A new study in mice and humans suggests GLP-1 medicines drive weight loss primarily by reducing fat, while preserving muscle function and easing fears about harmful muscle wasting.
Key takeaways
Fat, not skeletal muscle, accounted for most of the weight loss.
According to the study, GLP-1 medicines reduced body fat in obese mice much more than lean body mass, and fat and liver mass fell faster than skeletal muscle.
Muscle function was preserved better than some had feared.
Even though absolute muscle values fell slightly in mice, relative muscle mass and running performance improved. In the 12-week human pilot study, thigh muscle size decreased, but handgrip and knee extension strength did not worsen.
Loss of lean body mass is not the same as loss of skeletal muscle.
The study emphasizes that lean body mass includes bone, organs, and other tissues. It also found that liver mass decreased more than muscle mass, helping explain why body composition measures can overstate the impression of muscle loss.
Compared with calorie restriction, the drugs changed muscle biology in distinct ways.
Even with matched weight loss, GLP-1 treatment produced distinct muscle protein patterns: higher mitochondrial proteins in the non-immobilized leg and distinct remodeling signals in the immobilized leg.
Study: Weight loss with GLP-1 medicines does not result in a disproportionate loss of muscle mass or function in obese mice and humans. Image Credit: Corona Borealis Studio / Shutterstock
In a recent study published in the journal Cell Reports Medicine, researchers investigated concerns about potential skeletal muscle loss during treatment with GLP-1RA (glucagon-like peptide-1 receptor agonist) drugs. The study leveraged data from in vivo murine (mouse) model experiments and a human pilot trial, establishing that weight loss from these drugs is primarily driven by fat reduction rather than a disproportionate loss of muscle mass or function.
Study findings revealed that, while absolute muscle mass decreased slightly, the muscle-to-body-weight ratio improved in mice, and the lean-body-mass-to-body-weight ratio improved; relative muscle size was not negatively affected in humans. More importantly, the study found that GLP-1 drugs improved physical performance in mice, while preserving strength in human participants, thereby helping address prevalent concerns about their use in pharmacological weight-loss interventions.
Obesity Burden and GLP-1 Treatment Debate
Obesity is widely considered one of the most detrimental chronic metabolic conditions afflicting today’s human society. Public health reports indicate that the condition currently affects over 1 billion people worldwide, underscoring the urgent need for effective medical interventions.
The relatively recent development and introduction of GLP-1RA (glucagon-like peptide-1 receptor agonist) drugs like semaglutide and dual agonists like tirzepatide have provided an unprecedented pharmacological solution to unwanted weight gain, with documented weight reductions of 12-19% in non-diabetic patients.
Emergent clinical data, however, indicate that up to 40% of GLP-1-mediated weight loss is associated with reductions in lean body mass (LBM). These LBM findings have triggered an ongoing scientific debate, with critics worrying that such losses could result from reductions in skeletal muscle mass, thereby leading to frailty, particularly in older adults.
In contrast, supporters of GLP-1 drug use argue that LBM is an umbrella category that includes water-, bone-, and organ weight, not just skeletal muscle. Unfortunately, the present literature (primary data) directly measuring muscle function alongside mass during drug-induced weight loss has been sparse, leaving this debate unresolved.
Mouse and Human Study Design Methods
The present study aimed to address these ongoing concerns by conducting a comprehensive investigation comprising four preclinical in vivo mouse studies supplemented with data from a 12-week human pilot trial.
The animal studies utilized male diet-induced obese (DIO) mice to test various GLP-1 treatments, including semaglutide and tirzepatide. Echo magnetic resonance imaging (EchoMRI) was used to track baseline versus post-GLP-1 body composition, and subsequently to assess skeletal-muscle-associated performance metrics via grip strength and treadmill running tests.
Furthermore, the studies leveraged custom immobilization experiments to experimentally verify whether the drugs accelerated muscle loss during periods of limb disuse. These experiments used 3D-printed casts to restrict murine leg movements.
Finally, liquid chromatography-tandem mass spectrometry (LC-MS/MS) was performed to characterize the murine muscle proteome (the entire set of proteins expressed in the mouse muscular tissue) before and after GLP-1 administration.
The human pilot experiment comprised adult patients (n = 10) with clinical diagnoses of obesity and type 2 diabetes (T2D). These patients were administered escalating weekly doses of semaglutide (1 mg). Subsequently, the study leveraged B-mode ultrasonography to accurately measure the patients’ cross-sectional area (CSA) of their vastus lateralis (a key thigh muscle).
Additionally, participants’ strength relative to baseline was evaluated using maximum voluntary contraction (MVC) and handgrip dynamometry assays.
Fat Loss Dominates GLP-1 Weight Reduction
The study findings provided several lines of evidence against concerns about disproportionate muscle loss. In murine experiments, tirzepatide administration resulted in a 35% reduction in body weight (p < 0.0001). Notably, this weight loss was associated with a 73% reduction in fat mass, while LBM decreased by only 13%.
Furthermore, relative muscle mass estimates in experimental mice improved significantly (p < 0.05). Treadmill endurance test results corroborated these findings, demonstrating that obese mice on semaglutide regimens could run nearly as well as lean control mice.
Leg immobilization results revealed that GLP-1 drug-associated muscle loss did not differ significantly from that observed with calorie restriction matched for food intake or body weight. Finally, proteomic analysis showed that GLP-1RA treatment increased mitochondrial proteins compared with calorie restriction alone, suggesting that these drugs may specifically enhance muscle metabolic health.
In the human trial (12 weeks), fat loss was observed to account for ~70% of participants' weight reduction. While the absolute size of the thigh muscle decreased (p < 0.05), the participants' absolute and relative leg strength remained statistically unchanged from baseline measurements. Together, these findings imply that reductions in LBM are likely concentrated in non-contractile components rather than reflecting a clear decline in muscle function, though this was inferred rather than directly proven.
Implications for Muscle Preservation and Future Research
The present study concluded that weight loss via GLP-1 medicines does not result in disproportionate or pathological muscle loss in obese mice and humans. While slight reductions in absolute muscle mass were observed, concerns about loss-associated strength declines were addressed by observations of improved power-to-weight ratios and mobility gains in mice, alongside preserved strength in the small human cohort.
The researchers acknowledged that the study was limited by its small human sample size and the use of exclusively male mice. They emphasize that future work must determine if these observed benefits hold for older patients with pre-existing conditions like sarcopenia (age-related muscle loss) and other conditions associated with wasting or frailty.
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