Fueling for Running: What the Body Uses, When It Uses It, and How to Get It Right

Fuelling is one of the most influential factors in running performance, yet it is often approached with uncertainty. Many runners understand that nutrition matters, but lack clarity on what to consume, when to consume it, and how it interacts with training and racing.From an exercise science perspective, fueling is not simply about energy intake. It is about managing the availability of substrates that support muscular contraction, maintaining physiological stability, and delaying the onset of fatigue. The way the body produces and uses energy during running is predictable, and understanding this allows for more effective decisions around nutrition.

Energy Systems and Fuel Sources

During running, the body primarily relies on carbohydrates and fats as fuel sources. These are metabolised through aerobic and anaerobic pathways depending on intensity.At lower intensities, fat oxidation contributes a greater proportion of energy. This system is highly efficient and sustainable over long durations, but it produces energy at a slower rate. As intensity increases, the body shifts toward greater reliance on carbohydrates, which can be broken down more rapidly to meet higher energy demands.Carbohydrates are stored in the body as glycogen, primarily in the muscles and liver. These stores are limited. Once depleted, the ability to maintain intensity declines significantly. This is commonly referred to as “hitting the wall,” and it represents a point where carbohydrate availability is no longer sufficient to support the required effort.From a physiological standpoint, effective fueling aims to preserve glycogen, supplement carbohydrate availability during exercise, and maintain stable energy production.

Glycogen Depletion and Fatigue

One of the key limitations in endurance performance is the depletion of glycogen stores. Depending on intensity and individual variation, these stores can become significantly reduced within ninety minutes of continuous running.As glycogen levels decline, the body is forced to rely more heavily on fat metabolism. While this allows activity to continue, it reduces the capacity to sustain higher intensities. The result is a noticeable drop in pace, increased perceived effort, and a decline in overall performance.This process is not abrupt but progressive. Small reductions in carbohydrate availability accumulate over time, eventually reaching a point where performance can no longer be maintained.From a coaching perspective, this highlights the importance of both pre-run nutrition and in-run fueling. The goal is not simply to avoid depletion, but to manage energy availability so that performance remains stable throughout the effort.

Fueling Before Running

Pre-run nutrition plays a critical role in determining how much glycogen is available at the start of a session. Consuming carbohydrates in the hours leading up to a run increases glycogen stores and supports sustained energy production.From an exercise science perspective, the timing and composition of this intake influence digestion, absorption, and availability. Meals consumed two to four hours before running allow for proper digestion and minimise gastrointestinal discomfort. Smaller, more immediate intake may be used closer to the session if required.The type of carbohydrate also matters. Easily digestible sources are generally preferred, as they provide a rapid supply of glucose without excessive digestive demand.For shorter or lower intensity runs, pre-run fueling becomes less critical. For longer or more demanding sessions, it becomes a key factor in performance.

Fueling During Running

For runs extending beyond approximately sixty to ninety minutes, in-run fueling becomes increasingly important. This is where carbohydrate intake during exercise helps maintain blood glucose levels and delay glycogen depletion.From a physiological standpoint, the body can absorb and utilise a limited amount of carbohydrate per hour. This is often cited in the range of thirty to sixty grams per hour for most runners, with higher amounts possible through training and the use of multiple carbohydrate sources.Gels, sports drinks, and other easily digestible forms are commonly used because they provide carbohydrates in a form that is quickly absorbed and requires minimal digestion.The effectiveness of in-run fueling is not only dependent on quantity, but also on consistency. Regular intake at smaller intervals tends to be more effective than infrequent, larger amounts.From a coaching perspective, this is a skill that needs to be practised. The gastrointestinal system adapts to fueling strategies over time, and what works in training is more likely to work in racing.

Hydration and Fuel Interaction

Fueling and hydration are closely linked. Carbohydrate absorption is influenced by fluid availability, and dehydration can impair both digestion and performance.From an exercise science perspective, maintaining fluid balance supports cardiovascular function, thermoregulation, and nutrient transport. Even moderate levels of dehydration can increase perceived effort and reduce performance capacity.Electrolytes also play a role, particularly in longer events or hot conditions. They help maintain fluid balance and support neuromuscular function.Effective fueling strategies account for both carbohydrate intake and fluid needs, recognising that they operate as part of the same system.

Training the Gut

One of the most overlooked aspects of fueling is the adaptability of the gastrointestinal system. Just as muscles adapt to training, so too does the body’s ability to tolerate and absorb nutrients during exercise.From a physiological standpoint, repeated exposure to carbohydrate intake during running can increase absorption rates and reduce the likelihood of discomfort. This process is often referred to as “training the gut.”From a coaching perspective, this means that fueling strategies should be practised during training rather than introduced for the first time on race day. Consistency allows the body to adapt and reduces uncertainty.

Fueling and Different Types of Runs

Fueling requirements vary depending on the purpose and duration of the run.Short, low-intensity runs typically do not require additional fueling, as glycogen stores are sufficient to meet energy demands. In these cases, the focus is on overall nutrition rather than in-run intake.Long runs and endurance sessions place greater demand on energy systems and benefit from structured fueling strategies. These sessions also provide an opportunity to practice race nutrition.High-intensity sessions may require pre-run fueling to support performance, but in-run fueling is often less relevant due to shorter duration.From a coaching perspective, aligning fueling with the purpose of the session ensures that it supports, rather than interferes with, the intended adaptation.

Common Errors in Fueling

One of the most common errors is underfueling. Runners often underestimate how much energy is required, particularly during longer efforts. This can lead to early fatigue, reduced performance, and impaired recovery.Another common issue is inconsistency. Irregular intake during a run can result in fluctuations in energy availability, which are felt as changes in effort and performance.Overfueling can also be problematic, particularly if the gastrointestinal system is not accustomed to higher intake levels. This can lead to discomfort and reduced performance.From an exercise science perspective, effective fueling is about balance. It requires matching intake to demand and ensuring that the body can tolerate what is consumed.

Integrating Fueling Into Performance

Fueling is not separate from training. It is part of it.From a scientific standpoint, energy availability influences every aspect of performance, from muscular contraction to central nervous system function. From a coaching perspective, it is one of the most controllable variables available to a runner.When understood and applied correctly, fueling supports consistency, delays fatigue, and allows training adaptations to be fully realised.When neglected, it limits performance regardless of fitness.The difference is not in how hard you train, but in how well you support that training.And that begins with understanding how the body uses fuel.