Most of what is known about body composition on these medicines comes from trials. This is what it looked like in an ordinary clinic, measured four times over six months.
What happened
Researchers reviewed 388 adults with obesity treated with GLP-1s in a specialist endocrinology clinic, with multifrequency bioelectrical impedance scans at the start and again at six weeks, three months and six months (PubMed 42762981). The group was 40.7% women, with a mean age of 50.3.
At six months:
| Measure | Change |
|---|---|
| Body weight | down 11.3% |
| Fat mass | down 8.95% |
| Visceral fat area | down 40.6 cm2 |
| Skeletal muscle mass | down 1.46 kg |
The authors describe the muscle loss as modest relative to the fat loss, and the weight loss as predominantly driven by fat. Patients who stuck with treatment lost more fat than those who did not. Anxiety and binge eating behaviours improved, and adverse events were uncommon.
Reading the muscle number honestly
A 1.46 kg fall in skeletal muscle is not nothing, and it is a great deal better than the figure people usually quote.
It is also worth knowing what bioelectrical impedance is. It estimates body composition by passing a small current through you and inferring the rest, and it is sensitive to hydration, to when you last ate and to when you last exercised. It is not an MRI. For tracking a direction over months in a clinic it is a reasonable tool; for a precise gram count it is not.
A second paper from the same week is a useful companion. Writing in the same journal family on 17 September, Q Wang makes the point that muscle percentage is not muscle preservation (PubMed 42755165). If you lose a great deal of fat, muscle rises as a share of your body mass even while the absolute amount of it falls. A rising percentage on a smart scale can be perfectly consistent with losing muscle, and it is the kind of number that reassures people who should be paying attention.
So read kilograms, not percentages.
What it means for you
This is a study of people in a specialist clinic getting lifestyle support alongside the drug, which is exactly the setting where the ratio should look better than average. That is not a criticism of the finding; it is the finding. What you do alongside the medicine changes what comes off.
The two things with evidence behind them are unglamorous and both are within reach. Enough protein, at a level set by your body rather than your appetite. And resistance work twice a week, because protein without load is only half the signal.
What to do with this
The protein floor is the number that decides how much of your loss is fat. It comes from your weight and your dose, not from how hungry you feel.