The ABCs of Carb Loading

If you’ve spent any time around distance running, you’ve probably heard about carb loading. It’s often reduced to a throwaway line about eating more pasta the night before a race, but in reality, it’s a deliberate nutritional strategy grounded in exercise physiology. When done properly, carb loading allows you to line up on race day with a fuller “fuel tank,” giving you a measurable advantage in sustaining pace and delaying fatigue.

At its core, carb loading is about increasing the amount of glycogen stored in your muscles and liver. After you consume carbohydrates, they are broken down into glucose, which circulates in the bloodstream and is either used immediately for energy or stored as glycogen for later use. This stored form acts as a readily accessible energy reserve that your body can draw on during exercise.

Muscle glycogen is particularly important for running performance, as it provides a rapid and efficient fuel source during moderate to high-intensity efforts. Liver glycogen, on the other hand, plays a key role in maintaining stable blood glucose levels, especially during prolonged exercise. Together, these systems help ensure both your muscles and your brain have a consistent supply of energy as the race progresses.

Carb loading doesn’t make you fitter, faster, or stronger in the traditional sense. Instead, it allows you to express your existing fitness more effectively by reducing one of the primary limiting factors in endurance performance: fuel availability. Glycogen stores are inherently limited. Even in well-trained individuals, they can begin to decline significantly after approximately 90 to 120 minutes of sustained running. As these stores are depleted, the body increasingly relies on fat as a fuel source. While fat provides a large and virtually unlimited energy reserve, it is slower to convert into usable energy, particularly at higher intensities.

This shift in fuel utilisation is often what runners experience as a sudden increase in perceived effort, a drop in pace, or the classic “hitting the wall.” Carb loading helps delay this process by increasing the amount of glycogen available at the start of the race, allowing you to maintain a higher intensity for longer before fatigue begins to set in.

In addition to its effects on the muscles, adequate carbohydrate availability also supports brain function. The brain relies heavily on glucose, and reductions in blood glucose can impair concentration, decision-making, and pacing strategy. This is one of the reasons why fuelling strategies can influence not just physical performance, but also how clearly you think and respond during a race.

Carb loading is most effective when it takes place over the final 3 to 4 days before your race, typically alongside a reduction in training volume. This combination allows the body to store more glycogen than it would under normal training conditions, a process sometimes referred to as glycogen supercompensation.

During this period, carbohydrate intake is intentionally increased. General guidelines suggest aiming for approximately **8–12 grams of carbohydrate per kilogram of body weight per day**, with higher targets being more relevant for longer endurance events.

For many runners, this represents a significant increase compared to their usual intake, which is why structure and planning become important. Rather than relying on a single large meal, spreading carbohydrate intake across the day tends to be more effective. This approach supports digestion, helps maintain stable energy levels, and makes it easier to consistently meet intake targets without discomfort.

It’s also worth noting that carb loading should be practiced in training before race week. This allows you to identify which foods sit well, how your body responds to higher carbohydrate intake, and how best to structure your meals in a way that feels sustainable.

Different types of carbohydrates provide differing benefits, so it is helpful to understand the differences and choose the type of carbs that are best for your needs. Complex carbohydrates provide longer-lasting energy and can help to reduce fatigue, while simple carbohydrates provide a quick burst of energy. The type of carbohydrate you choose should depend on the intensity and duration of your activity. Complex carbs are the food source you'll want to eat while carb-loading since you won't need instantaneous energy, and they provide more fibre and nutrients. Conversely, simple carbohydrates are great to consume while racing for a rapid burst of energy. Depending on the duration of your run, a mixture of complex and simple carbohydrates can be beneficial during a race, especially when getting into ultra-marathon distances.

Earlier in the carb-loading phase, a mix of wholegrain and refined carbohydrate sources can be included. These foods provide a combination of energy, fibre, and micronutrients that support overall health. However, as race day approaches, it is generally beneficial to shift toward lower-fibre, more easily digestible carbohydrate sources. In the final 24 to 48 hours, many runners opt for simpler, lower-fibre options like white rice, white bread, and refined cereals to keep digestion predictable. This adjustment helps reduce gastrointestinal load and minimises the risk of discomfort during the race. It also allows for higher carbohydrate intake without excessive fullness.

Some examples of complex and simple carbohydrates include:

Hydration is closely intertwined with carbohydrate loading. Because glycogen storage involves water retention, adequate fluid intake supports the efficiency of the entire process.

Rather than dramatically increasing fluid intake, the goal is to maintain consistent hydration throughout the day. This can be achieved by drinking regularly and monitoring urine colour as a simple indicator of hydration status.

In warmer climates, or for runners with higher sweat rates, electrolytes become increasingly important. Sodium, in particular, helps regulate fluid balance and supports the retention of ingested fluids. Including electrolyte-rich drinks or slightly increasing sodium intake in the days leading up to the race can help optimise hydration status.

It’s important, however, to avoid overhydration. Excessive fluid intake without adequate electrolytes can dilute blood sodium levels, which may negatively impact performance and, in extreme cases, pose health risks.

Carb loading prepares your body for race day, but it works best when combined with an appropriate race-day fuelling strategy. The goal is to begin the race with high glycogen stores and then maintain energy availability throughout the event.

On the morning of the race, consuming a carbohydrate-rich meal approximately 2 to 4 hours before the start helps restore liver glycogen levels that have been reduced during the overnight fast. This meal should be familiar, easy to digest, and aligned with what you have practiced in training.

In addition to pre-race nutrition, carbohydrate intake during the race plays a critical role in sustaining performance. As exercise duration increases, maintaining blood glucose levels becomes increasingly important. This is typically achieved through easily digestible carbohydrate sources such as gels, chews, or sports drinks.

For shorter endurance events, a moderate intake of carbohydrates may be sufficient. As race duration extends, higher intake becomes beneficial, provided it has been practiced and tolerated in training. The key is to start fuelling early and continue at regular intervals, rather than waiting until fatigue becomes noticeable.

Carb loading is a simple concept, but its effectiveness lies in the details. It is not about drastically changing your diet overnight or overeating in a single sitting, but about gradually increasing carbohydrate intake in a structured and intentional way. When combined with appropriate hydration and race-day fuelling, carb loading allows you to maximise your available energy stores and better support the physiological demands of endurance running. Ultimately, it gives you the best possible platform to translate your training into performance when it matters most.