Hey there, I’m Matt.
If you’re reading this, there’s a good chance you’ve either started your journey with a weight loss injection, what everyone is calling the "fat jab", or you’re seriously considering it. Whether it’s Mounjaro, Wegovy, or Ozempic, these medications are changing lives, and I’ve seen some pretty incredible transformations.
I qualified as a coach in 2025, and since then, I’ve been working closely with people just like you at Muscles Matter. My goal is to help people get strong, feel mobile, and actually enjoy the body they live in. But here’s the thing: while these jabs are an amazing tool, they aren't a "set it and forget it" solution.
I’ve noticed a few common traps that people fall into when they start on the jab. These mistakes can slow down your progress, make you feel like rubbish, or, worst of all, cause you to lose the very thing we want to keep: your hard-earned muscle.
So, let’s dive into the 7 mistakes you might be making and, more importantly, how we can fix them.
1. You’re ignoring the "Muscle Tax"
When people talk about the "fat jab," the focus is usually on the number on the scale going down. And it does go down. Fast. But here is the catch: when you lose weight that quickly, your body doesn't just burn fat. It looks at your muscle mass and thinks, "I don't need all this expensive tissue right now," and starts breaking it down for energy.
I see this all the time. People lose three stone but end up feeling "soft" or weaker than before. This is what I call the "muscle tax." If you aren't actively protecting your muscle, you’re losing it along with the fat. This is a big deal because muscle is your metabolic engine. The less you have, the harder it is to keep the weight off long-term.
2. You aren't eating enough protein
This is probably the biggest hurdle for my clients on Mounjaro. The medication is literally designed to make you less hungry. It’s great for stopping the late-night fridge raids, but it also makes it really easy to under-eat.
When you aren't hungry, the last thing you want is a big chicken breast or a steak. So, people end up living on crackers, fruit, or tiny portions of whatever is lying around. If you don't hit your protein targets, your body has no choice but to scavenge your muscles for amino acids.
I always tell my clients: protein is non-negotiable. Even if you don't feel like a full meal, you’ve got to find ways to get it in. Think protein shakes, Greek yoghurt, or lean meats. If you want to dive deeper into how to balance this, check out my guide on strength training on Mounjaro.
3. You’ve stopped lifting weights (or never started)
I get it. When you're on the jab, the weight is falling off anyway, so why bother hitting the gym?
The truth is, resistance training is the "signal" your body needs to keep its muscle. When you lift weights, you’re telling your brain, "Hey, I’m still using these muscles, don't burn them for fuel!"
If you just do cardio or, worse, nothing at all, you’ll end up with what people call "skinny fat." You’ll be smaller, sure, but you won't have that toned, strong look, and your posture might even suffer. You don't need to spend hours in the gym, either. I’m a huge fan of The 45-Minute Method, which gets the job done without leaving you exhausted.
4. You’re eating out of habit, not hunger
This is a psychological one that I see a lot. These medications are powerful at suppressing appetite, but they don't automatically delete twenty years of habits.
Many people keep their usual meal times and portion sizes just because "that’s what I always do." They sit down for a massive Sunday roast or a large pasta dish, realise halfway through they are stuffed, but finish it anyway. This often leads to that "Mounjaro nausea" or feeling incredibly bloated.
The jab gives you a "pause button." Use it! Before you eat, ask yourself if you’re actually hungry. If you’re not, it’s okay to skip the snack or have a much smaller portion. Learning to listen to those new fullness cues is a massive part of the long-term success of this journey.
5. You’re neglecting fibre and water
Let’s talk about the "glamourous" side of weight loss jabs: the digestion issues. Constipation and bloating are the most common complaints I hear.
Because you're eating less, you're naturally taking in less fibre and less fluid from your food. Plus, the medication slows down your gastric emptying (which is how it keeps you full). If you don't consciously drink more water and stay on top of your fibre intake, things are going to get uncomfortable.
I always recommend my clients carry a water bottle everywhere and focus on getting some greens in at every meal, even if it’s just a handful of spinach wilted into some eggs. Staying hydrated is also key for recovery, especially if you're booking in for a sports massage to help with those training aches.
6. You’re injecting in the same spot every time
Did you know that where you jab matters? A common mistake is finding one "comfortable" spot on the stomach and sticking to it every week.
Over time, this can cause a buildup of scar tissue or fatty lumps under the skin, which can actually make the medication less effective because it isn't being absorbed properly. It’s really important to rotate your injection sites. Move between your stomach (staying at least 2 inches away from the belly button), your thighs, and even the back of your upper arms.
It might feel a bit weird to switch it up, but it keeps the tissue healthy and ensures you're getting the full benefit of the dose.
7. You’re treating the jab as the "whole solution"
I LOVE what I do because I see people transform their lives, but I have to be honest: the jab is a tool, not the whole toolbox.
If you use the medication to lose weight but don't change your relationship with movement or food, what happens when you stop? The most successful people I work with at Muscles Matter are the ones who use the "quiet head" the medication provides to build new, healthy habits.
They start personal training, they prioritise sleep, and they learn how to fuel their bodies properly. They treat this time as a "training camp" for the rest of their lives. If you’re just starting out and feeling overwhelmed, have a look at our Beginner’s Guide to Fat Loss, it’s a great place to get your head in the game.
Why muscle actually matters (The Clue is in the Name!)
At Muscles Matter, we don't just care about you being smaller. We care about you being better.
When you lose muscle, you lose strength. You lose the ability to move without pain. You lose that "spring in your step." I’ve spent a lot of my career as a sports massage therapist helping people fix aches and pains that come from being weak or immobile. Whether it’s through deep tissue massage or soft tissue therapy, I see the physical toll that neglecting your muscles can take.
If you’re on the jab, you have a golden opportunity to hit the reset button. Don’t waste it by just "disappearing." Build a body that is strong enough to support your new, lighter weight.
Ready to do this right?
If you’ve realised you might be making one (or all) of these mistakes, don't panic. It’s all part of the learning curve. The fact that you’re looking for info tells me you’re serious about your health.
If you want some professional help to navigate this, whether it’s through a structured training plan or some recovery work like cupping or massage, I’m here to help. You can check out how to book your session here.
Let’s make sure that when you reach your goal weight, you’re the strongest, healthiest version of yourself.
Stay strong,
Matt McArdle
Online Coach & Sports Massage Therapist
Muscles Matter
Please note: The views and advice shared in this blog are my personal opinions based on my experience and are not intended as medical fact or professional medical advice.
Please note: The views and advice shared in this blog are my personal opinions based on my experience and are not intended as medical fact or professional medical advice.
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