Semaglutide STEP UP: Thigh Muscle and Fat Infiltration

Key Finding Semaglutide reduced thigh skeletal muscle volume and muscle fat infiltration together, while sit-to-stand repetitions were preserved at 72 weeks — the muscle lost was partly fatty infiltration, not purely contractile tissue.

A secondary analysis of the phase 3b STEP UP trial offers a more detailed picture of what happens to muscle when people lose weight on semaglutide. Rather than relying on DXA-based lean mass estimates, this analysis used MRI to measure thigh skeletal muscle volume and — more importantly — the fat infiltrated within that muscle [PMID 42764399].

What STEP UP measured

STEP UP randomised 1,407 adults with a BMI of 30 kg/m² or higher to once-weekly subcutaneous semaglutide 7.2 mg, semaglutide 2.4 mg, or placebo for 72 weeks. This prespecified secondary analysis included 55 participants who had valid MRI body composition scans at baseline and follow-up. Because only 6 participants in the 2.4 mg arm had scans, the two semaglutide groups were pooled for the primary analysis.

The MRI protocol measured:

Adipose body tissue volume
Visceral adipose tissue volume
Lean body tissue volume
Thigh skeletal muscle volume
Thigh muscle fat infiltration (the percentage of muscle volume occupied by fat)
Physical function via the 30-second sit-to-stand test

What changed

The pooled semaglutide group showed significant reductions compared with placebo:

Adipose body tissue: −11.1 L (p < 0.0001)
Visceral adipose tissue: −1.5 L (p = 0.04)
Lean body tissue: −1.7 L (p = 0.16, not significant)
Thigh skeletal muscle volume: −1.1 L (p = 0.07, borderline)
Thigh muscle fat infiltration: −0.92 percentage points (p = 0.001)

The reduction in muscle fat infiltration is the finding worth attention. It suggests that the muscle volume lost was not purely contractile tissue — some of it was intramuscular fat that had been displacing lean muscle. A sensitivity analysis restricted to the 7.2 mg group showed similar results.

Physical function held steady

At baseline, participants averaged 13 sit-to-stand repetitions in 30 seconds. At week 72, both the semaglutide and placebo groups averaged 15 repetitions. The absolute number of repetitions was preserved, which is consistent with the idea that the muscle lost was not the muscle that powers functional movement.

Why this matters for the muscle loss debate

Most of the discussion around GLP-1 receptor agonists and muscle has focused on DXA-derived lean mass, which cannot distinguish between contractile tissue, connective tissue, and intramuscular fat. MRI-based measures of muscle fat infiltration add a quality dimension. If the tissue being lost is metabolically inactive fat stored within muscle, the functional and metabolic implications differ from losing contractile protein.

The study authors note that these findings are hypothesis-generating and call for larger MRI-based trials to assess how different weight loss interventions affect muscle composition and function over longer periods.

For a broader look at what the evidence says about GLP-1 agonists and muscle, see our earlier analysis of the muscle protein question and the SELECT frailty analysis. For the full semaglutide reference record, visit the Research Protocols archive.