Most body composition studies count kilograms. This one looked inside the muscle, and found something the scales cannot see.
What happened
STEP UP was a phase 3b trial that randomised adults with a BMI of 30 or more to once-weekly semaglutide at 7.2 mg, semaglutide at 2.4 mg, or placebo, for 72 weeks. This prespecified secondary analysis looked at MRI body composition and physical function (PubMed 42764399).
Against placebo, the semaglutide groups pooled:
| Measure | Difference against placebo | Significant |
|---|---|---|
| Adipose body tissue | down 11.1 litres | yes |
| Visceral adipose tissue | down 1.5 litres | yes |
| Lean body tissue | down 1.7 litres | no |
| Thigh skeletal muscle volume | down 1.1 litres | no |
| Thigh muscle fat infiltration | down 0.92 percentage points | yes |
On the sit-to-stand test, participants managed a mean of 13 repetitions in 30 seconds at the start and 15 at week 72, in both the semaglutide and placebo groups.
The finding worth understanding
Muscle fat infiltration is the fat marbled through the muscle itself rather than sitting on top of it. It rises with age and with obesity, and it tracks poorly with how big a muscle looks. Two people with identical thigh volume can have very different amounts of it, and the one with less is stronger.
So a fall of 0.92 percentage points, which was statistically significant while the changes in muscle volume were not, says the muscle got leaner as well as slightly smaller. That is a different story from the one usually told about these medicines, and it is consistent with the function result: nobody got worse at standing up out of a chair, and on average both groups got a little better.
Be careful with this one
The limitation is serious and the paper states it. Of 1,407 participants in STEP UP, only 55 had a valid baseline scan, and of those just 6 were on placebo.
Six people is not a comparison group in any robust sense. It is enough to generate a hypothesis and not enough to settle one, which is presumably why the authors close by saying further research into the effect of weight loss on skeletal muscle composition and function should be explored. The lean tissue and muscle volume results not reaching significance may reflect a genuinely small effect, or simply a substudy far too small to detect one.
Read it as a promising signal, not a conclusion. The direction is encouraging and the sample cannot carry much weight.
What it means for you
Two practical things.
Stop treating muscle as a single number. Size, quality and function are three different things, and this study moved in different directions on each. The one that matters day to day is function: whether you can get up off the floor, carry shopping, climb stairs without holding the rail.
Test it rather than guessing. The sit-to-stand test used here is one you can do at home: a dining chair, thirty seconds, arms folded, count how many times you can stand fully and sit again. Do it now, do it in three months, and you will know something about yourself that no scale will tell you.
What to do with this
Whatever the imaging says, protein and load are still the two things under your control. The floor comes from your weight and your dose.