By 

Giles Elmore (BSc Hons) MRSPH

It often comes to fruition at very predictable times, the last climb, the final surge home, or that final long drag into a headwind. All are moments where you require a little more power or intensity, and it simply isn’t there. It’s easy to put this down to fatigue or fitness. Quite often however, it’s fuel. Or more specifically, the type of fuel available at those moments.  

To understand why, you must look at how the body powers exercise. There are two primary fuels available to working muscle: fat and carbohydrate. Both are always present, and both contribute to energy production. However, they are not equal in how they perform.  

Fat is abundant. Even a lean cyclist carries tens of thousands of calories stored as fat. In theory, it represents an almost limitless energy supply. For long, steady efforts, this is ideal as fat can provide a consistent stream of energy for hours at a time. But there is a limitation. Fat is slow.  

Before fat can be used for energy, it must be mobilised from storage, transported through the bloodstream, taken up by muscle, and then shuttled into the mitochondria. Once there, it undergoes a series of steps; beta-oxidation, followed by entry into the Krebs cycle, before finally contributing to energy production. It’s a multi-stage process that takes time and relies heavily on oxygen. It produces a large amount of energy, but it does so gradually.  

At lower intensities, this works well. The demand for energy is modest, and fat can keep up. Carbohydrate behaves very differently.   

Stored as glycogen in the muscles and liver, it is broken down rapidly into glucose and fed directly into glycolysis, a much faster pathway for producing energy. From there, it can quickly enter the same energy-producing systems, but with far fewer steps to get started. The entire process is quicker, more direct, and requires less oxygen for the same output. In simple terms, carbohydrate is ready almost immediately.  

This difference becomes important as intensity rises. At easier efforts, fat contributes a large proportion of the energy supply. But as the intensity increases, the body shifts toward carbohydrate. This isn’t a preference, it’s a requirement. The demand for energy rises to a point where fat alone cannot supply it quickly enough and carbohydrate becomes the dominant fuel not because it is abundant, but because it is capable.  

This is where glycogen comes in. Glycogen represents the body’s readily available, high-speed fuel. But unlike fat, it is limited. Total stores typically equate to around 400–600 grams, roughly 2,000 calories. And during riding, particularly at moderate to high intensities, those stores are used relatively quickly. Cyclists can burn through 60 to100+ grams of carbohydrate per hour. A typical energy gel contains around 40 grams. In practical terms, that’s enough to support roughly 30 to 40 minutes of harder riding.  

As long as glycogen is available, the system works well. Power feels accessible, changes in pace are manageable, and the body can respond to the demands of the ride. But as glycogen availability drops, the situation changes. The body doesn’t stop producing energy, it simply shifts more heavily toward fat. And while fat can continue to supply energy, it cannot match the rate that carbohydrate once provided. That’s when you start to feel it. Not necessarily as a complete collapse, but as a gradual loss of sharpness. The last climb feels harder than it should.   

The ability to lift the pace fades. The legs feel flat, not because they’re empty, but because the fuel system supporting them has slowed down.  

You don’t run out of fuel. You run out of fast fuel.  

This is very relevant in real-world riding. Rarely is a ride perfectly steady. There are always changes; gradients, wind, positioning and accelerations. These moments depend on the ability to produce energy quickly. Without sufficient glycogen, they become increasingly difficult. Starting a ride under-fuelled also brings that point forward. With lower glycogen availability from the outset, reliance on fat increases earlier, and the limitations of that system appear sooner. The intensity you can sustain comfortably drops, and the margin for variation becomes smaller.  

Fat is still doing its job, it always is, but it cannot fully support the demands of higher intensity work. This doesn’t make fat a poor fuel, or carbohydrate a universally superior one. It simply means each has a specific role.   

Fat is exceptionally useful for long-duration, lower-intensity exercise, providing stability and endurance over time. But it is not designed for rapid energy delivery. That role belongs to carbohydrate.  

Which brings us to a simple but important distinction. Fat is for survival; Carbohydrate is for performance. Understanding that helps explain why different rides require different fuelling strategies. An easy ride can be supported largely by fat. A long ride requires a balance. But as intensity increases, carbohydrate becomes increasingly important. Not optional, but necessary.  

The body will always find a way to produce energy, but it cannot always produce it at the rate you want.  

And when you reach that final climb or final sprint, and glycogen has run low, that difference becomes clear.  

If you feel uncertain about whether your daily nutrition is truly supporting your health, performance, and long-term wellbeing, G2 Nutrition offers a highly personalised diet analysis with a bespoke 6-week optimisation programme, available for £200.  

This is a tailored, results-driven service designed to elevate your approach to nutrition—refining not only what you eat, but how your body performs, recovers, and thrives day to day.  

To enquire, please contact giles@g2nutrition.com or speak with me discreetly at MdV during your next visit.  

Clients of G2 Nutrition consistently experience: