Start with the uncomfortable part: you cannot reliably tell by feel. Muscle loss during weight loss is gradual, it is not painful, and it produces no distinctive sensation. Most of the signs circulated online are either late, ambiguous, or explained by something else entirely.
That does not mean nothing is knowable. It means the useful indicators are different from the ones people usually look for.
Signs that genuinely mean something
Losing strength on lifts you were previously doing comfortably. This is the most useful signal available without a scan. If a weight you handled for three sets a month ago now feels hard, and nothing else has changed, that is worth noticing. The caveat is that a large calorie deficit reduces performance for reasons that are not muscle loss — fatigue, low glycogen, poor sleep — so a single bad session means nothing. A trend over several weeks means something.
Losing weight faster than about 1% of body weight per week, sustained. Rate of loss is the input with the clearest relationship to lean tissue loss. It is also measurable, which the others are not. Three or more weeks running above that threshold is the pattern most associated with a poor split.
Eating very little protein without noticing. Not a symptom exactly, but the strongest predictor. If you have not counted your intake since starting the medication, there is a reasonable chance it is far lower than you assume — appetite suppression is not selective, and protein is the most filling thing on the plate.
Being over 60. Not a sign, but a reason to be more attentive. Older muscle is harder to maintain and the consequences of losing it are larger.
Signs that mean much less than you have been told
Feeling weak or tired. Extremely common on these medications, and almost always about eating less overall rather than about muscle specifically. It is worth mentioning to your prescriber, but it is not evidence of muscle loss.
A “skinny fat” appearance. Body shape changes as you lose weight, and skin does not always keep pace. What people identify as a muscle-loss appearance is frequently loose skin and a different fat distribution over a body that has lost a great deal.
Looking softer or flatter. Muscle glycogen and the water stored with it drop in a deficit, which makes muscle look less full without any of it being gone. This one is genuinely misleading and it panics a lot of people.
A bathroom scale’s body-fat reading going the wrong way one week. Bioimpedance is strongly affected by hydration. A single reading in either direction is noise.
The only way to actually know
Measurement. A DEXA scan is the reference method; a smart scale is much rougher but usable if you weigh under identical conditions every time. Either way it takes two readings at different times — one reading tells you where you are, not what changed. If you have two, our body composition calculator will work out exactly what share of your loss was lean tissue.
If you have no readings at all, the muscle check estimates the same figure from the factors that drive it. It is an estimate rather than a measurement and the page says so plainly — but it is built on the trial data and it will tell you which input is costing you most.
What to do if the answer worries you
Do not eat less. That is the instinct and it is the wrong one — reducing intake further while already losing lean tissue makes the problem worse, not better.
Raise protein towards the range for your weight, add two resistance sessions a week if you are not doing them, and if the weight is coming off very quickly, take that to your prescriber. Dose is their decision, not a calculator’s and not an article’s.