroys muscle") collapses a planning problem into a peptide problem. The peptide sets the deficit. What comes out of that deficit is set by what you eat and how you move.
Muscle loss is not a peptide effect. It is a planning gap inside the deficit the peptide creates.