Most people have heard phrases like fat burning zone, fuel your training or carb loading, but very few people understand what these things actually mean. You do not need a scientific background to understand the basics of how the body creates energy. You just need a simple explanation of ATP and the three main energy systems.
I see confusion around energy systems at times in my coaching work. When I am coaching runners and cyclists, it shows up as people under fuelling sessions, fading on hills or crashing late in long runs and rides. With my online weight loss coaching and support clients, it often shows up differently, through low energy, cravings, stalled progress or feeling like carbs are somehow the enemy. In reality, the same energy systems are at play in both training and fat loss. Understanding how they work removes a lot of unnecessary stress and makes food and training choices much simpler.
Once these concepts click, fuel in fitness and weight loss suddenly makes a lot more sense. You understand why some sessions feel easier than others, why you slow down on hills, why under fuelling leads to cravings and why both carbs and fats matter in different ways.
Let’s go through it…
What ATP Actually Is
ATP is the body’s main energy currency. Every movement, every muscle contraction, every thought and every heartbeat relies on ATP. The body cannot store much of it, so it has to regenerate ATP constantly.
Think of ATP like mobile phone battery. You use it all day and it needs constant topping up. Different types of exercise empty the battery at different speeds. This is why the body has several systems that work together to keep ATP levels topped up at all times.
The Three Energy Systems
All three systems operate at the same time, but one will dominate depending on the intensity of what you are doing.
1. The Phosphocreatine System
This is the fastest energy system and it provides immediate power for very short bursts of effort. It uses a compound called phosphocreatine that is stored inside the muscle.
You use this system for:
- Sprinting
- Short hill bursts
- Jumping
- Heavy lifting
- Fast accelerations
This system is powerful but brief. It only lasts a few seconds before the stores run out.
When these stores run out, it does not mean you are finished for the day. It simply means you cannot produce another all-out burst straight away. The phosphocreatine system recovers very quickly, usually within thirty seconds to a few minutes if you rest. This is why you can sprint, lift heavy, or accelerate hard again after a short breather, but not repeatedly without pause. The harder the effort, the longer the short recovery needs to be.
2. The Glycolytic System
The glycolytic system uses carbohydrate stored in the muscles as glycogen. It is used for moderate to high intensity activity where you are working hard but not sprinting.
You use this system for:
- Interval training
- Tempo runs
- Hard cycling efforts
- Hills
- High effort circuit training
This system produces lactate. Many people believe lactate causes soreness but it is actually a useful fuel that your body can recycle.
When this system is heavily used, you feel burning, fatigue, and a drop in power rather than a sudden stop. Recovery depends on how hard and how long the effort was. After a hard interval or hill session, this system can take several hours to fully recover, and sometimes up to a day if the session was very demanding. This is why back-to-back hard sessions often feel flat or sluggish unless recovery and fuelling are good.
3. The Oxidative System
This system uses a blend of carbohydrates and fats to create steady energy for long periods. It is slower than the other systems but can keep going for hours.
You use this system for:
- Long steady runs
- Long bike rides
- Walking
- Recovery sessions
Endurance athletes rely heavily on the oxidative system.
Because this system works more slowly and steadily, it does not “empty” in the same sharp way as the others. Instead, fatigue builds gradually as fuel stores drop and the body accumulates general tiredness. Recovery is more about refuelling and rest than waiting for the system itself to switch back on. With enough food and sleep, most people feel ready to go again within a day, but very long sessions can create fatigue that lingers for several days.
Carbs vs Fat: What Fuel Does The Body Prefer?
The body always burns a mix of carbs and fat. The ratio changes based on intensity.
- At lower intensity the body uses more fat
- At moderate intensity it uses a mix
- At high intensity it relies more on carbs
This does not mean one fuel is better than the other. It simply reflects how your body meets the demands of different types of training.
Fat Oxidation vs Fat Loss
A huge fitness myth is the idea that burning fat during a session means you are losing body fat. Fat oxidation means using fat for fuel during exercise. Fat loss means reducing stored body fat over time.
These are not the same thing.
You can burn fat during a session and still gain body fat if you are in a calorie surplus. You can burn mostly carbs during a hard run and still lose body fat if you are in a calorie deficit.
For most people, fat loss comes from overall energy balance, not the type of fuel used during a workout.
Hormones and Energy
Your energy systems decide how your body produces ATP, but your hormones influence how easily you can access and use that fuel.
The endocrine system is simply the body’s hormone system. Hormones act like messengers, helping different parts of the body communicate with each other. A few of the key ones involved in energy and weight loss are insulin, glucagon, cortisol, and the thyroid hormones often called T4 and T3.
These hormones do not replace your energy systems or override them. Instead, they influence how steady your energy feels from day to day and how smoothly fuel is moved around the body.
Insulin helps move fuel into storage after you eat, while glucagon helps release stored fuel when your body needs energy between meals or during exercise.
Thyroid hormones help regulate your overall metabolic rate, which affects how quickly energy is used. Cortisol plays a role in stress response and can influence cravings, appetite, and how tired or alert you feel.
Your energy systems still handle the actual production of energy. Hormones simply affect how easy or difficult that process feels in real life.
Why Carbs Matter For Gym-Goers, Runners and Cyclists
Carbs fuel harder work. When glycogen levels are low you may notice:
- Slower pace
- Heavier legs
- Fatigue early in the session
- Lower power output
- Cravings later in the day
- Reduced recovery
- More chance of injury
I often find that runners and cyclists do not struggle because they lack effort, they struggle because they lack fuel.
I see this all the time with clients who swear they are “eating fine”, then realise their lunch was a yoghurt and a banana before an evening gym session.
Under fuelling increases cravings, which often leads to overeating later. Proper fuelling for runs, cycling or the gym improves training quality, which supports better fat loss in the long run.
Carb Periodisation Explained Simply
You can think of it as matching your carbs to your training.
- Harder training days need more carbs
- Long distance days need more carbs
- Easy days can be normal
- Rest days can be normal
This does not always require tracking grams. It can be as simple as adding an extra bowl of oats before a long ride or having some pasta the night before a harder run.
How Creatine Supports Explosive Effort
Creatine helps top up the phosphocreatine system. This supports short bursts of high power output such as heavy lifting or short accelerations when running or on the bike.
Creatine does not burn fat. It does not affect weight loss directly. What it does do is improve strength, power and training quality, which can help you get more out of your sessions.
It is one of the safest and most researched supplements available.
Practical Fuel Guidance For Real Training Sessions
Here is a more detailed breakdown of how to fuel sessions based on demand.
Before Strength Training
A small amount of carbohydrate helps with performance.
Good options include:
- Fruit
- Oats
- A slice of toast with jam
- Yoghurt with honey
- A small bowl of cereal
You do not need a full meal. Just enough to avoid training on an empty tank.
Before Short Runs or Short Rides
Short sessions at easy to moderate intensity usually do not need specific fuelling if you have eaten normally.
Two to three hours after a meal works well. Sometimes you have to be practical though. Sometimes if I run or cycle early, I just have some jam and toast and crack on right away!
Before Long Runs or Long Rides
These sessions rely more on the oxidative system and need consistent fuel.
The day before, include:
- Pasta
- Rice
- Potatoes
- Bread
- Oats
On the day, have:
- Porridge
- Banana
- Toast with jam
- Cereal
- Bagel
If the session lasts more than ninety minutes you may benefit from taking carbs during the session.
During Long Sessions
If you train longer than about ninety minutes, you will likely benefit from fuelling during the session.
Examples include:
- Gels
- Chews
- Sports drink
- Flapjack
- Pieces of fruit
- Small sweets
Aim for thirty to sixty grams of carbohydrate per hour depending on intensity. Some science and pro-athletes go much higher than this now, but for most people this level of carbs will still work well.
After Training
Recovery needs both protein and carbohydrates.
Good combinations include:
- Chicken and rice
- Yoghurt and fruit
- Eggs on toast
- Porridge with protein powder
- Tuna pasta
- Cereal and milk
This helps replenish glycogen and repair muscle.
FAQs
Does eating carbs stop fat loss?
No. Fat loss comes from a calorie deficit. Carbs can help you train harder and recover better, which often improves consistency.
Is fasted cardio better for fat loss?
Not for most people – especially women. It might burn a higher percentage of fat during the session, but it does not improve overall fat loss. Many people also perform worse when fasted.
Why do I crash halfway through a run or ride?
This usually comes from low glycogen. If you start a long session under fuelled you are likely to hit the wall. Eating more the day before and adding carbs during the session usually solves this.
Do I need to avoid sugar completely?
No. Sugar is simply a fast acting carbohydrate. In a balanced diet it is not a problem and it can be useful during endurance sessions.
How To Put This Knowledge Into Action
If you want to fuel your training in a way that supports energy, performance and long term fat loss, I can help you create a plan that fits your lifestyle. You do not need extreme diets or complicated rules. You just need a simple structure that works for you.
If you want weight loss coaching or support with your sports performance, you can book a free consultation and find out how my coaching can help you.
