The Role of Carbohydrates for Endurance Athletes

Introduction to Carbohydrates and Endurance Performance

Carbohydrates are the primary fuel for intense activity. They are the cornerstone of energy provision for athletes, particularly those participating in endurance events. As a macronutrient, carbohydrates are stored in the form of glycogen in both muscle tissue (300-500g) and in smaller quantities in the liver (~80g), creating a readily accessible energy reserve. 

A diet rich in carbohydrate tops up these fuel tanks, while prolonged and intense physical activity depletes them. Fatigue during intense exercise is commonly associated with glycogen depletion, as the body no longer has the fuel to sustain higher intensities and therefore slows down. 

Athletes of all levels must understand the importance of carbohydrate intake. Knowledge of carbohydrate requirements, the specific timing of ingestion, and the various sources can create a competitive advantage and enhance training adaptations and performance.

Different Sources of Carbohydrates

Carbohydrates are found naturally in a wide variety of foods, each differing in their composition, glycemic index (GI), and nutrient profile. Refined and processed carbohydrates are absorbed rapidly and efficiently, as the energy is readily accessible due to the high carbohydrate content and lack of other nutrients. Sports drinks, gels, and chews are considered fast fuels, designed to provide carbohydrate rapidly during exercise, while foods like candy and low-fiber granola bars offer similar benefits in certain situations. 

Higher GI foods, such as white pasta, white bread, mashed potatoes, and white rice, are also absorbed quickly and can be effective for pre-event glycogen loading or during post-exercise recovery. In the context of an athlete's diet, freshly squeezed juices such as apple and orange juice might also benefit as they provide fast fuels combined with fluids. 

Whole grains such as oats, quinoa, and brown rice have less carbohydrate but more fiber, this combination provides energy over a longer period of time. Legumes and starchy vegetables like potatoes and sweet potatoes also serve as excellent sources of carbohydrates. Fruits, including bananas, apples, and berries, deliver natural sugars and essential vitamins and minerals. Non-starchy vegetables such as carrots and beets contribute moderate amounts of carbohydrates along with dietary fiber. 

Composition and Absorption of Carbohydrates

The composition of a carbohydrate significantly impacts the absorption, which can ultimately influence health and athletic performance. High-glycemic-index foods are absorbed quickly, providing an immediate source of energy. They can be described as fast fuels. On the other hand, low-GI foods are absorbed more slowly. These can be described as slow fuels, resulting in a sustained release of energy that is ideal for everyday meals away from training and competition.

Dietary fiber, protein, and fat content also play a role in modulating carbohydrate absorption. Fiber in particular, slows the digestive process, which can delay energy availability but promotes satiety, stabilizes blood sugar levels, and supports overall digestive health. Soluble fiber, found in foods like oats and apples, forms a gel-like substance in the gut that helps regulate blood sugar levels. Insoluble fiber, found in whole grains and vegetables, adds bulk to stools and aids in bowel regularity. Adequate fiber intake is associated with a reduced risk of heart disease, improved cholesterol levels, and better weight management.

However, in the context of endurance performance, reducing fiber intake may be beneficial before and during exercise. High-fiber foods can increase gastrointestinal (GI) discomfort, causing bloating, cramping, or a feeling of fullness that may hinder performance. Low-fiber, easily digestible carbohydrate sources, such as white rice, ripe bananas, or sports gels and drinks are often recommended in the pre-race meal and during exercise to provide fast fuel without GI distress.

Carbohydrate Requirements of Athletes

Interestingly, there is no strict physiological requirement for carbohydrate in humans; in fact humans can survive without ever consuming a carbohydrate. Most bodily tissues have the ability to use fatty acids as a fuel, while there are very few tissues that rely solely on glucose as a fuel source. Although under most conditions the brain relies exclusively on glucose, it cannot use fatty acids directly although it can use fatty acid derived ketone bodies. 

When carbohydrate intake is reduced, the body essentially becomes adapted to using these other fuel sources, and the little glucose that is required can be created within the body, in the liver and kidney specifically from sources such as lactate, pyruvate, amino acids and glycerol in a process known as gluconeogenesis. However, almost all bodily tissues will utilise carbohydrate, glucose as a fuel source if it is available, in this regard it is the bodies go to, or preferred energy source. As mentioned above it is also the predominant energy source for high-intensity activity. That said, carbohydrate intake should reflect the activity and high-intensity energy output of the individual on a given day.

The carbohydrate requirements of an athlete can vary dramatically, depending on factors such as training volume and intensity, duration, intended adaptation and more. It is generally recommended that endurance athletes consume between 6 and 10 grams of carbohydrates per kilogram of body weight per day. The lower end of this range is sufficient to meet the requirements of shorter duration, lower intensity or even non-training recovery days, with the upper range reserved for competition. 

 

Carbohydrate is often viewed as a fuel for muscle during high intensity activities. This claim still holds true, however, our understanding about the role of carbohydrate, especially the storage form – muscle glycogen has advanced exponentially over the last fifteen years (52, 53). Muscle glycogen is now viewed a “fuel sensor” and it is able to regulate the adaptations of our muscles to a bout of exercise. In another words, we can manipulate our muscle glycogen by altering our carbohydrate intake before each training session to achieve the desired goals of each training session and this approach is termed as “fuel for the work required”.

The concept of ‘fuel for the work required’ was developed in 2018 and essentially highlights the importance of adjusting carbohydrate intake to match the specific demands and requirements of training and competition. This has led to the most recent recommendations of periodized carbohydrate intakes, both day-to-day and within-day, to enhance adaptations, performance and recovery. 

Carbohydrate Periodization

Carbohydrate periodization refers to the strategic manipulation of carbohydrate intake in alignment with training and competition. This approach aims to optimize both performance and metabolic adaptations. Training with low carbohydrate availability can enhance the body's ability to oxidize fat as a fuel source, while competing with high carbohydrate availability ensures peak performance. 

Research by Impey et al. (2018) highlights the metabolic adaptations associated with low-carbohydrate training, while Stellingwerff et al. (2018) explored the benefits of balancing low-carbohydrate training with high-carbohydrate competition days. By aligning carbohydrate intake with specific training phases, athletes can maximize both endurance and recovery, while also managing body composition.

That being said, a low carbohydrate diet will compromise an elite athletes ability to train and maintain high exercise intensities and will likely impair both strength and speed, with no significant improvement in aerobic capacity and endurance performance. So this periodization framework must be thoroughly planned. 

Timing of Carbohydrate Intake

The timing of carbohydrate intake is critical for optimizing energy availability and performance. Carbohydrate ingestion stimulates a rapid increase in carbohydrate oxidation, the burning of carbohydrate as a fuel, particularly when fast fuels are ingested. However, the amount of carbohydrate that can be absorbed in the intestines is limited. This, combined with the knowledge that carbohydrate is the predominant energy source during high-intensity exercise and the fact that fatigue during exercise is associated with depleted carbohydrate stores makes the ingestion of carbohydrate in and around the exercise bout extremely effective.

During prolonged activity, ingesting 30 - 120 grams of carbohydrate per hour can help maintain blood glucose levels and delay fatigue. The dosage and type should reflect the duration and intensity of exercise and is discussed in more detail in our Carbohydrate Fuel Guide.

Post-exercise, rapid glycogen resynthesis is considered an essential component of recovery, particularly for athletes who train and compete regularly. Consuming 1.2 grams of carbohydrate per kilogram of body weight immediately post-exercise and then again at 1-hour intervals for the first 4-hours post-exercise produces the most rapid rate of refuelling. Carbohydrates combined with protein in a 3:1 ratio is likely to be favourable too. 

Benefits of Carbohydrates on Performance

The consumption of carbohydrate in sufficient quantities both before and during exercise is strongly associated with enhanced athletic performance in various domains.

Carbohydrate and Endurance Performance

Carbohydrates play a central role in sustaining endurance performance, both in the context of time to exhaustion and time trial studies. The consumption of 30 - 120 grams of carbohydrate per hour is able to maintain blood glucose, spare glycogen and prevent fatigue, allowing for the maintenance of higher exercise intensities and ultimately improved performance. However, carbohydrate intake during endurance exercise is only likely to benefit when exercise exceeds 70-minutes at high intensity. 

Carbohydrates and Strength Performance

Unlike endurance events or intense, repetitive sports such as football, resistance training is unlikely to deplete fuel stores in the muscle. Research suggests that a modest reduction of 24 - 40% of muscle glycogen is typical, even of higher volume strength sessions. Individuals who solely follow resistance training programmes should consume around 3-4 grams of carbohydrate per kilogram of body weight per day and allow sufficient time (24-48 hours) between sessions to refill the fuel tanks. However, if training fasted (8+ hours with no food/carbs) an additional 15 - 30 grams of carbohydrate before the session is likely to benefit. It is generally recommended that you sandwich your strength session between two mixed meal. 

Carbohydrates, Skill, and Cognitive Performance

The role of carbohydrates extends beyond physical performance. Glucose is the preferred fuel for the brain and low blood glucose is known to impair cognitive function such as attention and decision making. Therefore the consumption of 30 - 60 grams of carbohydrate per hour during exercise is known to maintain cognitive function and skill performance.

Carbohydrates and Recovery

Recovery is a critical aspect of athletic performance, particularly for those training and competing multiple times per week. Carbohydrates play an essential role in this recovery process. Studies have shown that co-ingestion of carbohydrates and protein in a 3:1 ratio enhances the insulin response, accelerating glycogen replenishment and muscle repair. 

In circumstances where rapid refuelling is required, if training or competing twice per day for example then athletes are encouraged to consume 1.2 grams of carbohydrate per kilogram of body weight immediately post-exercise and at regular intervals for the first 5-hours. This should be initiated as close to the end of the exercise bout as possible as even a small delay can reduce performance in the following session by up to 30%, even if that session is the following day. 

Research has shown that muscle damage from eccentric muscle contractions significantly impairs muscle glycogen resynthesis. That means it can take around 72-hours to completely restore muscle glycogen after downhill running or intense activity that involves rapid change of direction or decelerations. The addition of creatine monohydrate to a recovery mix is also known the enhance the rate of glycogen resynthesis, allowing athletes to refuel rapidly. 

Carbohydrates and Body Composition

Despite numerous social media claims, a recent Cochrane review found no difference in weight loss long-term between low carbohydrate diets and moderate carbohydrate diets. The original theory of low carbohydrate diet proponents was that carbohydrates raise insulin, and insulin is a fat storage hormone and prevents you from burning fat as a fuel so ultimately everything is stored as fat and carbs make you fat. This has subsequently been disproven, with the current evidence confirming that in the context of energy balance and sufficient protein, the carbohydrate content of the diet is unlikely to impact body composition.

Practical Recommendations

To maximize performance, endurance athletes should align their carbohydrate intake with their training and competition demands. Periodizing intake based on training cycles, optimizing timing to enhance performance and recovery, and selecting a balance of whole grains, fruits, and minimally processed options are essential strategies. Current research offers detailed carbohydrate planning frameworks, emphasizing the importance of individualization in dietary strategies. Be sure to take a look at our Fuel Bars whilst you are here.