Trials measure the scale. A newer real-world dataset measured body composition instead — and semaglutide and tirzepatide don't come out the same. Here's what roughly 8,000 patient records show, and why the drug isn't the only variable that matters.
Weight lost on a GLP-1 is never purely fat. Some of it is lean body mass — muscle, but also bone and organ tissue. In earlier trials this mostly showed up as a footnote: total weight down, a small and expected dip in lean mass alongside it.
It matters more than a footnote for two reasons. Muscle is metabolically active tissue that helps set your resting metabolic rate, so losing too much of it can work against long-term weight maintenance. And in older adults especially, rapid lean mass loss raises the risk of sarcopenia and reduced physical function — a real trade-off if it isn't managed.
A real-world analysis by health-data firm nference, posted ahead of peer review in April 2026, used body-composition scans to track roughly 8,000 patients — about 1,800 on tirzepatide and 6,200 on semaglutide. Patients on tirzepatide lost, on average, 1.1% more lean body mass at three months and 2% more at twelve months than patients on semaglutide.
The gap widened at the extremes. Among patients who lost more than 20% of their starting body weight, roughly 1 in 10 on tirzepatide saw a lean mass reduction exceeding 5% of total body mass, compared with fewer than 7% of semaglutide patients who lost the same amount of weight. The researchers didn't identify a biological explanation for the gap — tirzepatide targets both the GLP-1 and GIP receptors, semaglutide only GLP-1, but that mechanistic difference alone doesn't account for it.
A separate, smaller trial of semaglutide alone — the SEMALEAN study — found a similar pattern within one drug: an initial lean mass decline that leveled off well before month twelve, alongside a measured improvement in grip strength over the same year. Read together, both studies point the same direction: lean mass loss is front-loaded early in treatment, and it isn't fixed by the drug alone.
Reduced exercise tolerance was linked to greater lean mass decline in both drug groups — and the effect was more pronounced in patients on tirzepatide. Higher doses, longer treatment duration, and pre-existing musculoskeletal conditions each compounded the risk on both therapies.
Sources: nference real-world body-composition analysis (medRxiv preprint, posted April 2026, not yet peer-reviewed) — Greater lean-body-mass decline with tirzepatide than semaglutide in routine care; reported via European Medical Journal, 17 Apr 2026.
None of this is a reason to avoid either drug — both remain effective, and lean mass loss is a known, manageable part of any significant weight loss, whether from a GLP-1, bariatric surgery, or diet alone. It's a reason to treat resistance training and adequate protein intake as part of the regimen, not an afterthought, particularly if you're losing weight quickly or you're on tirzepatide at a higher dose.
It's also a reminder that this is an active, moving research area. The headline study here is a preprint, not yet peer-reviewed, and it's observational — it shows an association, not proof that tirzepatide directly causes more muscle loss than semaglutide in every patient. Expect the picture to sharpen as more body-composition data comes in.
This article summarizes emerging research on GLP-1 therapy and body composition. It is not medical advice. Talk to your prescriber before changing your regimen, and raise any concerns about strength, fatigue, or mobility changes with your care team.