The Untold Story Behind GLP-1 Weight Loss and Protein
Over the past year, GLP-1 medications have taken over almost every conversation about weight loss. Clients ask about them. Coaches debate them. Families look for guidance. And inside all of that excitement sits one awkward, unavoidable truth that keeps coming up: weight loss caused by GLP-1s often includes muscle loss, not just fat loss.
That theme has already come through in several of our previous pieces on this topic. In an effort to continue the much needed education on this subject, this article considers it further by looking at how these medications might affect adolescents and younger adults and how nutrition and training can support them.
Before going any further, this type of knowledge can be applied by those studying a personal trainer course thought educated programme design and client monitoring. The context matters because coaches are usually the first people clients turn to when their weight drops quickly but their strength doesn’t feel the same.
Another layer sits inside the world of nutrition education. The relationship between GLP-1s, appetite, protein intake and muscle health appears throughout the kind of work taught in a nutrition coach course, especially when appetite-reducing medications make consistent eating patterns harder to maintain.
And as GLP-1s become more common throughout every corner of the health and fitness sector, many professionals begin exploring fitness courses that help them stay updated on the science so they can guide clients with more confidence.
With that context in place, let’s get into the study.
A Quick Reset on GLP-1s and Muscle Loss
Most people are familiar with the general idea. GLP-1 receptor agonists like semaglutide lower appetite, slow gastric emptying and help stabilise blood glucose after meals. This combination makes sustained calorie deficits easier. Research consistently shows clinically meaningful reductions in bodyweight in people living with overweight or obesity. The Liraglutide obesity trials, the STEP semaglutide trials and follow-up work from cardiometabolic research groups all report very similar outcomes.
Alongside this, other studies have found that lean mass accounts for a notable portion of the weight lost. Some trials report around 20%. Others show closer to 40%. The numbers shift depending on dose, duration, baseline weight and how much movement or resistance training a person does during the intervention. Appetite drops sharply on GLP-1s, so total protein intake often falls too. When protein dips and training becomes inconsistent, muscle takes the hit.
Those findings explain why GLP-1s remain effective for lowering health risks linked to obesity but also raise concerns about long-term physical function. That tension becomes even more important in younger people, where bodies are still building towards peak muscle mass and bone density.
The New Study at a Glance
A recent MSc thesis by Brooke A. Morris (2025) takes this discussion in a useful direction by exploring how semaglutide affects muscle, fat and bone in an adolescent obesity model. The study uses mice, but the principles apply strongly to conversations around youth training, appetite, protein intake and long-term musculoskeletal development.
Here is the basic setup in simple terms. Male adolescent mice were raised on a high-fat diet to generate an obesity phenotype similar to what clinicians see in humans. After a period of acclimatisation, they were split into three groups:
- Western Diet + vehicle (control)
- Western Diet + semaglutide
- Western Diet + semaglutide + higher protein intake (a 60% increase from the standard diet)
The intervention ran for six weeks, which is an important window as this period aligns with adolescent development in mice, where muscle and bone growth remain active. Throughout the study, bodyweight, fat depots, muscle mass, bone volume and contractile muscle function were measured.
This gives a fuller picture of how GLP-1 medication interacts with a growing musculoskeletal system and how protein intake may buffer those effects.
What Happened to Bodyweight and Fat
The semaglutide group showed a clear reduction in bodyweight compared to the control group. That part isn’t surprising. GLP-1s reliably reduce energy intake and create steady weight loss in both clinical and preclinical research.
Interestingly, the pattern of fat reduction wasn’t uniform across different adipose depots. Subcutaneous fat around the hindlimb region decreased, while visceral fat showed limited change. This mirrors some human data where subcutaneous fat responds more consistently to pharmacotherapy than deeper visceral stores during early treatment phases.
This nuance matters for coaches because clients often expect rapid changes in waist measurements or midsection appearance, but the body sometimes shifts in a more compartment-specific way.
Muscle Size and Strength Took a Hit
Now comes the part that affects coaches most directly. The semaglutide-only group ended the trial with smaller limb muscles. Several individual muscles, including the soleus, gastrocnemius, plantaris, extensor digitorum longus and others, showed reductions in absolute muscle mass. The reductions matched the drops in overall bodyweight.
But the more important finding was functional. When maximal contractile force was measured, the semaglutide group produced less force. That doesn’t always happen with weight loss, but semaglutide’s appetite-suppressing effect likely reduced total protein intake and lowered available amino acids for muscle maintenance. Adolescence is normally a period of rapid musculoskeletal growth, so a sudden drop in substrate availability disrupts that process.
Another interesting point is that submaximal contractile function and mitochondrial respiration remained intact. So the internal machinery of the muscle wasn’t showing broad dysfunction. There was simply less muscle available to generate force.
This distinction offers a useful talking point for coaches. A client might still feel capable during everyday movement. They may still complete easy steps or basic tasks without noticing anything unusual. The shift becomes obvious during higher-intensity, higher-force actions such as lifting, pushing, sprinting, or any activity requiring strong contractile output.
What Happened to Bone
One unexpected finding came from the bone analysis. The semaglutide group actually showed increases in load-bearing bone volume. That result sits outside many people’s assumptions, because weight loss usually decreases bone loading and sometimes reduces bone density over longer periods.
This area is still early in research, so coaches should avoid drawing broad conclusions. But it does suggest that semaglutide does not automatically harm bone in younger models and may interact differently with bone turnover mechanisms under specific conditions.
For now, the important point to note is that bone did not decline in this study, which gives coaches room to focus on muscle without assuming bone is also moving in the same direction.
The High-Protein Twist
The most valuable part of the study comes from the third group which took semaglutide while on a high-protein diet. When protein intake increased by 60% using a blend of whey and casein:
- Overall bodyweight returned to a level similar to the control group
- Muscle mass across the measured muscles recovered
- Maximal contractile function climbed back to normal
- Fat mass still declined in meaningful ways
This combination shows that protein intake acts as a protective factor for muscle during GLP-1 treatment. The higher protein didn’t eliminate fat loss. It simply created the nutritional environment needed to preserve or restore muscle in a body going through rapid changes.
For coaches, this reinforces a theme that appears in many human trials as well. Protein matters during weight loss. Progressive resistance training is also very important. When appetite drops sharply and spontaneous movement declines, both need to be built in deliberately.
Why This is Especially Important in Young People
Adolescence and early adulthood are critical windows for physical development. Most people reach peak muscle mass and peak bone density in the late teens through to the mid-20s. The habits, nutrients and loading patterns established during these years create the foundation for strength, metabolic health and bone integrity throughout the rest of adulthood.
Increased adiposity and low-quality nutrition during adolescence can reduce the proportion of bodyweight made up of lean mass. Add pharmacotherapy on top of that and the system becomes more fragile. A drop in muscle during these years slows movement development, reduces the energy turnover of the muscle tissue that remains and may alter long-term physical confidence.
The study doesn’t provide human data, but it highlights a principle coaches already see in practice. When younger clients show interest in weight-loss medication under medical supervision, building a strategy around muscle maintenance should sit at the heart of the plan. That means structured training, targeted nutrition and active monitoring from day one.
Protein as a Practical Tool During GLP-1 Treatment
Protein is not a magic fix for everything, but this study shows it offers a meaningful buffer against muscle loss triggered by appetite suppression. Clients taking GLP-1s often feel full quickly and struggle to hit even basic protein targets, let alone higher amounts. Coaching support becomes a key tool here.
Breaking protein targets into light, manageable meals works well. Liquid protein can help on days of very low appetite. Fortified foods, consistent meal routines and placing protein near training windows all support better outcomes. The most important job is making sure protein becomes a non-negotiable part of the routine, even when overall food intake is low.
Training Still Matters, A Lot
Medication can lower appetite. It cannot build muscle. It cannot strengthen tendons. It cannot improve coordination. It cannot stimulate bone the way gravity and impact do.
A structured resistance training routine becomes the anchor for clients using GLP-1s. For younger clients, training also helps restore confidence in movement, upgrades neuromuscular coordination and supports growth that would otherwise slow during weight loss.
A simple framework works well here. Two to three weekly full-body resistance sessions using major movement patterns. Slow, steady progression. Impact-appropriate conditioning. A movement routine that feels supportive, not punishing. And regular check-ins that track strength, not simply scale weight.
How Coaches Can Approach These Conversations
Clients often arrive with a clear goal, which is generally, they want to lose weight. Many feel relieved once medication starts working. But the job of a coach includes explaining that muscle and bone form the long-term foundation for a strong, capable body and those structures need support during rapid weight loss.
Parents appreciate the same message when their adolescent is involved. The aim isn’t to discourage the use of GLP-1 medication under medical care. The aim is to frame the conversation around health, growth, movement, confidence and physical development.
When framed this way, clients understand why proteins, training sessions and regular monitoring matter just as much as the number on the scales.
A Few Final Thoughts
This study uses animals, so it cannot answer every question about human health. But it gives us as coaches and young clients something important to think about. GLP-1 medications can be part of a weight-management plan for people living with obesity, including adolescents under specialist care. And at the same time, the musculoskeletal system still needs attention.
Protein intake, resistance training and a focus on functional capacity make that possible. Many clients begin these medications without guidance on how to keep their muscle strong. Coaches fill that gap. Nutrition professionals fill that gap. A thoughtful, well-structured plan helps clients feel safer and more confident throughout the process.
The next wave of research will eventually show how these findings play out in humans. Until then, the principle stands. The goal isn’t just lighter bodies. The goal is stronger, healthier, more capable bodies that continue to grow and thrive.
Reference
- Morris, B. A. (2025). Anti-obesity pharmacotherapy-induced loss of muscle mass and function is rescued by modulating dietary protein consumption in a model of adolescent obesity. York University. Click here to review the full research article.
- Haase, C. L. et al. (2021). Weight loss and risk reduction of obesity-related outcomes in 0.5 million people: evidence from a UK primary care database. International Journal of Obesity, 45, 1249–1258. Click here to review the full research article.
- Powers, S. K. et al. (2016). Disease-induced skeletal muscle atrophy and fatigue. Medicine & Science in Sports & Exercise, 48, 2307–2319. Click here to review the full research article.
- Josse, A. R. et al. (2011). Increased consumption of dairy foods and protein during diet- and exercise-induced weight loss promotes fat mass loss and lean mass gain. Journal of Nutrition, 141, 1626–1634. Click here to review the full research article.
- Hughes, V. A. et al. (2002). Longitudinal changes in body composition in older men and women. American Journal of Clinical Nutrition, 76, 473–481. Click here to review the full research article.
- Wing, R. & Phelan, S. (2005). Long-term weight loss maintenance. American Journal of Clinical Nutrition, 82, 222S–225S. Click here to review the full research article.
Use Exercise to Shape the Future Health of Young People
When you work with young people, their bodies aren’t just changing, they’re building the foundation they’ll carry into adulthood. Our Exercise for Adolescent Clients course gives you the skills to support them properly at this crucial stage. A recent study on adolescent obesity and GLP-1 medication found that six weeks of treatment reduced limb muscle mass across multiple muscles and lowered maximal force output in growing bodies. That means strength and functional capacity can slip quickly when appetite drops or training fades. The same research showed that targeted nutritional strategies restored muscle size and strength, reinforcing just how responsive adolescent physiology is when you get the training and lifestyle support right. This course helps you understand those developmental demands, create age-appropriate programmes and coach young clients with confidence, clarity and care.
Exercise for Adolescent Clients Course – Distance Study
Guide Clients With Smarter, Evidence-Led Nutrition
Supporting clients with nutrition goes far beyond calorie targets and the Level 4 Nutrition Coach Course is designed for professionals who want to give accurate, evidence-led advice with real-world impact. Recent research in an adolescent obesity model showed that semaglutide sharply reduced spontaneous food intake during treatment, which meant overall protein availability dropped and muscle mass declined across several major muscle groups. The same study highlighted how powerful nutrition can be when used well; increasing dietary protein by 60% restored muscle size and strength even during ongoing pharmacotherapy. These findings reinforce how closely nutrition shapes body composition, energy and performance, especially when appetite shifts or clients follow structured weight management plans. This course helps you dive into those nutritional dynamics, understand how diet interacts with physiology and guide clients with confidence, clarity and care.
Nutrition Coach Course – Distance Study
