For anyone who has ever taken part in endurance exercise, whether a long bike ride through the countryside, a half marathon along city streets, or even a demanding hike in the Lake District, one truth quickly becomes apparent: energy management is everything. The body is not a machine with limitless reserves—it operates on finely tuned systems that provide fuel at different speeds and under different conditions. Understanding how energy is processed during movement is not simply an exercise in sports science; it is the difference between a comfortable finish and grinding to a halt halfway through.
In this article, we will explore the fascinating mechanics behind energy production during exercise. From the way the body taps into its fuel stores to why digestion changes once you’re on the move, we’ll uncover the science that helps athletes stay energised. Along the way, we’ll highlight where fast-absorbing fuels such as cycling gels fit into this intricate picture, without overstating their role. By the end, you’ll see why knowing a little about energy processing can make your workouts or events not only more effective but also far more enjoyable.
The body is equipped with several energy systems, each designed to fuel movement in different ways. These systems work together, switching dominance depending on the intensity and duration of the activity. While the science can sound complicated, the principles are surprisingly intuitive once broken down.
Glycogen is the stored form of carbohydrates, kept primarily in the muscles and liver. Think of it as the body’s version of a rechargeable battery, ready to supply rapid energy when demand suddenly spikes. During high-intensity efforts—such as sprinting up a steep hill on a bike—glycogen is the go-to source. However, these stores are limited. For the average active adult, glycogen might fuel around 90 minutes of continuous, moderate-to-high effort before running dangerously low.
While glycogen is fast and efficient, fat is the body’s long-term savings account. Fat stores are abundant—even the leanest athletes have tens of thousands of calories tucked away. The catch is that fat takes longer to convert into usable energy, making it less suitable for explosive efforts. It shines during steady, prolonged activity such as long-distance cycling or endurance runs. This is why pacing is so important: too much intensity too soon means the body relies heavily on glycogen and burns through it quickly.
Although protein is not designed as a primary fuel source, it can be called upon in extreme situations where glycogen is depleted, and fat alone isn’t enough. This usually occurs during very long or under-fuelled exercise. The downside is that protein breakdown can compromise muscle tissue, which is hardly ideal for anyone aiming to perform at their best.
All of these fuels—glycogen, fat, and occasionally protein—are ultimately converted into adenosine triphosphate (ATP), the body’s universal energy currency. Every muscle contraction, from pedalling a bike to lifting a glass of water, relies on ATP. The body constantly breaks down and regenerates ATP, making the energy systems less like separate machines and more like layers in a flexible network.
It’s easy to forget that the body has priorities. When you’re sitting at a table with a meal, digestion takes centre stage. But once you start exercising, particularly at higher intensities, the body diverts its resources to the muscles and lungs, leaving the digestive system running at reduced capacity. This shift can have a profound effect on what, when, and how you eat or drink during physical activity.
During exercise, blood is redirected away from the stomach and intestines to support the working muscles. This ensures oxygen and nutrients reach where they’re needed most but means the digestive system receives less attention. As a result, heavy or complex foods that would normally digest without issue can suddenly feel uncomfortable, sitting in the stomach like a lead weight. This is one reason athletes are advised to stick to lighter, more easily digested options while on the move.
The higher the intensity, the less tolerant the digestive system becomes. A brisk walk may not cause any problems for eating a snack, but try wolfing down a sandwich during a hill climb on a bike and the discomfort will be immediate. Even fluids can feel challenging to process if effort levels are very high, leading to that familiar sloshing sensation in the stomach.
Because digestion slows, fuelling strategies need to adapt. Foods that are quick to break down and deliver energy become invaluable. This is why items such as bananas, dried fruits, and specifically designed sports products like cycling gels can be helpful mid-session. They bypass much of the digestive effort required for bulkier foods, providing usable energy without overwhelming the system.
Despite best intentions, many athletes encounter issues such as bloating, cramping, or nausea during longer events. These problems are rarely caused by one factor alone—they’re usually the result of a combination of high intensity, poor timing, or unsuitable food choices. Learning how your body reacts during exercise is as important as physical training itself, often referred to as “training the gut.”
When energy is needed quickly, the body benefits from fuel that can be absorbed and used without delay. This is where fast-absorbing carbohydrate sources play a vital role. They’re designed to provide an almost immediate lift, preventing fatigue from setting in during demanding activity. For athletes and recreational exercisers alike, understanding how these fuels work can make a noticeable difference in performance and comfort.
Fast-absorbing fuels are typically made from simple sugars such as glucose, fructose, or maltodextrin. Unlike complex carbohydrates found in whole grains or vegetables, these sugars are broken down and absorbed swiftly, delivering energy to the bloodstream within minutes. Once available, they help maintain blood sugar levels and supply working muscles with the glucose required to keep moving efficiently.
Because digestion slows during physical activity, the body often struggles to handle bulky or fibre-rich foods. Fast-absorbing fuels bypass much of this strain by providing energy in small, concentrated forms. This is why a slice of toast isn’t practical mid-race, but a sports drink or a cycling gel can provide the same core benefit in a more manageable way. They act as a lifeline to delay fatigue and help athletes maintain pace, particularly in endurance sports.
These fuels come in several forms, each with its own strengths:
Athletes often combine these options depending on the event, using drinks in early stages, gels in more intense moments, and chews as variety to break monotony.
The effectiveness of fast-absorbing fuels depends not just on what you take, but when. Using them too early can cause unnecessary spikes in blood sugar, while leaving it too late risks “hitting the wall.” Practising timing in training helps athletes learn when their body needs that quick top-up most. Products like cycling gels are designed with these scenarios in mind, offering rapid support exactly when it’s required.
While the body’s main energy systems are well understood, a number of less obvious influences can alter how efficiently we use fuel during exercise. These hidden factors often explain why performance feels inconsistent from one day to the next, even when training routines remain the same.
Heat places an extra burden on the body. As core temperature rises, more blood is directed to the skin to aid cooling, leaving less available for the muscles and digestive system. This not only increases fatigue but also makes fuelling more difficult. Conversely, exercising in colder conditions may improve endurance by slowing glycogen depletion, but it can also lead to underestimating fluid and energy needs.
At higher altitudes, oxygen levels drop, forcing the body to rely more heavily on carbohydrate metabolism because it requires less oxygen than fat burning. This means glycogen stores are depleted more quickly than at sea level. For cyclists or runners tackling mountainous regions, this shift can dramatically change fuelling requirements.
Psychological stress affects hormone balance, particularly cortisol and adrenaline, which can influence how the body mobilises energy. Pre-race nerves or everyday pressures may cause an athlete to burn through glycogen faster or experience digestive upset. Managing stress becomes part of effective energy management, alongside training and nutrition.
An often-overlooked factor is the impact of training on energy use. Well-trained athletes typically develop better “metabolic flexibility”—the ability to switch between carbohydrate and fat burning more efficiently. This adaptation delays the onset of fatigue and reduces the reliance on rapid refuelling options such as cycling gels, allowing performance to be sustained for longer.
Just as muscles can be trained to ride faster or run further, the digestive system can also adapt to handle the demands of endurance exercise. Many of the stomach problems athletes experience during long events—such as cramps, nausea, or bloating—are not inevitable. Instead, they often come down to a lack of preparation in fuelling strategies. “Training the gut” is about teaching the body to tolerate, absorb, and make use of fuel while on the move.
The gut is not naturally primed to digest food during intense exercise, as blood flow is prioritised to working muscles. Without adaptation, introducing fuel mid-session can cause discomfort. By practising with specific foods or sports products in training, athletes help their digestive system learn to process them more smoothly, reducing the likelihood of issues on race day.
Adaptation requires consistency and patience. Athletes often begin by introducing small amounts of carbohydrate during shorter training sessions. Over time, these amounts are increased until the gut can comfortably handle the intake required for long rides, marathons, or triathlons. This progressive approach mirrors physical training: start small, build steadily, and strengthen the system.
Not all fuels are equally easy to tolerate. Simple carbohydrates, such as those found in bananas, dried fruits, or specially formulated products like cycling gels, tend to be more digestible during exercise. Experimentation is essential, as individual tolerances vary. What works for one runner or cyclist may leave another struggling, so testing different options in training is critical.
Gut training is not just about science; it’s also about habit and routine. Practising fuelling strategies during training helps athletes know exactly when and how to eat or drink during an event. It builds confidence, turning nutrition from a potential source of worry into a reliable tool for maintaining energy and performance.
Understanding how the body processes energy during exercise is more than an academic curiosity—it is a practical tool for anyone aiming to get the best from their performance. By recognising the roles of glycogen, fat, and even protein, athletes can plan training and nutrition strategies with greater precision. Appreciating why digestion slows when you’re on the move explains why lighter, faster fuels can make the difference between comfort and discomfort. Meanwhile, awareness of hidden factors such as heat, altitude, and stress reminds us that energy use is never just about the food we eat.
Perhaps the most empowering lesson is that the gut itself can be trained, just like the legs or lungs. Experimenting with timing, fuel types, and quantities helps reduce the risk of mid-race mishaps and allows athletes to fuel with confidence. Whether using everyday foods or purpose-designed options like cycling gels, the key is preparation and practice.
So, the next time you head out on a long ride, run, or hike, consider this: are you training your energy systems as well as your muscles? With the right knowledge and a thoughtful approach, managing energy can transform endurance from a battle of survival into an experience of control, flow, and even enjoyment.