GLP-1 drugs – Drugs like semaglutide and tirzepatide help many people lose weight, but scientists are increasingly focused on a hidden tradeoff: a substantial share of the weight lost can be lean body mass, including muscle. For older adults—already vulnerable to age-relate
Walk into the conversation around Ozempic or Wegovy and the numbers tend to dominate: pounds shed, body mass reduced, metabolic health improved. But in clinics and labs, another set of measurements has been getting harder attention—how much muscle disappears when weight drops.
Among people losing weight on GLP-1 drugs such as Wegovy and Ozempic. researchers estimate that about 25 to 40 percent of the pounds shed is lean body mass. including muscle. For older adults, that isn’t a minor inconvenience. Their muscle is already under pressure from age-related loss that contributes to falls and bone fractures. Preserving muscle matters not just in later life. researchers note. but earlier too: the more muscle someone has in their 30s. the more they retain in their 60s. Losing any amount can amplify risks tied to declines in physical and metabolic function.
Every weight-loss intervention—including diets. bariatric surgery. and GLP-1 receptor agonist medicine such as Wegovy—melts muscle alongside fat to some degree. Now. with GLP-1 drugs widely used. scientists are racing to develop experimental medications aimed at weight loss that spares muscle as much as possible.
Muscle, after all, isn’t just strength on display. These bands of fibrous tissue help store and metabolize glucose and can be used to burn energy. Muscle mass also predicts mobility later in life. Between the 20s and 80s, people gradually lose around 30 percent of their muscle. Research suggests GLP-1 drugs may speed up that decline.
Clinical trials and estimates involving semaglutide—sold as Wegovy and Ozempic—and tirzepatide—sold as Zepbound and Mounjaro—suggest that within a few years of starting treatment, people may experience a loss of muscle mass equal to 20 years of age-related decline.
That risk lands on a population already facing two converging realities: older people have diminished muscle, and they also have a high rate of GLP-1 prescriptions. Add those together and the concern becomes less abstract—strength and function could erode faster than the body is accustomed to.
Not everyone views the issue the same way. Some researchers argue the amount of muscle lost during GLP-1 treatment is broadly proportional to the weight lost. and that may not be a health issue. But the counterpoint remains stark for older adults: they start with less muscle and are more likely to be prescribed GLP-1 drugs. which could turn proportional loss into a practical hit to physical resilience.
Complicating the picture further, new research awaiting peer review suggests there is a link between GLP-1 drugs and increased risk of osteoporosis.
Drugmakers have a clear motive to solve a problem that threatens to tarnish a success story. Eli Lilly—the maker of Zepbound and Mounjaro—is developing a muscle-retention drug called bimagrumab. The approach is built around myostatin, a protein that suppresses muscle growth. Mutations in the gene encoding myostatin can make the protein less effective. and animals born with such changes can develop extraordinary muscle. Such mutations and unusual muscle growth have also been documented in at least one human.
Myostatin binds to activin type II receptors on muscle cells, where it acts as an “off switch” that stops muscle development. Bimagrumab blocks these receptors, disabling the switch and allowing muscles to expand.
Its target may not be limited to muscle. According to Steven Heymsfield. an obesity and body-composition expert at Pennington Biomedical Research Center in Louisiana. bimagrumab also blocks activin type II receptors on fat cells. That action appears to reduce fat mass, he says. Heymsfield consults for Eli Lilly and has been involved in the company’s bimagrumab trials.
The strongest available signal comes from a phase 2 clinical trial funded by Eli Lilly and published in Nature Medicine. In that study, combining bimagrumab with semaglutide produced a 22 percent reduction in body weight over 72 weeks. But the distribution of weight lost tilted sharply: 92 percent of the weight lost was fat. compared with 76 percent of that lost on semaglutide alone.
People taking bimagrumab not only maintained more muscle than those receiving semaglutide by itself—they actually grew new muscle fibers. Grip strength also followed the same direction: those taking both drugs had the highest gains.
Still, the story is not a clean win. In early trials of bimagrumab. older adults developed more muscle but didn’t show meaningful improvements in grip strength. walking speed. or endurance. Dimitris Papamargaritis. an obesity researcher at the University of Leicester in England. interprets that outcome as possibly suggesting the drug may be “just making the muscle bigger without the metabolic and physical function benefits.”.
Additional studies that combine bimagrumab with tirzepatide are underway, and so are studies tied to other experimental approaches.
Researchers are also pursuing a different class of muscle-retention drugs: SARMs, or selective androgen-receptor modulators. These synthetic compounds are designed to activate the body’s androgen receptors in a way similar to testosterone. a hormone that can trigger muscle growth. But testosterone acts throughout the body, potentially creating bad side effects for organs such as the heart and the prostate. SARMs are engineered to act primarily in muscle and bone. with the hope of promoting muscle growth with fewer side effects than testosterone.
A 2025 review found that SARMs are correlated with improvements in physical performance and body composition, but the safety profile remains unclear.
Until new medications are proven safe and effective in more human trials. the most evidence-backed tools for protecting muscle are straightforward: resistance exercise and adequate protein intake. Ian Neeland. a cardiologist and obesity expert at Case Western Reserve University. puts it plainly: “The vast majority of older adults can tolerate [GLP-1] medications just fine.” For those taking the drugs. his recommendation is roughly 1.2 to 1.6 grams of protein per kilogram of body weight per day. about double the usual recommendation. He also says patients should engage in resistance training for about a third of their overall exercise time.
The through-line in this fast-moving field is uncomfortable: weight loss is working, but muscle is part of what’s disappearing. Now the goal is to keep the benefits—while making sure the body keeps what it needs to move, metabolize, and stay independent.
Ozempic Wegovy semaglutide tirzepatide Zepbound Mounjaro GLP-1 muscle loss bimagrumab myostatin SARMs osteoporosis protein intake resistance training