The short answer
A SURMOUNT-1 substudy found that most weight lost with tirzepatide was fat, but lean mass also decreased. The number on the scale did not describe the whole change.
Looking beyond the scale
Researchers compared body scans at the beginning and at week 72 in 160 participants: 124 pooled across tirzepatide doses and 36 receiving placebo. This was a small subset of the larger trial.
With tirzepatide, fat mass decreased by 33.9% and lean mass by 10.9%. With placebo, the corresponding changes were 8.2% and 2.6%. Each percentage refers to that tissue's own starting amount; they should not be added together.
Percent of each tissue’s starting amount
Original chart based on the study. Values are reported group averages.
| Group or measure | Average result |
|---|---|
| Tirzepatide: fat mass | −33.9% |
| Placebo: fat mass | −8.2% |
| Tirzepatide: lean mass | −10.9% |
| Placebo: lean mass | −2.6% |
Why the distinction matters
Lean mass is not a direct measurement of muscle alone. The analysis also did not establish which diet or exercise plan best preserved it. It did not include direct physical-performance testing.
The useful question is broader than “How many pounds?”: how did body composition and everyday function change?
Sources
Look and colleagues, Diabetes, Obesity and Metabolism, 2025. Funded by Eli Lilly. Full paper · DOI.