Carbs, Fats, and Protein: How Your Body Uses Fuel for Energy

The Basics of Energy Systems

When we think about fitness, training, and performance, we often focus on sets, reps, and workouts. But behind the scenes, your body is a finely tuned engine that relies on three macronutrients—carbohydrates, fats, and proteins—to produce energy. Each of these fuels plays a different role depending on exercise intensity, duration, and whether the activity is aerobic (with oxygen) or anaerobic (without oxygen).

Understanding how these fuels work isn’t just interesting—it’s essential if you want to optimise performance, recovery, and body composition. This knowledge also forms a core part of the Level 3 Personal Training qualification, giving trainers the tools to explain nutrition and energy systems clearly to clients.

Every movement we make—from walking to sprinting to lifting weights—requires energy. That energy comes in the form of adenosine triphosphate (ATP), the body’s energy currency.

Because the body only stores a tiny amount of ATP, it must constantly regenerate it through different energy systems:

• ATP-PC system (anaerobic, very short duration, high power)

• Lactic acid system (anaerobic, short duration, moderate power)

• Aerobic system (with oxygen, long duration, lower power)

The macronutrient you rely on depends largely on which of these systems is dominant during your activity.

Carbohydrates as Fuel

Carbohydrates are your body’s preferred high-intensity fuel. Stored as glycogen in the muscles and liver, carbs are quickly broken down into glucose to supply ATP.

• Anaerobic use: During high-intensity, short bursts (e.g. sprinting, heavy lifting), carbohydrates provide energy through anaerobic glycolysis, producing ATP rapidly but also generating lactate as a by-product, which can limit performance.

• Aerobic use: In longer sessions (running, cycling, circuits), carbohydrates can also be oxidised with oxygen for sustained energy, especially as exercise intensity rises.

Key takeaway: If you’re training hard and fast, carbs are king. This is why athletes carb-load before endurance events and why low-carb diets can compromise high-intensity performance.

Fat as Fuel

Fat is the body’s primary low-intensity, long-duration fuel. It’s stored abundantly in adipose tissue and muscles, making it a near-limitless source of energy.

• Aerobic use only: Fat cannot fuel anaerobic activity because it requires oxygen to be broken down. This means it’s the dominant energy source in activities like walking, hiking, and steady-state cardio.

• Slow but efficient: Although fat oxidation is slower than carbohydrate metabolism, it provides a huge amount of ATP per gram, making it perfect for endurance and recovery.

Key takeaway: If your workout is lower in intensity and longer in duration, fat will be your body’s main fuel. That’s why aerobic base training taps into fat stores and supports weight management.

Protein as Fuel

Protein is the body’s emergency backup fuel. Its main role is to build and repair muscle tissue, enzymes, and hormones—not to supply energy. However, in certain circumstances, protein can contribute to ATP production.

• Aerobic use in extreme conditions: When glycogen is depleted (long endurance events, starvation, or very low-carb diets), protein can be broken down into amino acids and converted into glucose via gluconeogenesis.

• Not ideal: This process is inefficient and can lead to muscle breakdown, which is why nutrition strategies for athletes focus on preserving protein for recovery, not fuelling.

Key takeaway: Protein is your body’s last resort for energy. Prioritise it for muscle repair and recovery by consuming enough dietary protein daily.

No Macronutrient Is “Bad”

One of the biggest myths in fitness and nutrition is that a single macronutrient is the enemy. For years, fat was demonised. More recently, carbs have been labelled as the reason people gain weight. The truth? No macronutrient is inherently bad.

• Carbohydrates provide fast energy and fuel high-intensity performance.

• Fats support hormone production, brain function, and long-duration energy.

• Protein repairs and builds muscle tissue, as well as supporting recovery.

When it comes to fat loss, the science is simple: it’s not about cutting out carbs or fats, it’s about creating a calorie deficit. That means consuming fewer calories than you burn.

You can achieve this deficit by:

• Reducing overall carbohydrate intake

• Lowering dietary fat intake

• Or a combination of both

As long as protein intake remains sufficient to preserve lean muscle, the specific balance of carbs and fats comes down to personal preference, lifestyle, and performance goals.

Practical Example for PTs

Imagine you’re working with two clients:

• Client A: Loves bread, rice, and pasta. For them, a moderate-to-high carb diet works best, so reducing calories by lowering dietary fat might be the easiest approach.

• Client B: Enjoys avocado, nuts, and cheese but doesn’t crave carbs. For them, keeping fats moderate and lowering carb intake may be more sustainable.

Both clients can achieve fat loss because the underlying principle is the same: calories in vs calories out. What differs is the route to creating that calorie deficit.

Key takeaway: There’s no single “bad” macronutrient. Sustainable fat loss comes from managing total calorie intake, not villainising carbs or fats.

Aerobic vs Anaerobic Fuel Use

Practical Applications for Trainers and Athletes

1. Match fuel to activity – High-intensity training relies on carbs, while low-intensity, longer sessions rely more on fat.

2. Periodise nutrition – Endurance athletes may increase carbs before key sessions, while fat adaptation strategies can be useful for aerobic base training.

3. Protect protein – Adequate protein intake supports recovery and muscle growth. Aim for 1.6–2.2 g per kg bodyweight daily.

4. Hydration matters – Energy metabolism is less efficient when dehydrated. Always pair nutrition with proper hydration strategies.

5. Individualise – Every client is different. Trainers should consider lifestyle, goals, and tolerance when giving nutritional advice.

Why This Matters for Fitness Professionals

For personal trainers and coaches, explaining these energy systems in simple terms helps clients understand why their diet matters as much as their training. It also gives authority and confidence when discussing performance and recovery strategies.

This topic links directly to both the Level 3 Certificate in Personal Training and the Level 4 Certificate in Strength and Conditioning, making it a core piece of knowledge for career progression.

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Next Steps

1. Apply this knowledge: Track how your training intensity changes your body’s reliance on carbs vs fat.

2. Subscribe: Get more science-backed fitness blogs and podcasts from Storm Fitness Academy.

3. Advance your career: Check out our Level 4 Strength and Conditioning course and other qualifications to deepen your expertise, and complete the contact form below if you’d like to speak to us for guidance and discuss your options.

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