This article was originally written on September 6, 2017 and fully updated on November 3, 2022.
Carbohydrates are the primary fuel source for high-intensity endurance efforts. They are stored in the muscle and the liver as glycogen. Unlike fat, carbohydrates cannot be stored in large quantities in the body. After a high-intensity effort of about two hours, glycogen stores are depleted. This is why it is important for endurance athletes to consume carbohydrates regularly after their workouts.
Often, marathon runners experience severe fatigue at around the 30th km. This manifests as a strong desire to stop running. It’s what’s known as “hitting the wall”. This happens because the body’s glycogen stores in the liver and muscles are depleted and fat becomes the main fuel. Fat is a less effective fuel than glycogen, because it requires a lot more oxygen to produce the same amount of energy. That’s why running speed decreases dramatically if fat becomes the main source of fuel. Fortunately, with adequate nutritional intake, it is possible to delay this phenomenon or even prevent it from occurring altogether.
Following a very high carbohydrate diet before an endurance event lasting more than 90 minutes, such as a half marathon, marathon or Ironman, can improve performance by 2 to 3%.
The idea is to modify your diet and training to maximize the amount of glycogen in your muscles. A proper protocol followed faithfully will almost double your muscle’s glycogen content. Note that this practice is useless for events lasting less than 90 minutes.
|Sedentary||80-90 mmol/kg of muscle|
|Endurance athletes||130-135 mmol/kg of muscle|
|Athletes that applied the carb loading protocol||210-230 mmol/kg of muscle|
Protocols for glycogen overloading have evolved over time. While a supercompensation protocol may allow to achieve maximum muscle glycogen levels, studies show that the rate of glycogen breakdown is faster when muscle stores are very high. According to the most recent research, the recommendation is to reduce training volume the days before the race (what’s known as tapering) and adopt a carbohydrate rich diet (7 to 12 g of carbs per kg body weight) for two to three days before the race, or for a day if it is a highly trained athlete who already has a relatively high carbohydrate diet. The longer the target event, the higher the carbohydrate requirements (for example, one could aim for 7 to 10 g of carbohydrates per kg of body weight before a half-marathon versus 9 to 12 g of carbohydrates per kg of body weight before a marathon).
It is important to understand that the idea is not to eat more calories the days leading up to a race. In fact, you probably need to eat less, since energy expenditure is reduced. Rather, it’s about replacing foods high in fat and protein with foods high in carbohydrates. For example, instead of eating eggs, nuts, cheese and avocados, you can choose waffles with maple syrup, fruit, dates and bagels. A common mistake athletes make is eating a big plate of spaghetti the day before a race, when it would be better to choose foods high in carbohydrates throughout the day and avoid overeating at dinner. In short, a pasta meal can be a good choice the day before a race, but it is not beneficial to eat a large portion, on the contrary, it can make you feel sluggish afterwards. Meals that contain other types of starches such as rice or potatoes are also very good choices.
It is not so easy to reach the carbohydrate recommendations required to optimize glycogen overload, which is why it is important to plan your meals and snacks well. For example, a runner who weighs 70 kg will need to consume approximately 630 g of carbohydrates per day before running a marathon. To give you an idea of what that represents, a medium fruit or a slice of bread contain about 20g of carbohydrates. A personalized and precisely calculated plan will help you optimize your glycogen stores and your performance.
To get your three-day personalised carb loading plan adapted to your intolerances and dietary preferences, go to carb loading for endurance sports.
1. Burke et coll. (2019) International Association of Athletics Federations Consensus Statement 2019: Nutrition for Athletics. International Journal of Sport Nutrition and Exercise Metabolism; 29:73-84.
2. Jeukendrup. How to fuel for a marathon. https://www.mysportscience.com/post/how-to-fuel-for-a-marathon
3. Jeukendrup. Carb loading: what is new? https://www.mysportscience.com/post/2016/05/12/carb-loading-what-is-new