Do GLP-1 medications cause muscle loss?

Some reduction in lean body mass does occur, but research shows the loss is not disproportionate to overall weight loss, and relative muscle strength typically improves. The often-cited 'up to 40%' figure refers to lean body mass — which includes organs, bone, and water — not just skeletal muscle.

Did the 2026 study show that strength gets worse on GLP-1s?

No. In both the mouse and human arms of the study, relative muscle strength improved despite some absolute decrease in mass. In mice, running performance also improved. In the 10-person human trial, handgrip and knee extension strength did not worsen over 12 weeks.

What does 'lean body mass' actually include?

Lean body mass is everything that isn't body fat — including skeletal muscle, bone, organs, skin, and water. A reduction in lean body mass during weight loss doesn't mean muscle is being lost at the same rate. In the 2026 study, liver mass loss in mice actually exceeded muscle mass loss.

Should I still worry about muscle loss on GLP-1s?

Some caution is still warranted, especially if you're older or have lower baseline muscle mass. The 2026 study was a proof-of-concept trial with a small human sample (10 people) and male-only mice — researchers explicitly noted that older adults and those with pre-existing sarcopenia need further study.

What can I do to protect my muscle while on GLP-1 medications?

Prioritize protein intake (roughly 80–100g daily is a common starting point, adjusted for body weight and activity) and include resistance training. These two habits help preserve functional muscle during any weight loss — regardless of what any single study shows.

How does GLP-1 muscle loss compare to calorie restriction alone?

The 2026 study found that GLP-1 medicines modify the skeletal muscle protein profile differently than calorie restriction alone, which may have long-term implications. But the practical finding is that functional strength is maintained — similar to what's seen with well-managed dietary weight loss.

GLP-1 Muscle Loss and Strength: A 2026 Study

Written by the GLP-1 Coach editorial team. Last updated August 20, 2026.

If you've spent any time in GLP-1 communities, you've probably seen the number: up to 40% of weight lost on medications like Ozempic or Wegovy could be lean body mass, not fat. It's a real figure — it comes from the STEP-1 trial published in The Lancet — and it understandably makes people nervous. But a study published in Cell Reports Medicine in March 2026 adds important context that changes how to read it.

The headline: GLP-1 medicines do cause some loss of lean mass, but it's not disproportionate to normal weight loss, and relative muscle strength tends to improve. Here's what the research actually found — and what it means for how you should be thinking about protein and training.

What did the 2026 study find?

The study, published March 19, 2026 in Cell Reports Medicine and conducted by researchers at the UC Davis Innovation Institute for Food and Health, combined four preclinical experiments in obese mice with a proof-of-concept clinical trial in humans.

In mice: Tirzepatide produced roughly 35% body weight reduction. Fat mass fell 73% — a substantial drop. Lean body mass fell 13% — a much smaller drop. Critically, liver mass loss actually exceeded muscle mass loss. And when the researchers tested running performance, mice treated with semaglutide could run nearly as well as lean control mice — suggesting that functional capacity improved despite some absolute loss of mass.

In humans: The trial included 10 adults with obesity and type 2 diabetes over 12 weeks. Thigh muscle size decreased (a statistically significant reduction), but handgrip strength and knee extension strength did not worsen. Absolute and relative leg strength remained unchanged.

The core finding: relative muscle mass and strength improve even when absolute numbers tick down — because the weight coming off is so heavily skewed toward fat and organ mass, not muscle.

What does "lean body mass" actually mean?

This is where the 40% figure gets misread. Lean body mass is not the same as skeletal muscle. It's everything that isn't body fat — including skeletal muscle, yes, but also bone, organs (especially the liver), skin, connective tissue, and water. When the STEP-1 data showed that up to 40% of weight loss was from "lean body mass," it wasn't saying 40% was pure muscle. The 2026 study demonstrated, in fact, that liver mass changes outpaced muscle mass changes in mice on GLP-1 therapy.

That doesn't make the STEP-1 finding irrelevant — lean mass matters broadly for metabolic health — but it changes the alarm level considerably. A statistic framed as "you might be losing mostly muscle" is not what the full body of evidence shows.

Does running performance actually improve?

In the mouse arm of the study, yes. Mice treated with semaglutide showed improved endurance on treadmill tests relative to their body weight. This is functionally what matters: can you do more with the body you have? If relative strength and endurance increase as weight comes down, that's a meaningful outcome even if the absolute quantity of muscle dips slightly.

This distinction — absolute vs. relative — is worth holding onto whenever you see muscle mass numbers in headlines. A lighter body carrying the same strength capacity is, in practical terms, a stronger body.

What are the caveats?

The study was a proof-of-concept, not a definitive population-level answer. The human arm had only 10 participants. The mouse experiments used exclusively male animals. The researchers were direct about this limitation: older patients and those with pre-existing sarcopenia or wasting conditions need further study. These findings are most reassuring for middle-aged adults without significant baseline muscle loss. If you're older or already managing lower muscle mass, the picture may differ — and that's a conversation to have with your prescriber.

The study also found that GLP-1 medicines affect the skeletal muscle protein profile (proteome) differently than calorie restriction alone — a finding whose long-term significance isn't yet fully understood.

Does this mean protein and training don't matter?

No. This finding is not an argument for skipping protein goals or resistance training — it's an argument against panic. The two highest-leverage habits during GLP-1 therapy remain:

Protein: Roughly 80–100g daily is a common target for most adults, adjusted up for higher body weights or activity levels. When appetite is suppressed, protein tends to drop first — tracking it gives you the data to catch that before it compounds into a metabolic problem. Use the protein calculator to get a personalized starting point.
Resistance training: Even 2–3 sessions per week of compound movements (squats, deadlifts, rows, presses) sends a strong signal to preserve muscle during a calorie deficit. This applies whether you're on a GLP-1 medication or not.

For a deeper look at how to protect muscle specifically during GLP-1 treatment — including what kinds of training help most and what to watch for — see the muscle preservation guide.

Concerned that low protein during a plateau might already be causing muscle loss? The plateau muscle-loss connection is worth reading alongside this.

The bottom line

Bottom line: The "up to 40% could be lean mass" figure is real but routinely misread — lean body mass includes organs, bone, and water, not just muscle. A 2026 Cell Reports Medicine study found that relative muscle strength improves on GLP-1 medications even when absolute mass dips, and running performance improved in mice. The study was small and didn't cover older adults or those with sarcopenia, so some individual-level caution remains warranted — but the evidence points away from disproportionate or pathological muscle loss. Protein and resistance training remain the right response, from a place of prevention rather than alarm.

Final Thoughts

The 40% figure travels fast because it sounds alarming. The fuller picture is that GLP-1 medications appear to preferentially reduce fat and organ mass, with functional muscle strength largely maintained — at least in the populations studied so far. That's not a green light to ignore protein or skip resistance training; it's context for why those habits are protective rather than panic-driven.

If you're in the middle of GLP-1 treatment and concerned about muscle, the most useful things are concrete: track your protein with the protein calculator, read the muscle preservation guide, and monitor your strength levels over time. That data will tell you far more than any single headline.

This article is educational and not a substitute for medical advice. Talk to your healthcare provider about your individual body composition goals and any concerns about muscle mass during GLP-1 therapy.

Source: Brocchi A et al. "Weight loss with GLP-1 medicines does not result in a disproportionate loss of muscle mass or function in obese mice and humans." Cell Reports Medicine, March 19, 2026. PubMed