If you're on Ozempic, Mounjaro, or Wegovy — or thinking about starting — you've probably heard the big number: people are losing 15 to 22% of their body weight on these medications. And that's real. The clinical trials back it up.
But here's what most GPs don't have time to explain in a 10-minute appointment: not all of that weight you're losing is fat.
Research from the major clinical trials — the STEP trials for semaglutide and the SURMOUNT trials for tirzepatide — shows that between 20 and 40% of total weight lost on GLP-1 medications can come from lean mass, which includes your muscle tissue. That's not a side effect buried in the fine print. That's a fundamental change to your body composition that affects your metabolism, your strength, your energy, and whether you keep the weight off long-term.
As a personal trainer on Sydney's North Shore, I'm seeing more and more clients walk through our door who are on these medications. They're losing weight — but they're also losing strength, feeling weaker, and noticing their body doesn't look or feel the way they expected. The good news? This is fixable. And it starts with understanding what's actually happening inside your body.
of weight lost on GLP-1 medications like Ozempic and Mounjaro can come from lean mass — not fat — according to STEP and SURMOUNT clinical trial data.
What Are Ozempic, Mounjaro, and Wegovy? A Quick Breakdown
All of these drugs belong to a class called GLP-1 receptor agonists. They mimic a hormone your gut naturally produces after eating, which signals your brain to feel full, slows stomach emptying, and helps regulate blood sugar. The net effect is that you eat less, you feel satisfied sooner, and you lose weight.
| Medication | Active Ingredient | How It Works | Avg. Weight Loss | Status in Australia |
|---|---|---|---|---|
| Ozempic | Semaglutide | GLP-1 agonist (single receptor) | ~15% body weight | TGA-approved for type 2 diabetes; off-label for weight loss |
| Wegovy | Semaglutide (higher dose) | GLP-1 agonist (single receptor) | ~15% body weight | TGA-approved for weight management |
| Mounjaro | Tirzepatide | Dual GLP-1 + GIP agonist | ~21–22% body weight | TGA-approved for weight management & type 2 diabetes |
| Saxenda | Liraglutide | GLP-1 agonist (daily injection) | ~8% body weight | TGA-approved for weight management |
The Muscle Loss Problem: Why It Matters More Than You Think
Muscle isn't just about looking toned. It's your body's metabolic engine. Every kilogram of muscle tissue burns significantly more calories at rest than a kilogram of fat. When you lose muscle, your basal metabolic rate drops — meaning your body needs fewer calories to function. This is the exact mechanism behind the weight regain that happens when people stop taking GLP-1 medications.
Data from the SURMOUNT-4 trial showed that participants who stopped tirzepatide regained roughly half the weight they'd lost within a year. And when weight comes back, it tends to come back as fat — not muscle. So you end up in a worse body composition than where you started: less muscle, same or more fat, slower metabolism.
What This Means for You
If you're taking Ozempic, Mounjaro, or Wegovy without doing structured resistance training, you're almost certainly losing muscle along with fat. This makes your results harder to maintain and your metabolism slower over time. The medication handles appetite — but only training protects your muscle.
What Your Doctor Probably Isn't Telling You
This isn't a criticism of GPs. They're working in 10-to-15-minute consultations and managing complex chronic conditions. But here's what typically gets missed in the prescribing conversation:
- 1The prescription isn't a complete solution. These medications are designed to be used alongside a structured diet and exercise program. The TGA and the FDA both state this clearly. But in practice, most patients receive the medication without a concrete exercise plan.
- 2Not all exercise is equal on these drugs. Walking and general cardio are great for cardiovascular health, but they do very little to prevent muscle loss. If you're in a significant calorie deficit — which GLP-1 medications create — cardio without resistance training can actually accelerate muscle breakdown.
- 3Your protein needs change dramatically. When you're eating less (which is the whole point of the medication), every meal matters more. You need to prioritise protein at levels most people aren't hitting — and that requires deliberate planning, not just eating less of what you normally eat.
- 4The medication is a tool, not a destination. The story of these medications is about improving metabolic health — not just dropping kilograms. Blood pressure, blood lipids, visceral fat, inflammation markers — these all improve. But only if the weight you're losing is the right kind of weight.
How to Train While on Ozempic or Mounjaro: A Personal Trainer's Protocol
1. Resistance Training 3–4 Times Per Week (Non-Negotiable)
This is the single most important thing you can do to protect your muscle mass. Research consistently shows that people who combine GLP-1 medication with structured resistance training retain significantly more lean mass than those who don't. We're talking about progressive overload — not just going through the motions with light weights.
2. Prioritise Compound Movements
Squats, deadlifts, rows, presses, lunges — these multi-joint exercises recruit the largest muscle groups and give you the most bang for your time. When you're eating less and your energy is lower, efficiency matters. Compound movements build and preserve more muscle per minute of training than isolation exercises.
3. Manage Your Energy and Recovery
GLP-1 medications reduce your appetite, which means you're likely eating fewer calories than before. That affects your energy in the gym and your recovery between sessions. A good trainer adjusts your program accordingly — managing training volume, intensity, and rest periods so you're stimulating muscle without overtraining.
4. Hit Your Protein Targets — Every Single Day
If you're losing weight on a GLP-1 medication and training to preserve muscle, you likely need closer to 1.6 to 2.0g of protein per kilogram of body weight per day. Practical protein sources: eggs, Greek yoghurt, lean chicken breast, fish, cottage cheese, tofu, and legumes. When your appetite is suppressed, every bite needs to count.
5. Monitor Body Composition, Not Just the Scale
The number on the scale doesn't tell you what you're losing. At PSI Health, we track body fat percentage, lean muscle mass, and key health markers over time — not just kilograms. This is how you know whether your weight loss is actually healthy weight loss.
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Ozempic With a Trainer vs. Without: What the Difference Looks Like
| Without Structured Training | With PSI Health |
|---|---|
| Lose 15–20kg, but up to 40% comes from muscle | Lose 15–20kg with significantly more fat loss and muscle retained |
| Metabolism slows as muscle mass drops | Metabolism stays higher — muscle acts as your calorie-burning engine |
| Feel weaker, less energy, skinny fat appearance | Look toned and strong, improved energy and functional strength |
| High risk of regaining weight after stopping medication | Sustainable results because your body composition has genuinely changed |
| No accountability or structured nutrition plan | Tracked body composition, daily accountability, and personalised nutrition guidance |
What If You Don't Want to Take Weight Loss Medication?
This is important to say: you don't need Ozempic or Mounjaro to lose weight. The fundamentals of sustainable weight loss haven't changed: a moderate calorie deficit, sufficient protein intake, progressive resistance training, consistent sleep, and a support system that keeps you accountable.
At PSI Health, we've helped clients lose 20, 25, even 30+ kilograms without medication — using the same evidence-based approach that forms the foundation of our training programs. The key differentiator isn't the medication. It's the accountability, the structure, and the expertise.
Frequently Asked Questions
Can I exercise while taking Ozempic or Mounjaro?
Yes — and you should. Both the TGA and FDA recommend combining GLP-1 medications with diet and exercise. Resistance training specifically is critical for preserving muscle mass during the rapid weight loss these medications produce. Start with 3 sessions per week and focus on compound movements.
How much muscle do you lose on Ozempic?
Clinical trial data from the STEP trials shows that lean mass can account for 20–40% of total weight lost on semaglutide. Structured resistance training and adequate protein intake significantly reduce muscle loss.
Is Mounjaro better than Ozempic for weight loss?
Clinical trials show Mounjaro (tirzepatide) produces greater average weight loss — up to 22% of body weight vs. about 15% for Ozempic (semaglutide). However, both carry similar muscle-loss risks. Your GP can help determine which is right for your situation.
Do I need a personal trainer if I'm on weight loss medication?
While not strictly required, a personal trainer dramatically improves the quality of your results. The medication handles appetite suppression, but it doesn't build muscle, preserve your metabolic rate, or ensure the weight you're losing is primarily fat.
What happens when you stop taking Ozempic?
Research shows that participants who stopped tirzepatide regained approximately half the weight they'd lost within a year. This is why building and preserving muscle during treatment is so important — it gives your metabolism the best possible foundation to maintain results long-term.
How much does Ozempic or Mounjaro cost in Australia?
Mounjaro costs approximately $280–$700 per month depending on dose. Speak with your GP for the latest pricing and eligibility information.
Puneet Sharma
18× Gold Medal Champion · Founder, PSI HealthPhysique and mindset transformation coach on Sydney's North Shore. Specialising in sustainable weight loss, body transformation, and personalised training for busy professionals. 158 five-star Google reviews.
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