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
References3 sources
How to read these sources
This article uses primary sources and reviews to separate mechanism, human evidence, and context.
- Human Trial
Massachusetts Medical SocietyOnce-Weekly Semaglutide in Adults with Overweight or Obesity Read source
Used Here ForGrounding the muscle-loss question in the STEP 1 trial and its DEXA body-composition substudy.
Good ForHuman body-composition data measured directly in trial participants.
Not ForPredicting an individual's own muscle-to-fat loss ratio.
New England Journal of Medicine 384(11):989-1002 - Human Trial
Diabetes, Obesity and Metabolism
WileyBody composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study of adults with obesity or overweight Read source
Used Here ForAdding the tirzepatide body-composition data from the SURMOUNT-1 DXA substudy.
Good ForHuman body-composition data measured directly in tirzepatide trial participants.
Not ForPredicting an individual's own muscle-to-fat loss ratio.
Diabetes, Obesity and Metabolism 27(5):2720-2729 - Review
Obesity (Silver Spring)
Wiley / The Obesity SocietyNutritional priorities to support GLP-1 therapy for obesity: A joint Advisory from the American College of Lifestyle Medicine, the American Society for Nutrition, the Obesity Medicine Association, and The Obesity Society Read source
Used Here ForGrounding the practical protein and resistance-training guidance for preserving lean mass.
Good ForPractical nutrition guidance for people on GLP-1 medicines.
Not ForA personalized protein or training plan.
Obesity (Silver Spring) 33(8):1475-1503