# GLP-1 weight loss and muscle: what the numbers mean

Weight loss can include fat and lean mass. Body-composition studies and strength measures answer different questions about muscle health.

Canonical: https://velri.co/blog/glp1-muscle-loss-protect-lean-mass-ex-athlete
Publisher: Velri (https://velri.co/about)
Published: 2026-06-22
Updated: 2026-09-30
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Weight loss with semaglutide or tirzepatide can include both fat and lean mass. Their prescribing information describes greater loss of fat mass than lean mass. That is a reason to pay attention to physical function as well as the scale. [1][2]

## What did the tirzepatide body-composition study find?

A SURMOUNT-1 substudy used DXA scans in 160 participants with measurements at the start and at 72 weeks. In the tirzepatide group, approximately three-quarters of the weight lost was fat mass and one-quarter was lean mass. The proportions were similar in the placebo group. [3]

These are averages from a selected substudy, not a prediction of one person's result. They describe body composition, and the scan itself is not a test of how much weight someone can lift or how easily they can climb stairs. [3]

## Why function belongs beside the scan

A review of diet-induced weight loss found that changes in muscle mass do not necessarily translate into the same change in strength. It also described improvement in overall physical function as fat mass decreased. [4]

That supports looking at both composition and ability. A lower lean-mass estimate and declining strength are different observations; their combination provides more information than either alone. [3][4]

## What supports muscle during weight loss?

The review found that physical activity helps preserve muscle mass during weight loss, with resistance exercise also improving strength. Adequate protein intake was part of its recommended approach; more protein was not automatically associated with better strength. [4]

These findings come largely from diet-induced weight loss research and support discussing nutrition and activity as part of treatment. [4]

A useful follow-up can discuss actual food intake, training, and familiar tasks that have become easier or harder. If reduced appetite or digestive symptoms make adequate eating difficult, that belongs in the treatment review too. [1][2]

## Sources
[1] DailyMed - WEGOVY- semaglutide injection, solution
WEGOVY- semaglutide tablet (2026). https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=ee06186f-2aa3-4990-a760-757579d8f77b
[2] DailyMed - ZEPBOUND- tirzepatide injection, solution
ZEPBOUND- tirzepatide injection, solution
ZEPBOUND KWIKPEN- tirzepatide injection, solution (2026). https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=487cd7e7-434c-4925-99fa-aa80b1cc776b
[3] Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study of adults with obesity or overweight. (2025). https://pubmed.ncbi.nlm.nih.gov/39996356/
[4] Preserving Healthy Muscle during Weight Loss. (2017). https://pubmed.ncbi.nlm.nih.gov/28507015/

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