GLP-1 Muscle Loss: What the Protein Research Says

Caroline Apovian, M.D., co-director of the Center for Weight Management and Wellness at Brigham and Women's Hospital, explains the mechanism behind GLP-1 muscle loss in a Mass General Brigham Grand Rounds presentation: "When you lose weight, you are decreasing caloric intake in terms of proteins, carbohydrates, and fats. The brain needs glucose. Fat by itself cannot be turned into glucose, and that is the major reason why muscle loss is inevitable when you decrease caloric intake."

Weight loss paradigms universally induce the loss of lean body mass. Calorie restriction does it. Bariatric surgery does it. GLP-1 medications do it. With one out of every eight Americans now taking a GLP-1 medication, and the number on incretin-based therapies having increased 587% in the last five years according to the American Diabetes Association, several major studies on GLP-1 muscle loss and protein landed within the past 12 months: the ENDO 2025 Haines data linking protein intake to muscle preservation on semaglutide, the BELIEVE trial testing bimagrumab as a muscle-sparing add-on, and a Hong Kong genetic analysis of 800,000 individuals confirming favorable body composition changes.

How much lean mass are people losing?

In the pivotal trial that led to approval of semaglutide for treatment of obesity in adults, 40% of the reduction in body weight was due to loss of lean mass, according to a review by the Obesity Medicine Association. The pooled treatment groups in the tirzepatide pivotal trial lost 10.9% of their lean mass, which the OMA notes is similar to the semaglutide data when comparing the overall percentage of weight lost versus lean mass lost. The OMA described these figures as comparable to bariatric surgery and caloric restriction without specifying the exact numbers from those older studies.

A meta-analysis of 36 studies published in the International Journal of Obesity found that after three months, participants lost about 9% of their starting weight, with only a modest decrease in lean body mass. After 12 months, reductions in visceral adipose tissue and fat mass were far larger than lean body mass loss.

Mir Ali, M.D., a bariatric surgeon and medical director of MemorialCare Surgical Weight Loss Center, told Medical News Today his clinical experience matches: "Patients primarily lose fat when using these medications. While some muscle loss may occur, the majority of the weight loss is fat loss."

The Hong Kong genetic analysis, published in Diabetes, Obesity and Metabolism, confirmed that while both lean and fat mass decrease during GLP-1 treatment, the reduction in fat mass is more substantial, resulting in what the researchers called an overall favorable change in body composition.

Higher muscle loss predicted worse blood sugar outcomes in the Haines study

A study presented at ENDO 2025 by Melanie Haines, M.D., of Massachusetts General Hospital and Harvard Medical School, followed 40 adults with obesity over three months. Twenty-three were prescribed semaglutide; 17 followed a diet and lifestyle program. In the semaglutide group, participants who lost more muscle saw less improvement in blood sugar control, measured by HbA1c levels.

Haines stated: "Losing too much muscle may reduce the benefits of semaglutide on blood sugar control. This means preserving muscle during weight loss with semaglutide may be important to reduce insulin resistance and prevent frailty in people with obesity."

The study found semaglutide produced lean-mass loss similar to the diet-only group, while identifying three risk factors within the semaglutide cohort: older age, female sex, and lower protein intake.

Douglas Ewing, M.D., medical director of the Center for Weight Loss and Metabolic Health at Hackensack University Medical Center, explained the metabolic stakes: "Muscle is more metabolically active than fat. A significant loss of muscle mass can lower a person's metabolic rate, making it more challenging to maintain weight loss in the long run."

The protein targets from current research

In the ENDO 2025 data, lower protein intake was the single dietary variable directly linked to greater muscle loss in the semaglutide group. Haines stated: "Older adults and women may be more likely to lose muscle on semaglutide, but eating more protein may help protect against this." The study did not quantify a specific gram-per-day threshold separating the higher-loss group from the lower-loss group. Larger trials with protein intake as a controlled variable, rather than an observed covariate in a 40-person study, would be needed to establish that number.

Evidence-based guidelines for patients on tirzepatide (Zepbound) recommend 1.2 to 1.5 grams of protein per kilogram of ideal body weight daily, substantially higher than the standard RDA of 0.8 grams per kilogram.

For someone whose ideal body weight is 70 kg (about 154 pounds), that range works out to 84 to 105 grams of protein per day. For someone at 90 kg (about 198 pounds), it is 108 to 135 grams.

The distribution also matters. Fella Health recommends 25 to 35 grams of protein per meal for optimal muscle protein synthesis. Three meals at 30 grams each gets you to 90 grams before snacks.

The OMA review reinforces this, noting that reduced caloric intake from GLP-1 medications can lead to nutritional deficits that impair muscle maintenance and function, and that adequate protein and micronutrient intake combined with resistance training may help mitigate the risk.

Resistance training alongside protein

Mass General Brigham researchers reported that combining a high protein diet and consistent exercise with GLP-1 treatment showed the greatest benefit in preserving bone and muscle mass, compared to diet alone or high protein diet alone. Patients who exercised at the start of treatment and throughout had the best outcomes for maintaining lean body mass.

Research presented at the European Congress on Obesity (ECO 2025) reported that adults taking GLP-1 drugs retained muscle while losing weight by combining strength training with adequate protein intake.

Dr. Apovian also noted that eating a low carbohydrate, protein-sparing diet causes the body to develop ketosis, where ketones feed the brain instead of glucose derived from muscle. "Ketosis blocks the glucose-alanine cycle, which means it blocks the extrusion of branch chain amino acids coming from muscle. So it protects your muscle mass," she explained.

Working around appetite suppression

The SURMOUNT-1 trial showed patients on tirzepatide lost an average of 15 to 21% of body weight over 72 weeks, primarily through reduced caloric intake. GLP-1 side effects like nausea and early satiety make elevated protein targets harder to reach in practice. Fella Health notes that patients with chronic kidney disease may require lower protein intake and should consult their healthcare provider before increasing consumption.

Cold, soft, low-volume foods tend to be more tolerable during nausea: Greek yogurt (about 170g for 17g of protein) and cottage cheese (about 113g for 14g of protein) deliver protein without large plates or heavy cooking. Two servings of each across a day add up to over 60 grams. Fish and eggs round out the total. On days when solids are not tolerable at all, a whey or pea protein shake prevents a zero-protein day.

The timing structure from the research, 25 to 35 grams spread across three meals per Fella Health's guidelines, is more effective for muscle protein synthesis than loading a single meal. If appetite limits you to two meals, both need to center on protein. How much protein do you actually need? covers how to calculate the number based on ideal body weight.

The BELIEVE trial and what comes next

The BELIEVE Phase 2b trial, presented at the American Diabetes Association's 85th Scientific Sessions in Chicago, tested bimagrumab (a monoclonal antibody targeting activin type II receptors) in combination with semaglutide in 507 participants.

With semaglutide alone, 71.8% of total weight loss came from fat mass. Adding bimagrumab raised that to 92.8%. Total weight loss increased too: 22.1% for the combination versus 15.7% for semaglutide alone and 10.8% for bimagrumab alone.

With bimagrumab alone, 100% of weight loss was attributed to fat mass, and participants saw a 2.5% increase in total lean mass. Steven Heymsfield, M.D., Professor at Pennington Biomedical Research Center and lead author, stated: "These insights indicate that it is not only possible to achieve substantial fat loss, but also to preserve, or even enhance, lean mass in the process."

The researchers are now conducting studies of bimagrumab in combination with tirzepatide to evaluate its impact on both efficacy and safety. No results from those trials have been published yet.