Today’s paper is a deep look at the SEMALEAN study, which examined how semaglutide affects body composition, muscle function, and resting energy expenditure over 12 months in 115 adults living with obesity.
Find the full article here: Alissou, Mathieu, et al. “Impact of Semaglutide on fat mass, lean mass and muscle function in patients with obesity: The SEMALEAN study.” Diabetes, Obesity and Metabolism (2025).
For clinicians worried that GLP-1-related weight loss might lead to muscle deterioration or functional decline, this study offers new insight—some reassuring, some still uncertain.
Why Was This Study Done?
We know GLP-1 medications reliably produce weight loss through a combination of appetite suppression, reduced gastric emptying, and decreased overall intake (Read my full GLP1 deep dive here).
But fewer studies have used gold-standard assessment techniques—like DXA and indirect calorimetry—to understand how GLP-1s affect:
- Lean mass
- Functional strength
- Risk of sarcopenia
This matters clinically: lean mass loss does not always equal strength loss, but severe declines can impact mobility, metabolic health, and quality of life.
Study Design
This was a prospective, longitudinal, real-world cohort study. In other words, clinicians followed their patients starting semaglutide and collected standardized measures at baseline, 7 months, and 12 months.
Who Was Included?
Participants had to have:
- BMI ≥ 40
- ≥1 obesity-related comorbidity (OSA, HTN, CVD, dyslipidemia)
- Documented failure of lifestyle interventions
Almost 69% were female, average age 52 years, and mean BMI 46.3.
Notably: 49% had sarcopenia at baseline, which is surprisingly high for a relatively young cohort.
Who Was Excluded?
- Active cancer
- History of pancreatitis
- Discontinued GLP-1 therapy before 7 months
- Incomplete follow-up data
The last two exclusions limit generalizability: patients who discontinued treatment due to severe side effects are not represented.
Semaglutide Dosing
All participants titrated semaglutide from 0.25 mg weekly to 2.4 mg by week 16.
What Data Was Collected?
- Weight and waist circumference
- DXA scans
- Resting energy expenditure (indirect calorimetry)
- Handgrip strength (dominant hand, highest of 3 trials)
- Bioelectrical impedance (total and third-space water)
- Sarcopenia diagnosis (via appendicular skeletal muscle mass/body weight ratio + grip strength using European Working Group on Sarcopenia (EWGSOP2) guidelines)
Dropouts
115 participants enrolled; only 9 dropped out:
- 7 for GI side effects
- 1 for gallstone-related cholecystitis
- 1 for worsening renal function in pre-existing CKD
This low dropout rate supports feasibility but also reflects the exclusion criteria.
What Did the Researchers Find?
1. Body Composition
- Total fat mass decreased ~19%.
- Lean mass also decreased, particularly in the first 7 months.
- Lean mass stabilized between months 7–12, with an overall ~3 kg loss.
- Appendicular muscle mass and total water also stabilized by month 7.
- Lean-mass–to–body-mass ratio increased, suggesting proportionate preservation.
Clinical interpretation:
Early lean mass loss appears typical during weight reduction, but stabilization suggests a non-progressive pattern—not consistent with accelerated sarcopenia.
2. Muscle Function & Sarcopenia
- Handgrip strength increased at 7 and 12 months.
- Sarcopenia prevalence fell from 49% → 33%.
- Among those with sarcopenia at baseline, 22% no longer met criteria by 12 months.
- But 5% developed sarcopenia during the study.
Clinical interpretation:
This is the most important finding: muscle strength improved despite some lean mass loss. The functional gains may help mitigate lean mass reductions.
3. Resting Energy Expenditure (REE)
- REE dropped by ~244 kcal/day at month 7
- Then increased by ~140 kcal/day at month 12
Clinical interpretation:
Early metabolic adaptation is typical during weight loss. The partial rebound suggests stabilization as participants approached a new steady state.
Key Takeaways
- In younger adults (~50 years) who tolerate semaglutide, weight loss was accompanied by improved muscle function and lower rates of sarcopenia.
- Some individuals did develop sarcopenia, and without a control group we cannot determine whether semaglutide, weight loss, or unrelated factors were responsible.
- The study did not assess physical activity or lifestyle habits, limiting the ability to separate medication effects from behavior changes.
- More research is needed in older adults, who are at higher risk for sarcopenia and functional decline.
