Losing muscle mass while losing weight: 8 signs and what to do

Losing muscle mass comes with losing weight. How much varies enormously: in a systematic review of six semaglutide trials covering 1,541 participants, the share of lean mass in total weight lost ranged from almost 0 to about 40 percent.

That spread is the whole story. Not whether you lose muscle, but how much.

What strikes us here is that almost nobody measures it. People weigh themselves daily and know after six months exactly how many kilos are gone, and nothing about what those kilos were made of. Below are eight signs that the muscle share is running high.

1. The scale drops faster than your clothes loosen

Muscle is denser than fat, so a kilo of muscle takes up less room than a kilo of fat. If weight is falling fast while your trousers barely loosen, that points towards loss that is not coming from fat tissue.

A tape measure around your waist says more here than the scale does.

It is the cheapest check available and hardly anyone runs it.

2. Your strength drops faster than your weight

With healthy weight loss, strength dips slightly, simply because you are moving less mass. If your strength falls further than your weight, say 20 percent less on the bar against 8 percent of body weight gone, that fits muscle loss better.

Note down one exercise and track it. One number, every month.

3. Everyday movements take more effort

Stairs, getting up from a low chair and carrying shopping are more sensitive to strength loss than most people expect. If these get harder while your body weight is going down, that is worth attention.

This sign counts for more the older you are.

4. You feel colder and tire sooner

Less muscle tissue means less heat production at rest. Feeling cold and tired also comes with an energy deficit itself, so this sign does not point one way only.

The same complaints turn up with an underactive thyroid, which does not simplify matters. We described that overlap in our piece on thyroid and weight loss.

5. Your protein intake has quietly halved

This is the most underestimated point. Appetite falls so sharply on GLP-1 medicines that protein intake goes down with it, without anyone deciding to do so. In research, spontaneous energy intake was 35 percent lower than on placebo.

The Dutch nutrition authority Voedingscentrum uses roughly 0.8 grams of protein per kilo of body weight per day as a starting point for adults. In the weight-loss literature, a higher intake is named as a factor that helps preserve muscle mass.

Work it out once for an ordinary day. The answer surprises most people.

6. There is no strength stimulus left

Without loading, the body has little reason to hold on to muscle tissue during an energy deficit. In the review by Cava and colleagues, resistance exercise emerges as the factor that, alongside protein, links most clearly to preserving muscle mass.

The Dutch Health Council, Gezondheidsraad, names muscle and bone strengthening activity at least twice a week for adults in its physical activity guidelines.

7. Your creatinine falls with it

This is the sign almost nobody mentions. Creatinine is formed from creatine in muscle tissue, and how much you produce relates to how much muscle you carry. Lose a lot of muscle and your creatinine can fall.

That has an awkward side effect. eGFR is calculated from creatinine, so a lower creatinine makes kidney function look better.

Take two men who both lose 12 kilos over six months. In the first, creatinine drops from 88 to 84 micromol per litre. In the second, from 95 to 72.

On the scale they did the same thing. In blood they did not. In the second, that trend fits substantial muscle loss better, and his calculated eGFR now looks more flattering than his kidneys are.

That is the number I find most interesting in this whole piece, and precisely the one nobody looks at.

8. It is going very fast

The larger the weekly energy deficit, the larger the muscle share in what comes off. In STEP 1, participants lost an average of 14.9 percent of body weight over 68 weeks, and that is a pace at which composition matters.

Speed is the only factor on this list you can turn down directly, in consultation with the doctor prescribing the medicine.

What you can follow alongside

Blood testing does not measure muscle mass. It does show a few things that sit around it.

What you can do with this

If I tracked one thing, it would not be the scale but the combination of a tape measure and a single strength exercise. Those two together catch what weight alone hides.

If you do test, look at the trend in creatinine rather than a single value, and put it next to albumin. Both sit in the extended panel. Discuss a falling creatinine with your GP before drawing conclusions yourself, because there are more explanations than muscle loss alone.

Every blood test result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.

References

• Bikou A, Dermiki-Gkana F, Penteris M, et al. A systematic review of the effect of semaglutide on lean mass: insights from clinical trials. Expert Opin Pharmacother, 2024. PMID 38629387.
• Cava E, Yeat NC, Mittendorfer B. Preserving healthy muscle during weight loss. Adv Nutr, 2017. PMID 28507015.
• Wilding JPH, Batterham RL, Calanna S, et al. Once-weekly semaglutide in adults with overweight or obesity (STEP 1). N Engl J Med, 2021. PMID 33567185.
• Gezondheidsraad, physical activity guidelines.
• Voedingscentrum, protein.