GLP-1s and Muscle Loss: What the Evidence Shows

Thirty-eight to thirty-nine percent. That is the share of total weight lost that registered as “lean mass” on the body-composition scans in STEP-1, the landmark semaglutide trial published in the New England Journal of Medicine in 2021.^¹ The trial enrolled 1,961 adults with obesity. Average weight loss was about 15% of body weight over 68 weeks. And roughly four out of every ten pounds lost, according to the scan, came from the body’s lean tissue.

That number has been on every morning show, in every health influencer’s Instagram post, and in the text message your brother-in-law sent you last week. For a medication that is making weight loss manageable for millions of people, 38% sounds like a reason to stop.

Before you do, look at what the scan actually measured.

What does a DXA scan call “lean mass”?

Dual-energy X-ray absorptiometry (DXA) passes two low-dose X-ray beams at different energies through the body and separates what it finds into three compartments: bone, fat, and everything else.^² That third compartment, labeled “lean soft tissue,” is the number reported as “lean mass” in the GLP-1 trials. It includes skeletal muscle. It also includes water, glycogen, organ tissue, connective tissue, and fat stored inside muscle fibers. DXA cannot tell these apart.

DXA assumes a constant tissue hydration of about 73% water in the lean compartment. That assumption holds in a stable-weight adult, but not when people are losing weight. The same-day precision for DXA lean-mass measurement runs approximately 0.3 to 2%, which means a shift of a few hundred grams in either direction can fall inside the noise of the test itself. When someone drops 15% of their body weight over 68 weeks, glycogen, water, organ mass, and inflammatory fluid all shift at the same time.

During GLP-1-assisted weight loss, several things happen inside that third compartment that have nothing to do with losing skeletal muscle. Glycogen stores drop, and each gram of glycogen carries roughly three grams of water with it. Inflammation resolves, releasing fluid from tissue. Intramuscular fat shrinks. Stored liver fat decreases. All of these register on a DXA scan as lean-mass loss, but none of them are the muscle tissue the headlines are about.

The same measurement issue runs in both directions. DXA inflates apparent lean-mass gain during periods of training, carbohydrate loading, or creatine supplementation, and deflates apparent lean mass during low-carbohydrate diets, fasting protocols, or rapid weight loss. Controlled studies have shown that glycogen loading alone can shift DXA leg lean mass by 1 to 2% within days, and creatine supplementation independently increases total body water by roughly 1.4 percent, both with zero change in contractile protein.³⁷ A reader who is skeptical of “GLP-1s cause muscle loss” headlines should be equally skeptical of “this program added 5 pounds of muscle in 8 weeks” headlines. The instrument is the same and its limitations are the same. What it sees as a kilogram of lean tissue gain after a heavy training block with full glycogen stores is not a kilogram of new contractile protein, any more than what it sees as a kilogram of lean tissue loss on a GLP-1 is a kilogram of departed muscle.

How much of the DXA lean-mass loss is actually liver?

The single largest contributor to DXA-measured lean-mass loss during GLP-1 treatment is probably the liver, which counts as lean tissue on DXA alongside muscle. Its mass scales directly with metabolic disease: in individuals with obesity, and especially those with metabolic-associated steatotic liver disease (MASLD, historically called “fatty liver”), the liver can carry more than a kilogram of excess glycogen, intrahepatic triglyceride (fat), water, and structural protein above its lean baseline. When a GLP-1 clears steatosis (fat), reduces inflammation, and shrinks the organ back toward its normal size, that entire reduction gets counted as lean-mass loss on the scan.

In the Langer 2026 mouse study, liver mass change accounted for roughly 71% of the body-weight-change signal on day 7. Muscle mass change in the untreated leg accounted for only 9%. The liver was doing about eight times more of the weight-change work than the muscle was.^²⁵ In the survodutide arm (a GLP-1 and glucagon receptor dual agonist), liver mass dropped 55% while muscle loss in the casted leg was comparable across semaglutide, survodutide, and pair-fed controls. The drug itself did not produce additional muscle atrophy beyond what the weight loss produced.

Mouse data tells you where to look; human data tells you if the signal crosses the species barrier. Survodutide and retatrutide phase 2 clinical trials in adults with MASLD and MASH show 60 to 80% relative reductions in intrahepatic triglyceride.^26,27,30 In a patient starting with fat in their liver, the hepatic compartment alone can account for a meaningful fraction of the total “lean mass” change that DXA reports. That fraction is metabolic improvement being miscounted by the instrument as muscle loss.

The two datasets converge on the same point: much of what DXA labels as lean-mass loss on a GLP-1 is liver tissue responding to metabolic disease, not muscle responding to the medication. STEP-1 showed something that rarely gets mentioned: the placebo group also showed DXA-measured lean-mass loss despite minimal weight change. That pattern is more consistent with systematic measurement artifact than with a drug-specific catabolic effect.^¹

What is “3 kg of lean mass loss” made of?

For a representative DXA-measured “lean mass loss” of 3kg in an adult with obesity using GLP-1s, the compartments that make up that number break down approximately as is shown in Table 1.

Table 1. Body Compartment Contributions to GLP-1-Assisted Weight Loss

Note: Bounded estimates derived from published compartment physiology and body-composition modeling.^{4,6,25,26,27,31} Ranges should be read as reasonable bounds, not exact values.

The highlighted row at the bottom is the one the headlines are about. It is the smallest or second-smallest contributor. The rest is metabolic improvement, substrate turnover, or measurement artifact. When someone is said to have lost “3 kg of lean mass on Ozempic,” the contractile muscle portion of that number is roughly 200 to 500 grams. The scan called the other 2.5 to 2.8 kg lean-mass loss, but none of it was contractile muscle.

What happens when you use a better measurement tool?

SURMOUNT-1 tested the dual GLP-1/GIP agonist tirzepatide in adults without diabetes over 72 weeks. Average weight loss on the highest dose was about 21%. DXA showed roughly 25% of the weight lost was lean mass.^³ The drug is different, the total weight loss is larger, and the lean-mass ratio is better, but the DXA limitation is the same.

The SURPASS-3 MRI substudy went a step further.^⁴ Researchers looked at the body composition changes produced by tirzepatide using MRI, which can distinguish contractile muscle from fat inside the muscle. DXA cannot make that distinction. The tirzepatide group showed a significant reduction in intramuscular fat.

Intramuscular fat is a byproduct of long-standing obesity and metabolic disease. It accumulates between and inside muscle fibers, lowers strength per unit of muscle cross-section, worsens local insulin sensitivity, and is independently associated with physical disability and mortality in older adults.^⁵ It is the muscle-tissue equivalent of fatty liver. When it shrinks during treatment, the muscle works better, but DXA counts it as lean-mass loss all the same.

How do body composition changes on GLP-1s compare to diet-only interventions?

Almost no GLP-1 trial includes a “pair-fed” or matched-Caloric-restriction control group. Without one, “muscle loss from weight loss” cannot be cleanly separated from “muscle loss from the drug.” The few preclinical studies that have run this comparison, including the Langer 2026 data, show that the drug does not add incremental muscle loss beyond what matched weight loss produces.^²⁵ The public-facing claim “GLP-1s cause muscle loss” would require a pair-fed human trial to validate, and that trial does not exist. What does exist is a body of evidence showing that the lean-mass trajectory on a GLP-1 tracks the lean-mass trajectory of equivalent Caloric restriction. The drug is not uniquely catabolic to muscle, but as an effective weight-loss tool, the weight loss involves some lean-mass change by definition.

For context, in a standard Caloric deficit without medication, the often-cited ratio for body composition change is roughly 75% fat to 25% lean mass lost.^⁶ 

STEP-1’s 38% sits above that baseline as an outlier, while SURMOUNT-1’s roughly 25% on DXA and the semaglutide arm in REDEFINE 1  came in at 28%.^3,28 The retatrutide phase 2 body-composition substudy showed lean-mass loss in line with the total weight loss, without evidence of drug-specific muscle catabolism.^²⁹ Across the broader trial landscape, the numbers cluster near the same 25% that dieting alone produces.

What’s wrong with the 25% benchmark?

The 25% “quarter rule” was itself derived from studies that used DXA-era methods with the same compartmental issues described above.^⁶ Those studies did not separate water, glycogen, liver mass, and intramuscular fat from contractile muscle. The benchmark is an aggregate of every non-fat compartment; it does not measure actual muscle protein loss.

GLP-1 medications resolve conditions that load the lean compartment disproportionately. A patient with MASLD may start treatment carrying a kilogram or more of excess intrahepatic triglyceride, glycogen, and water that DXA counts as lean tissue. A patient with systemic inflammation carries fluid that DXA counts as lean tissue. When the medication resolves those conditions, the lean-compartment number drops faster than it would in a metabolically healthy person dieting without medication, even if the contractile muscle loss is the same or less.

The mechanistically correct prediction is that GLP-1s should look worse than dieting on DXA-measured lean mass precisely because they improve the metabolic conditions that inflate the lean compartment. Whether they are worse on contractile muscle is a question that requires MRI, direct dissection, or some other instrument with better accuracy to answer. The few studies that have used those tools, including the Langer data, do not show disproportionate muscle loss. 

Does muscle function get worse on a GLP-1?

The thing that determines whether someone falls, fractures a hip, loses independence, or dies earlier is how the muscle works, not a proxy of it measured on an X-ray. 

The SEMALEAN study followed 106 adults with obesity (mean BMI 46, mean age 52, roughly half meeting sarcopenic obesity criteria at baseline) on max-dose semaglutide for 12 months, with DXA and handgrip dynamometry at baseline, seven months, and twelve months.^⁷ Fat mass dropped 18%. Lean mass dropped about 5% and stabilized after month 7. Handgrip strength improved by an average of 4.5 kg, and the prevalence of sarcopenic obesity fell from 49% to 33%. No structured exercise was prescribed.

A 2024 review in Circulation summarized the skeletal-muscle data across multiple GLP-1 trials and concluded that the observed changes appear “adaptive rather than pathological.”^⁸

In March 2026, Langer and colleagues published a direct test of the muscle question in Cell Reports Medicine, albeit in mice.^²⁵ The study combined four preclinical experiments with a proof-of-concept clinical trial in adults with obesity. Across every model and compound tested, GLP-1 medications predominantly reduced body fat. Absolute lean mass did decrease, but the muscle-to-body-weight ratio improved, meaning the body carried a higher proportion of muscle after treatment than before. The liver accounted for more of the lean-tissue loss than skeletal muscle did. In the clinical trial, maximum knee-extension and handgrip strength remained unchanged despite significant reductions in muscle size.

In the tirzepatide arm, DXA-measured lean body mass dropped 13%, but when the researchers dissected individual muscles and weighed them directly, the actual loss was closer to 10%, and relative muscle mass per unit of body weight improved. DXA overstated the muscle loss by roughly a third. Smaller gastrocnemius mass predicted roughly 37% of running performance variance; total lean body mass predicted almost none of it. The title of the paper states the conclusion directly: “Weight loss with GLP-1 medicines does not result in a disproportionate loss of muscle mass or function.”

Sarcopenia is not a synonym for “lost some muscle.” The 2019 European Working Group revision (EWGSOP2) consensus defines it as a syndrome of low muscle strength and impaired physical function.⁹ Strength is the primary diagnostic criterion. Mass dropped to a confirmatory role. Physical performance tests (gait speed, chair-rise time) mark the severe stage. A separate U.S. group, the Sarcopenia Definition and Outcomes Consortium (SDOC), went further: pooling data from more than 18,000 participants across eight epidemiologic studies and six randomized trials, SDOC found that DXA-measured lean mass did not predict falls, fractures, mobility limitation, or mortality once other factors were accounted for. The SDOC definition relies on grip strength and walking speed. Mass drops out of the diagnosis entirely.³²

In the PURE study of 139,691 adults across 17 countries, every 5 kg decrease in grip strength was associated with a 17% higher risk of cardiovascular death and a 7% higher risk of non-cardiovascular death.²³ Grip strength is a proxy for systemic muscle function. Training grip directly can inflate this number without earning the systemic health the grip number tracks. Training the whole body with compound resistance exercises does.

Most of the headlines about GLP-1 users “developing sarcopenia” are using the word loosely to mean “lost DXA-measured lean mass during rapid weight loss.” That is not what sarcopenia is.

When SEMALEAN measured sarcopenia by its actual clinical definition, prevalence fell during treatment.^⁷ The medication was a net positive for the sarcopenia question in the population most at risk for it.

What about the “don’t take a GLP-1 if you have sarcopenia” narrative?

Adults with sarcopenic obesity carry a double burden of excess fat and poor muscle function. The dominant driver of their decline is the excess fat, not the dysfunctional muscle mass. Reducing the fat load improves the ratio of function to body weight even if some absolute lean mass is lost along the way. That is what SEMALEAN showed. Sarcopenic obesity prevalence dropped from 49% to 33% and handgrip strength improved. These participants were not enrolled in a structured resistance-training program, which makes the result more striking.

If anything, the data argue for GLP-1 therapy in individuals with sarcopenic obesity, paired with resistance training when possible. For a complete treatment of the sarcopenia question, screening tests, and what moves the needle, see our sarcopenia article.

What determines whether you lose fat or muscle?

A body in an energy deficit is, in effect, running a triage on every tissue it carries. Muscle is metabolically expensive. If a muscle is never loaded, never asked to produce force, never stressed in any meaningful way, the body treats it as expendable tissue. The deficit pulls from both fat and lean mass, and the lean goes faster than it has to.

When muscle is loaded regularly, muscle protein synthesis rises roughly 30% above maintenance rates even during a deficit.¹⁰ The training sends a signal that the tissue is necessary and that the body should protect it. The deficit still pulls fat, but stops pulling muscle at the same rate.

A 2018 meta-analysis of resistance training during caloric restriction in older adults with obesity found that the training groups preserved roughly 93% of their lean mass compared with diet-only controls.¹¹ A separate controlled trial comparing caloric restriction alone, exercise alone, and the combination found that the diet-only group lost about 16% of their weight as lean mass. The diet-plus-exercise group lost close to none.¹²

Preliminary data from the T-REX trial, which combined a GLP-1 agonist with a structured resistance-training program, landed in the same range: the training arm cut fat-free-mass loss roughly in half compared with medication alone.¹³ That is consistent with what the broader resistance-training literature has shown for 30 years. GLP-1s are not uniquely catabolic. They produce the same body-composition trajectory as equivalent weight loss from any other cause, and the same intervention that protects muscle in any deficit protects it here.

The BELIEVE trial paired semaglutide with bimagrumab, a medication that blocks the body’s internal brake on muscle growth. The combination group lost 22% of body weight, and 93% of that loss was fat.¹⁴ The QUALITY trial added enobosarm, a medication that activates muscle- and bone-building signals similar to testosterone but only in those tissues, to semaglutide in older adults with obesity. The combination group lost less than 1% of their weight as lean mass over 16 weeks, compared with 34% in the semaglutide-alone group.¹⁵ Both trials are early-stage and neither is available as a prescription. However, they do confirm that the lean-mass loss seen on GLP-1s is a feature of the energy deficit, not the drug, and it responds to interventions that signal muscle preservation.

Training is that signal for most people. It is available, it is cheap, and it works.

Why does training matter more than protein intake?

You have probably seen dozens of posts telling you to eat 2.0 or 2.2 grams of protein per kilogram on a GLP-1, and almost none telling you what training protocol to follow. The implicit message is that protein is the primary lever and training is optional, but the data say the opposite.

Without a training stimulus, higher protein intake produces only modest improvements in lean-mass retention during a deficit and little of that is likely contractile muscle tissue. The body has no reason to build or maintain muscle if no demand is being placed on it, regardless of how much raw material is available. With a training stimulus, even moderate protein intake preserves most of the muscle that matters, because the mechanical signal tells the body the tissue is necessary and the protein supplies the amino acids to rebuild it.

If you do one thing, lift weights. If you do two things, lift weights and get enough high-quality sleep. If you do three things, hit your protein target. That is the order.

What counts as resistance training?

Resistance training is physical activity in which muscles generate force by contracting against a load. The load can be a barbell, a machine, a dumbbell, or bodyweight. For the purpose of preserving muscle during a deficit, what matters is that the last two or three repetitions of each set are genuinely hard and that the load can be progressed over time.

Walking does not qualify. Yoga does not qualify. Pilates and low-load bodyweight circuits technically fit the definition because they do produce muscle contraction against a load, but the strongest recent meta-analyses do not support Pilates for meaningful strength or muscle-power gains in adults.¹⁶ If you enjoy Pilates, do it for its own reasons. Do not substitute it for the work that preserves muscle, bone, and strength while on a GLP-1.

What’s a good starting exercise program?

For someone new to the gym or returning after time away, a program adapted from our Beginner Prescription or Beginner Template is the right starting point.

Two to four sessions of resistance training per week using a variety of self-selected exercises that span the major movement patterns: squat, hinge, horizontal push, vertical push, horizontal pull, and vertical pull. Perform two to three challenging (but not maximal) working sets of four to twelve reps per movement pattern to start, using a load that gets you within two to four reps of failure – meaning you could do two to four more reps if you had to, but you stop before that. Take two to four minutes of rest between sets on the compound lifts. Progressive overload that follows adaptation: as you get stronger, add weight or reps.

A full-body session at that volume takes 30 to 60 minutes. For most people on a GLP-1, particularly those who are new to lifting weights, that is enough work to signal the muscle to stick around. 

Table 2. What each combination does for muscle on a GLP-1.

Training is the entry point. Protein amplifies what training does.The medication is the behavior-change tool that makes the deficit sustainable, not the intervention that determines what is lost during the deficit.

How should you train through the side-effect window?

GLP-1 medications can cause nausea, particularly in the first few weeks after starting and after each dose increase. Total training load can be reduced during the worst symptom windows: fewer sets, lower intensity, fewer accessory movements. A light session is worth more than a skipped one because the signal to the body matters more than the absolute load. Keep the signal on.

Most of the nausea resolves within four to six weeks as the body adjusts. If you are new to lifting, starting during a dose-increase week is harder than it has to be. A coach or a well-structured template skips the trial-and-error. Our coaching service and training templates are built for this kind of transition.

How much protein do you need on a GLP-1?

Aim to consume about 1.2 to 1.6 grams of protein per kilogram of body weight per day. The previous 0.8 g/kg Recommended Dietary Allowance (RDA) recommendation was set to prevent nitrogen deficiency in sedentary adults and was never designed to preserve muscle during weight loss or support training. The 2026 federal dietary guidelines raised the protein floor to 1.2 to 1.6 g/kg for adults.¹⁸ For someone on a GLP-1 who is actively losing weight and training, 1.6 g/kg is the working target.

Total daily intake matters more than per-meal distribution, and training matters more than either. Without a training stimulus, higher protein intake produces only modest improvements in lean-mass retention during a deficit. With training, even moderate protein preserves most of the muscle that matters.

For an individual weighing 180 pounds (about 82 kg), 1.6 g/kg is approximately 130 g of protein per day. For 220 pounds (about 100 kg), roughly 160 g. Intakes in the 2.0 to 2.4 g/kg range are unlikely to cause harm but are also unlikely to produce meaningful additional benefit in most individuals on a GLP-1. The data behind the 1.6 g/kg target come from a 2018 meta-analysis, which found that the benefit of additional protein on resistance-training outcomes (strength and hypertrophy) flattened at about 1.6 g/kg/day.¹⁷ For a step-by-step calculation specific to your weight and goals, use our macronutrient calculator or our caloric needs calculator.

Does protein distribution matter?

Protein distribution does not seem to have a large effect on body composition or response to exercise. The total daily protein intake, as well as the rest of the dietary pattern, matter for more than how much protein is consumed per meal. 

When looking at the body’s response to dietary protein, each meal needs enough of the branched-chain amino acid leucine (about 2 to 3 g) to trigger a full muscle-protein-synthesis response. This translates to roughly 25 to 40 g of protein per meal, which can be from an animal or plant-based source. The long-term effects of “optimizing” each protein distribution across three to five meals per day, separated by three to five hours, is unknown. 

What’s more likely is that any observed benefit that comes from dosing protein at each meal stems from the displacement of ultra-processed-foods and getting enough total protein per day. If you can spread protein across three to five meals, good. If some days you back-load most of it into dinner because that is when your GLP-1-suppressed appetite shows up, also good.

How to hit your protein target with a suppressed appetite?

One of the hardest parts of the protein target on a GLP-1 can be physically eating it. A few tactics that may work for you:

1. Front-load protein at your largest meal. For many people on a GLP-1, appetite is best early in the day. Hitting 40 to 50 g at breakfast or lunch takes pressure off the rest of the day.
2. Eat the protein first. Fullness arrives faster than expected on these medications. Make sure what gets down is the protein, not the rice.
3. Use a protein shake when solid food is not tolerable. A whey or plant-based shake with 20 to 35 g of protein is easy to get down when the stomach is rebelling.
4. Choose lean, protein-dense sources. Chicken breast, fish, Greek yogurt, eggs, low-fat dairy, tofu, and legumes pack more protein per Calorie than protein-with-fat combinations, and they are easier to eat on a suppressed appetite. 

For an in-depth treatment, see our Guide to Protein article. If hitting the target through whole food is difficult, a whey concentrate is the most cost-effective tool, though some individuals prefer whey isolate, as it can be easier on the stomach. Our own product, Whey Rx, is a whey protein isolate, low in Calories, and is third-party tested.

What about sleep and creatine?

Getting enough high quality, restorative sleep is important for not only health, but also weight management. Individuals vary, but seven to nine hours of sleep is the target. Sleep restriction increases appetite, impairs recovery, and shifts the partitioning of weight lost during a deficit toward lean mass and away from fat. Individuals who are sleep-deprived lose more lean mass and less fat during weight loss than individuals who sleep adequately. 

The one supplement that earns a mention is creatine monohydrate. When dosed at three to five grams per day (or 0.03-0.05 g/kg), taken whenever convenient, it produces small but reliable improvements in strength and hypertrophy on top of what training alone provides. Skip the loading phase. Buy monohydrate from a third-party-tested manufacturer. For the full walk-through, see our creatine on GLP-1 and the complete guide to creatine articles. Everything else in the weight-loss supplement aisle is underpowered, unregulated, or both.

How do you know if you are losing too much muscle?

Some muscle loss occurs with any form of sustained weight loss, but losing “too much” muscle is unlikely for the vast majority of people on GLP-1s. Some people will want to track what’s happening anyway. The exact number doesn’t change the approach, lift, eat enough protein, sleep, but if you want data, the tools available have real limitations. 

Measuring the amount of muscle that’s been lost is challenging, especially in the short to medium term. DXA is not the answer due to the reasons discussed previously. If the lean-mass loss limitations are kept in mind, it can be useful to trend body composition every 6 to 12 months, but it is not useful for month-to-month monitoring.

The disagreement between DXA and direct imaging is not just about magnitude. It can run in opposite directions on the same person. In a 2019 study comparing DXA-measured thigh lean mass against MRI-measured thigh muscle volume across a resistance-training intervention, only 46% of scans agreed within 3%. Some participants showed muscle growth by MRI but apparent muscle loss by DXA on the same intervention.³⁸ The takeaway for someone monitoring their own body composition is that an individual DXA scan, especially one taken without standardized pre-scan conditions, may not reflect the direction of change in the underlying muscle, let alone the magnitude. This is part of why training performance and waist circumference are more useful tools for monitoring than scan-derived lean mass numbers.

BIA (the body-fat scale at the gym or the handheld bioelectrical impedance device) is even less reliable. BIA passes a small electrical current through the body and estimates fat percentage from the resistance the current encounters. The estimate is highly sensitive to hydration: morning versus evening readings on the same person can swing by 2 to 3 percentage points of body fat, and a glass of water or a meal can move it further. Compared to gold-standard testing, BIA has an average error of around 3 to 5% body fat, large enough to hide meaningful short-term change.²⁴ If you have a body-fat scale, you are getting a number that is precise enough to trend over months but not accurate enough to act on week to week.

MRI can isolate contractile muscle from intramuscular fat, but MRI is not a practical monitoring tool for most people due to the cost and availability. The three measurements that matter for someone on a GLP-1 are simpler, cheaper, and more informative:

Weekly: waist circumference at the navel. 

Measure the waist first thing in the morning, after going to the bathroom, but before eating or drinking. Waist circumference tracks visceral adipose tissue, the fat compartment that drives metabolic disease. A slow, steady decrease in waist circumference is the single best sign that the weight you are losing is the weight you want to lose. 

Rolling eight weeks: the weight-to-waist ratio. 

Track the ratio of weight lost to waist circumference change as a proxy for fat and lean mass losses over a rolling eight-week window. High-quality weight loss on a GLP-1 with training and adequate protein typically lands near 0.6 to 0.8 kg of body weight lost per centimeter of waist. A ratio creeping above 1.0 over eight to twelve weeks is a warning sign that the weight being lost is not coming primarily from fat, which is typically due to insufficient resistance training, protein, and/or sleep over this time frame.

Every four to six weeks: training performance. 

Provided someone is lifting weights, they should continue to see improvements in their performance over time. If the working sets feel the same or easier over a four- to six-week window, the contractile muscle is apparently doing well. If performance is going down and not recovering, that suggests something is off in the training program, the individual’s muscle, and/or their energy intake. 

GLP-1 use notwithstanding, determining the cause of a performance decline can be challenging. Systematically working through the following can help:

• Are you sleeping?Are you getting seven to nine hours most nights? If not, consider speaking with your doctor or sleep specialist for help.  
• Are you fueling your workouts with enough Calories? A deficit that has crept too low from continued GLP-1 appetite suppression can look exactly like muscle loss. Low energy availability from under-eating is a more common cause of performance decline on these medications than most people realize. While the deficit produces the weight loss, eating a little bit more (a small 200 Calorie snack) before and after a workout can be useful.
• Is it the training? Do sessions feel fresh and easy (under-training), or grindy and demoralizing (overreaching)? The fix is different for each. The training must be matched to you, the individual, and we have a free guide to help if your exercise program isn’t working for you. 

When appropriately prescribed, losing too much muscle is unlikely on GLP-1 medications. Nonetheless, tracking the waist, weight loss to waist change ratio, and training performance can be useful for self-monitoring. Still, stopping the medication is rarely the answer. Changing what is done around it usually is.

Is bone loss a bigger concern than muscle loss?

Bone loss when using GLP-1s, like muscle loss, has been overstated. While muscle pulls on bone, and any intervention that removes mechanical load from the skeleton carries a theoretical risk of bone-density loss, the apparent decrease in bone mineral density from GLP-1 use is not unique. People who lose weight by any means see the same changes, though this does not mean they are at a higher risk of falls or fracture. 

A recent abstract presentation reviewed the electronic medical records of GLP-1 users with Type 2 Diabetes and obesity. It’s been generating  headlines because it suggested an elevated risk of developing osteoporosis.¹⁹  A few caveats. First, reviewing medical records does not prospectively measure fractures or falls as primary outcomes. Next, the differences in risk between GLP-1 users and matched controls are small, i.e. less than a 1% increase in absolute risk for osteoporosis when on a GLP-1. Out of every 1,000 GLP-1 users in the study, 41 were diagnosed with osteoporosis over five years. Out of every 1,000 matched controls, 32 were. That nine-person difference is what the headlines called a 29% increased risk. Lastly, a recent study followed adults with obesity on liraglutide over 52 weeks and showed that the group that paired the medication with structured exercise preserved bone mineral density, while the medication-alone group had measurable decreases at the hip.²⁰ 

The lever that protects muscle during a deficit is the same lever that protects bone: resistance training.

What about retatrutide and survodutide: do the newer drugs hit muscle harder?

Retatrutide is a GLP-1/GIP/glucagon triple agonist. Survodutide is a GLP-1/glucagon dual agonist. Both are in late-stage clinical development and both add glucagon receptor (GCGR) activation on top of the GLP-1 backbone. Glucagon receptor agonism works predominantly in the liver: it drives glycogenolysis (glycogen breakdown), fatty acid oxidation, suppressed lipogenesis (creation of triglycerides), and gluconeogenesis (making new sugar). For any process that requires amino acids, the liver pulls from amino acids in the blood before needing to draw from muscle. GLP-1s, meanwhile, reduce food intake and total amino acid availability. With more amino acids being used and fewer amino acids coming in, the theoretical concern is straightforward: adding a glucagon agonist to a GLP-1 should accelerate muscle breakdown.

The empirical data do not support that concern at the doses currently being tested.

The retatrutide phase 2 body-composition substudy showed lean-mass loss in line with the total weight loss, without evidence of the medication causing extra lean mass losses.^²⁹ In the Langer 2026 data from mice, survodutide produced the largest liver-mass reduction of any compound tested (a 55% drop), but tibialis anterior muscle loss in the casted leg was comparable across semaglutide, survodutide, and pair-fed controls who did not get the medication.²⁵ This shows that the glucagon receptor  activation did not add incremental muscle atrophy beyond what matched weight loss produced. 

While theoretically concerning, existing data suggest that combination agents using glucagon agonism do not produce greater-than-expected contractile muscle loss. That said, they are still in active development and will need more study to confirm this early signal. 

Who should be on a GLP-1?

Current guidelines recommend GLP-1s for adults with obesity (BMI at or above 30), or adults with overweight (BMI at or above 27) plus a weight-related comorbidity such as type 2 diabetes, hypertension, fatty liver disease, or sleep apnea, for whom the cardiometabolic benefit of the medication exceeds its risks. 

The standard framing treats the risk of the medication as the only risk in the equation and ignores the risk of leaving obesity untreated. That framing is incomplete.

What does diet and exercise alone achieve?

Lifestyle change (diet and exercise) alone produces modest weight loss and then fails to hold for most people, even when the program, the coaching, and the personal effort continue.

The Diabetes Prevention Program randomized 3,234 adults with overweight or obesity and impaired glucose tolerance to metformin, placebo, or intensive lifestyle. At twelve months, nearly two-thirds of the lifestyle group had lost at least 5% of their body weight. By the ten-year follow-up, more than half of that weight was regained despite ongoing program engagement.³³

The Look AHEAD study enrolled 5,145 adults with overweight or obesity and type 2 diabetes into an intensive lifestyle intervention that continued for years. The group averaged an 8.6% loss and 68% hit the 5% threshold at one year. By year four, fewer than half of those who achieved clinically significant loss had maintained it.³⁴

The POUNDS Lost trial ran 811 adults through four different macronutrient patterns at a 750-Calorie-per-day deficit. The specific diet did not matter. Only 15% of participants lost 10% or more of their body weight.³⁵ 

A meta-analysis of 29 U.S.-based lifestyle trials with professional intervention and at least two years of follow-up found that more than half of the lost weight was regained within two years. By five years, more than 80% was regained, with average sustained loss settling around 3%.³⁶

The pattern is consistent across decades of research. About half of people achieve clinically significant weight loss in year one. Roughly one in four hits 10% and few do better than that. Five years out, about one in ten is still holding a clinically meaningful result. The other nine have drifted back toward baseline. These are not failures of willpower. Instead, the body pushes back on weight loss through increased hunger, metabolic adaptation, and hormonal shifts that make sustained weight loss without pharmacological help extraordinarily difficult for most people with obesity.

Compare that to semaglutide, where about 86% of STEP-1 participants achieved at least 5% weight loss, or tirzepatide, where average total weight loss on SURMOUNT-1 approached 21%.^1,3 These are not equivalent interventions. The effect size differs by roughly an order of magnitude. 

Regain after discontinuation is not a failure of the drug or willpower. It is the same underlying disease reasserting itself, the way it does with lifestyle alone — even if people continue “doing” the lifestyle. 

What are the side effects of GLP-1 medications?

Nausea is the most common side effect and is usually transient, peaking during dose titration and fading within a few weeks. Constipation can occur and should be managed proactively from the first week. Gallstone risk is elevated, though this is true of any method of rapid weight loss, not a GLP-1-specific phenomenon. Pancreatitis is rare and, in trial data, has not occurred at rates higher than placebo. Some patients experience injection-site reactions or a small heart-rate elevation of a few beats per minute. These side effects are real and deserve clinical attention, and they should be managed by a physician experienced in obesity medicine.

What are the risks of leaving obesity untreated?

Untreated obesity increases the risk of cardiovascular disease, type 2 diabetes, obstructive sleep apnea, metabolic-associated steatotic liver disease (fatty liver), chronic kidney disease, multiple cancers (endometrial, esophageal, liver, kidney, breast), osteoarthritis, and dementia. Lifespan is also shortened. Any conversation about GLP-1 risk that leaves these out is one-sided.

The SELECT trial showed that semaglutide reduced major adverse cardiovascular events (heart attack, stroke, cardiovascular death) in adults with obesity and established cardiovascular disease, without requiring a diabetes diagnosis.²¹ The FLOW trial showed kidney benefits with semaglutide use in patients with type 2 diabetes and chronic kidney disease.²² 

Improvements in hard clinical outcomes like cardiac events, kidney disease, and death are commonly observed with GLP-1 use, and are due to both the direct effects of treating the obesity, and indirect effects of the medications themselves.

Who should not use GLP-1 medications?

Individuals with a personal or family history of medullary thyroid cancer, multiple endocrine neoplasia type 2, or a history of severe pancreatitis should not use GLP-1s. There are other considerations for GLP-1 use that should be discussed with the prescribing physician.

One question that has recently come up is whether it is okay to use GLP-1 medications for cosmetic weight loss, i.e. for those who do not have obesity. This is a challenging discussion because there’s little available data in this group. Individual considerations of risks and benefits, acknowledging that many of these are unknown for those without obesity, should also be done with a trained medical professional.

Individuals who cannot access the medication through a legitimate prescription channel have another set of concerns. Compounded semaglutide, compounded tirzepatide, and research-grade “peptide stacks” sold through online clinics and messaging groups carry unpredictable sterility, potency, and composition. We advise against them regardless of price difference.

The full picture

Given how important muscle is to health, function, and longevity, it’s reasonable to be concerned about losing too much of it. All interventions that result in weight loss cause some muscle to be lost, but it does not appear to be excessive with GLP-1s. When the non-muscle components of lean mass loss are accounted for — liver fat, intramuscular fat, glycogen, water — the actual contractile muscle loss appears to be about the same as diet-induced weight loss, if not less. 

Pound for pound, GLP-1 medications do not cause more muscle loss than dieting alone. People on them lose more weight, which is the reason the absolute numbers look larger. In individuals with obesity, muscle function typically improves after treatment, not despite the weight loss but because of it.

Regardless of the weight loss intervention people choose, we strongly recommend lifting weights. Regularly loading the body helps reduce muscle mass losses by half (or more), as well as preserving bone mineral density, and improving function. To support your training, weight management, and overall health, ensure you’re getting enough high quality, restorative sleep (7-9 hours), eating enough protein (1.2-1.6 g/kg/day), and take your creatine monohydrate (3-5 g/day) if you want to.  

GLP-1 medications are behavior change tools for creating the Calorie deficit. You’ll have to handle the muscle yourself.

Should I get a coach? 

Coaching can be helpful for many, though individuals on GLP-1s specifically may benefit from the guidance of an exercise and nutrition professional. 

Barbell Medicine Coaching pairs you with a physician-led team that can coordinate with your prescriber, design your training, and adjust your nutrition to match the medication’s appetite effects. It is the fastest way to get a muscle-preserving program dialed in on a GLP-1.

Barbell Medicine  training templates are the do-it-yourself option. The Beginner Template is built for individuals starting resistance training for the first time. The accompanying article includes all the background information you’ll need to get started, as well as a free 4-week program. 

Listen to the full podcast episode where we walked through this material with Dr. Spencer Nadolsky, an obesity-medicine specialist who prescribes these medications clinically every day.

References

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