We often think of diet in terms of body weight. Eat too much, gain weight. Eat less, lose weight. Simple, right? Unfortunately, that’s a dangerous oversimplification. Behind the number on the scale lies a far more complex picture of what’s happening inside your body, especially to your muscles and bones.
If your diet is poor, low in nutrients, extreme in restrictions, or simply unbalanced, you could be losing more than just fat. You might be shedding valuable muscle mass and even weakening your bones, without realising it. This blog explores how a bad diet can silently erode your strength from the inside out, why understanding how your body burns energy matters more than you think, and why tracking your body composition with a DEXA scan is absolutely essential.
Not All Weight Loss is Good Loss
Let’s start with an uncomfortable truth: not all weight loss is healthy. While fat loss might be the goal, it often comes at the cost of muscle and even bone density, especially when the diet isn’t done properly. This is especially common with fad diets, crash diets, or overly restrictive eating plans.
Weight loss without considering body composition is a bit like throwing away the contents of your handbag to make it lighter, only to realise you’ve chucked your wallet, keys and phone. Sure, the bag weighs less, but it no longer does its job.
Muscle mass and bone density aren’t optional extras — they are vital components of a healthy, strong body. Lose too much of either, and you risk a drop in your metabolism, poor physical function, and long-term health issues like osteoporosis or frailty.
How a Bad Diet Destroys Muscle
Muscle isn’t just for bodybuilders. It’s essential for everyday function, stability, and metabolic health. But muscle is metabolically expensive — it burns calories even when you’re at rest. So when you cut calories too far, your body sees muscle as a luxury it can’t afford.
Here's how bad diets wreck muscle mass:
1. Not Enough Protein
Protein is crucial for muscle maintenance and repair. If your diet is too low in protein, especially when losing weight, your body lacks the building blocks to hold onto muscle. This is common in low-calorie diets where quantity trumps quality.
2. Excessive Calorie Deficit
Going too low on calories, particularly below your resting metabolic rate (RMR), pushes your body into conservation mode. It slows metabolism, burns through glycogen, and turns to muscle tissue for energy. You may see quick results on the scale, but much of that is muscle, not just fat.
3. No Resistance Training
A bad diet combined with no resistance training is a perfect recipe for muscle loss. Strength training signals the body to preserve muscle. Without it, and without enough fuel, your muscles shrink — a process known as muscle catabolism.
Carbs, Fats and Muscle Loss: Understanding Energy Use
To understand how diet affects muscle mass, we need to look at how your body uses fuel.
Your body burns both fat and carbohydrates (glucose) for energy. Which one it uses more depends on the intensity of activity and what’s available. When resting or doing light activity, you’ll mostly burn fat. As intensity increases, especially during exercise, you start burning more carbs.
When carbohydrates are scarce and fat stores aren’t readily accessible (as in fasted states or extreme diets), the body starts to break down muscle to create glucose through a process called gluconeogenesis. Yes, your body will eat your own muscle to keep your brain and vital organs running.
This is especially true during:
Low-carb diets with little protein
Intermittent fasting without resistance training
Very low-calorie diets for extended periods
When the body needs quick energy and it’s not coming from food, muscle tissue becomes fair game.
That’s why the optimum time to have a carbohydrate snack is immediately after exercise. This is when your muscles are depleted of glycogen and are most receptive to replenishment. A quick, healthy carb source after your workout helps restore your energy stores and reduces the risk of your body breaking down muscle tissue for fuel later on. At BodyView, we provide a Vo2 Fat Max test, cutting out the guess work. This test spells out your opitum fat burning zones, without cutting in to your carbohydrate stores.
Vo2 Fat Max Test Example Report
The Role of Bone in Metabolism and Health
Now let’s talk about bones. You might not think of bones as active tissue, but they are. They’re constantly being broken down and rebuilt in a cycle called bone remodelling. To do this properly, your body needs nutrients, calcium, vitamin D, vitamin K2, magnesium and protein — all of which are often lacking in bad diets.
Here’s how poor nutrition affects bone health:
1. Calcium Deficiency
Calcium is essential for bone strength. If your diet is low in calcium (common in dairy-free or restrictive diets), your body pulls calcium from your bones to keep blood levels stable. Over time, this weakens bones.
2. Low Vitamin D
Vitamin D helps your body absorb calcium. Without it, even if you consume enough calcium, your body can’t use it effectively. Many people in the UK are deficient in vitamin D, especially in winter.
3. Acidic Diets and Bone Loss
Diets high in processed foods, sugar, and animal proteins can increase the acid load on the body. To buffer this acidity, the body may draw alkaline minerals like calcium from bones, further depleting bone density.
4. Weight Loss and Bone Density
Weight-bearing helps maintain bone strength. When you lose weight, especially rapidly, you reduce the load on your bones. Combined with poor nutrient intake, this accelerates bone loss — particularly in older adults.
The Vicious Cycle: Less Muscle = Weaker Bones
Muscle and bone health are closely linked. The stronger your muscles, the more stress they put on bones, which in turn makes bones denser and stronger. Lose muscle, and bones lose this stimulus.
So, if your diet leads to muscle loss, it also leads to bone weakening. Add in poor nutrient intake, and you’ve got a dangerous mix for long-term skeletal health.
Why You Might Not Notice Until It’s Too Late
The worst part? Muscle and bone loss are often invisible.
You might be thrilled to see the scales drop but unaware that you’re becoming metabolically slower, weaker, and more fragile. Unlike fat loss, which you can see and measure, bone and muscle loss can go undetected for years.
By the time you feel weaker or notice changes in posture, balance, or strength, the damage is often done.
Track Your Progress With a DEXA Scan
This is where technology comes in. A DEXA Body Composition scan and Bone Density scan can help you see the full picture, not just how much weight you’re losing, but what kind of weight.
Are you losing fat or muscle?
Are your bones staying strong?
Is your fat loss coming from the right areas?
These are vital questions, especially when starting a new health or weight loss plan. A DEXA scan at BodyView gives you precise data before and after any dietary or fitness programme. You’ll know exactly how your body is changing, and if you need to make adjustments to protect your long-term health.
The BodyCompPro report
The BodyCompPro report is the gold standard in DEXA body composition analysis — and it’s available exclusively at BodyView. Developed with athletes and performance-focused individuals in mind, this advanced report goes far beyond basic DEXA data. It provides a precise regional breakdown of fat and lean mass, muscular symmetry, visceral fat, and trend tracking over time. With professional benchmarking against athletic norms, BodyCompPro gives you the most detailed, actionable insights available, empowering you to optimise your training, monitor progress with accuracy, and achieve measurable results like never before.
DEXA BodyCompPro Example Report
If You’re Using Mounjaro, Be Extra Careful
Medications like Mounjaro (tirzepatide) are helping many people lose weight, especially those struggling with obesity or type 2 diabetes. However, rapid weight loss, particularly with appetite-suppressing drugs, comes with increased risk of muscle and bone loss if not managed carefully.
Use a DEXA scan to monitor your muscle and bone mass
At BodyView, we strongly recommend a DEXA scan if you’re using Mounjaro or any other weight loss medication. With such medications, weight can drop quickly, but without careful planning, it may not be the kind of weight you want to lose.
How to Protect Muscle and Bone While Losing Fat
Thankfully, you don’t have to choose between fat loss and health. You can lose fat while protecting (and even gaining) muscle and bone density, if you’re smart about it.
1. Prioritise Protein
Seek advice for your ideal protein per kg of body weight daily during weight loss. This supports muscle maintenance and keeps you feeling full.
2. Include Resistance Training
Lift weights 2–4 times a week. Strength training is the best way to signal your body to keep muscle and bones strong.
3. Don’t Slash Calories Too Far
Stay within a moderate calorie deficit! Use an RMR (resting metabolic rate) test to determine your baseline needs.
4. Supplement Wisely
Consider supplements like vitamin D3, magnesium, and a quality multivitamin, especially if your diet lacks variety.
5. Eat Whole Foods
Choose nutrient-dense foods: vegetables, lean proteins, whole grains, nuts, seeds, and dairy (or calcium-rich alternatives). Avoid ultra-processed junk that offers calories without nutrition.
6. Refuel After Exercise
Don’t skip your post-workout carbs. A small snack, like a banana, oat bar, or rice cake, helps replenish your glycogen stores, supports recovery, and prevents your body from breaking down muscle for fuel.
7. Monitor Your Body Composition
Get a DEXA scan at BodyView before and after your programme. Knowing exactly how your muscle, fat, and bone levels are changing will keep you informed and in control.
A bad diet might make you lighter, but it can also make you weaker. Losing muscle and bone is a hidden danger that often gets overlooked in the rush to drop fat. But your long-term health, strength, metabolism, and independence depends on preserving what really matters.
Don’t let a number on the scale fool you. Healthy weight loss is about body composition, not just weight. Eat to fuel, lift to grow, and nourish your bones from the inside out.
Whether you're following a diet, using medication like Mounjaro, or training hard at the gym, protect your body with science.
A DEXA scan at BodyView gives you the knowledge you need to lose fat the right way and stay strong for life.