Carb loading is one of the most powerful tools available to endurance athletes, and one of the most commonly misunderstood throughout running nutrition. When done correctly, it allows you to start race day with full energy stores, helping you maintain pace, delay fatigue, and finish strong. When done poorly, it can leave you feeling sluggish, bloated or under-fuelled.

The key to successful carb loading isn’t excess or restriction, but timing, consistency and simplicity. Here, we are going to look through what carb loading is, how it can be used and why you should do it when preparing for a marathon.

Carb loading is the process of increasing carbohydrate intake in the days before an endurance event in order to maximise muscle glycogen stores. Glycogen is the primary fuel source for moderate-to-high intensity exercise, and once it’s depleted, performance drops sharply.

For events lasting longer than around 90 minutes, starting with full glycogen stores can be the difference between finishing strong and hitting the wall. Carb loading increases the amount of glycogen stored in your muscles before race day, giving you a larger energy reserve to draw from once the race begins.

Carb loading is most beneficial for athletes taking part in:

• Marathons and ultra runs

• Half marathons at higher intensities

• Long-distance cycling or triathlon events

• Any endurance event lasting 90 minutes or more

For shorter races or low-intensity efforts, carb loading is usually unnecessary, as it is a performance tool rather than an everyday nutrition strategy.

Modern carb loading is more refined for marathon training than the old-school approach of depletion followed by extreme overeating. Most runners benefit from starting 48–72 hours before race day.

During this window:

• Training volume is reduced (tapering)

• Carbohydrate intake increases

• Fat and fibre intake are moderated to aid digestion

Because the training load is lower, more of the carbohydrate you eat is stored as glycogen rather than burned off.

Carbohydrate needs vary, but most endurance athletes aim for approximately 7–10 grams of carbohydrate per kilogram of bodyweight per day during the carb-loading phase.

That amount may sound high, but it becomes manageable when spread across regular meals and snacks. The goal isn’t to force food, but to gently shift the balance of your diet towards carbohydrates while reducing unnecessary fibre and fat.

The goal of carb loading is to increase carbohydrate intake without overwhelming your digestive system. This means choosing foods that are easy to digest and low in fibre and fat.

Good carb-loading options include:

• White rice, pasta and potatoes

• Oats and porridge 

• White bread, bagels and wraps

• Fruit, smoothies and fruit juice

• Energy bars, chews and sports nutrition

If you are looking for a great carb source, then overnight oats are highly recommended, as they are easy to make and perfect for carb loading.

Carb loading doesn’t end the night before your event. On race morning, a carbohydrate-rich breakfast eaten two to three hours before the start helps top up liver glycogen after an overnight fast. The best option is something familiar that you’ve used in training. If nerves affect appetite, liquid or semi-solid carbohydrate options can be easier to tolerate than large meals.

As with all race-day nutrition, carb loading should be practised ahead of your goal event. Testing quantities, foods and timing during training removes guesswork and reduces stress when it matters most.

Carb loading doesn’t need to be extreme or complicated to be effective. By increasing carbohydrate intake in the final two to three days before your event, while keeping food choices simple and familiar, you give your body the energy reserves it needs to perform at its best.

Done well, carb loading is quiet, controlled and confidence-building, and when race day arrives, you’ll be glad you took the time to get it right.