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
References
- 01STEP-1 DEXA substudy (Wilding et al. 2021, *New England Journal of Medicine*, PMID 33567185). SURMOUNT-1 DXA substudy: Look et al. 2025, *Diabetes, Obesity and Metabolism* (PMID 39996356). 2025 nutrition advisory: Mozaffarian D et al., *Obesity* (Silver Spring), 33(8):1475-1503 (PMID 40445127).Source line — see article bodyPMID