Muscle Loss on GLP-1s: What It Is, Why It Happens, and Why It Matters

“Clinically, our goal isn't simply to make the number on the scale go down. It's to maximize fat loss while preserving muscle.”Dr. Jonathan Snipes, Medical Director

1. What does “lean mass” actually mean?

Everything in your body that is not fat. Body weight splits into two parts: fat mass, and lean mass. Lean mass includes skeletal muscle, but also organs, water, bone and connective tissue.

That distinction matters when you read headlines. A scan showing ten pounds of lean mass lost does not mean ten pounds of muscle lost. Some of it is water, and some is the extra organ and connective tissue a larger body carried to support itself. That is why good studies measure strength and function as well as lean mass.

2. Why does GLP-1 weight loss affect muscle at all?

Because any sustained calorie deficit does. GLP-1 medications work mainly by reducing appetite, so you eat less, and when the body runs short on energy it draws on both fat and lean tissue. Dieting and surgery do the same. What is different about GLP-1s is how much weight people lose and how quickly, which is why the question comes up so often.

Two things can make it worse:

• Too little protein. When appetite drops sharply, protein is often the first thing to fall off the plate.
• Moving less. Muscle that is not asked to work is muscle the body is willing to give up.

3. How much muscle do people actually lose?

Less than the headlines suggest, but not zero. The best-measured example is a substudy of SURMOUNT-1, the main tirzepatide obesity trial, where 160 participants had DXA body-composition scans at the start and after 72 weeks:

The split was the same on placebo, so the medication did not change what was lost, only how much. A 2026 paper in Cell Reports Medicine reached a similar conclusion from four mouse studies and a small human trial: absolute muscle mass and strength dipped slightly, but muscle relative to body weight improved, and patients kept their strength.

Not every estimate is this reassuring. Some researchers put the lean share as high as 45% of weight lost. The fair summary is that most of what you lose on a GLP-1 is fat, some is lean mass, and the amount varies from person to person, which is exactly why it is worth paying attention to.

4. Why does keeping muscle matter so much?

Dr. Snipes gives four reasons skeletal muscle deserves protecting:

• Strength. Carrying groceries, getting up off the floor, keeping up with your kids.
• Mobility. Balance and joint support, which matter more every decade.
• Metabolic health. Muscle is one of the body's main places for clearing glucose from the blood.
• Keeping the weight off. Muscle supports the energy you burn and your ability to stay active after treatment is tapered or stopped.

“Preserving lean mass can actually help patients emerge from treatment healthier and more functional, rather than just lighter.”

That is the standard we hold ourselves to. A lower number on the scale is the means. Better health and function are the goal.

5. What did the new Northwestern study find?

That an experimental RNA drug roughly halved lean-mass loss during semaglutide weight loss, in mice. The study, from Dr. Joseph Bass's lab at Northwestern University, was published in the Proceedings of the National Academy of Sciences in September 2026. It is the research Dr. Snipes reacts to in the video.

The drug is an antisense RNA therapy, developed with Ionis Pharmaceuticals, that switches off a gene called ZFP423 in fat tissue. ZFP423 acts as a brake that keeps white, energy-storing fat from turning into beige fat, which burns energy as heat. With the brake off, the mice burned more energy.

The likely reason it works: semaglutide cuts how much you eat, and the RNA therapy adds a second route by raising how much you burn. With two levers instead of one, the body does not need as deep a food deficit, and gives up less muscle.

6. What does that study not tell us yet?

“The study is preclinical at this time, so I would want to see some human trials showing efficacy, appropriate dosing, safety, meaningful long-term preservation of mass, but I think it's extremely encouraging for the future.”

• It is a mouse study. Many treatments that work in mice never work in people. The Northwestern team's next step is testing similar therapies in human cells, not yet in patients.
• There is no human dose. Nobody knows how much would be needed, or whether the effect carries over at all.
• Safety is unknown. Turning up the body's heat production is a large metabolic change that needs careful study.
• Long-term effect is unknown. Weeks of results in mice do not show muscle stays protected over months or years in people.

This therapy is not available anywhere, including from us. Be wary of anyone claiming to sell something like it.

It is not the only line of research. Earlier in 2026, a JCI Insight study found ketone ester supplements blunted semaglutide-related muscle loss in mice, and a study in Diabetes found female mice were largely resistant to it. Both are preclinical too.

7. What can you do about it today?

While the science works on muscle-sparing drugs, the approaches with the best evidence are the unglamorous ones:

• Put protein first at every meal, especially on low-appetite days.
• Do resistance training two to three times a week. Weights, bands and bodyweight all count.
• Track body composition, not just weight, where you can.
• Do not chase faster loss than your plan calls for. Talk to your provider before pushing a dose up.
• Tell your care team if you notice weakness, fatigue or trouble with everyday tasks.

Optional: adding a growth hormone peptide

Some patients want to do more than the basics. For them, we offer the option of adding CJC-1295/Ipamorelin alongside a GLP-1. It is not a GLP-1 and does not cause weight loss on its own. It asks your pituitary to release more of your own growth hormone, and growth hormone drives IGF-1, one of the body's main signals for building and repairing muscle.

It is dosed in the evening, 5 nights a week, to line up with the large growth hormone pulse your body already releases in early sleep. Here is what the evidence does and does not show:

• It raises growth hormone and IGF-1 in people. In a placebo-controlled trial in healthy adults, CJC-1295 raised growth hormone 2 to 10 times and IGF-1 1.5 to 3 times. That trial used the long-acting DAC form, while our prescription uses CJC-1295 without DAC, which acts for a shorter time.
• Raising growth hormone has increased lean mass in other trials. In a 2-year trial in adults aged 60 to 81, a different growth hormone secretagogue (MK-677) raised IGF-1 to young-adult levels and added about 1.1 kg of fat-free mass while the placebo group lost mass. It did not improve strength or function.
• No trial has tested CJC-1295/Ipamorelin for muscle, or alongside a GLP-1. Whether it protects muscle during GLP-1 weight loss is a reasonable hypothesis, not a proven result.
• It can nudge blood sugar up. Growth hormone opposes insulin, and the MK-677 trial saw fasting glucose rise and insulin sensitivity fall. That matters if you are on a GLP-1 for diabetes or prediabetes, so your provider reviews your glucose history before prescribing.

Treat it as an add-on to protein and strength training, never a replacement for them. Your provider decides whether it fits you, and the full dosing and evidence breakdown is in our CJC-1295/Ipamorelin guide.

If nausea is what is keeping protein off your plate, our guide to stopping GLP-1 nausea covers the dose and timing fixes. For choosing a medication in the first place, see semaglutide vs tirzepatide.

8. The bottom line

Muscle loss on GLP-1s is real, manageable and worth taking seriously. The goal of treatment is to leave you healthier and more capable, not just lighter. The Northwestern study is an encouraging sign that muscle-sparing weight loss drugs may be coming. Until they are proven in people, the tools that work are protein, strength training and a care team that watches more than the scale. For patients who want more, a growth hormone peptide is an option to discuss with your provider.

Want a plan built around fat loss and muscle? Book a consultation with Dr. Snipes, or see our weight loss options.

References

1. Look M, Dunn JP, Kushner RF, et al. Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study of adults with obesity or overweight. Diabetes Obes Metab. 2025;27(5):2720-2729. doi:10.1111/dom.16275
2. Langer HT, Gilmore NK, Hayden CMT, 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 Rep Med. 2026;7(3):102665. doi:10.1016/j.xcrm.2026.102665
3. Thorne A, Waldeck N, Rose J, et al; Bass J. An RNA thermogenic therapy to preserve lean mass and enhance metabolic health during GLP-1 weight loss. Proc Natl Acad Sci U S A. 2026. doi:10.1073/pnas.2618845123
4. Abuetabh Y, Schmidt MA, Naganuma M, et al. Semaglutide-induced loss of skeletal muscle mass is blunted by co-administration of ketone esters. JCI Insight. 2026;11(15). doi:10.1172/jci.insight.201810
5. Rout S, Karasawa T, Watanabe S, et al. Females are completely resistant to semaglutide-induced muscle loss in ob/ob mice. Diabetes. 2026;75(9):1584-1595. doi:10.2337/db26-0229
6. Teichman SL, Neale A, Lawrence B, et al. Prolonged stimulation of growth hormone (GH) and insulin-like growth factor I secretion by CJC-1295, a long-acting analog of GH-releasing hormone, in healthy adults. J Clin Endocrinol Metab. 2006;91(3):799-805. doi:10.1210/jc.2005-1536
7. Nass R, Pezzoli SS, Oliveri MC, et al. Effects of an oral ghrelin mimetic on body composition and clinical outcomes in healthy older adults: a randomized trial. Ann Intern Med. 2008;149(9):601-611. doi:10.7326/0003-4819-149-9-200811040-00003