How do pro athletes consume above 100g of carbs per hour.

With recent record-breaking marathon times a lot of runners are digging deeper into what it takes to improve their own running times.

Most runners only seem to manage 60-90g of carbs before experiencing some gut issues, which begs the question: How are pro runners able to consume more than 100g of carbs.

"During running and other endurance events, the sympathetic nervous system is activated..."

Firstly, why are carbs necessary during running races:

Carbohydrates are the body’s primary and most efficient fuel source during endurance running, particularly as intensity rises. While fat also contributes to energy production, it cannot be broken down quickly enough to sustain higher-effort running, making carbohydrates essential for maintaining pace and performance. The body stores carbohydrates as glycogen in the muscles (roughly 300–700 g, depending on muscle mass) and liver (around 80–120 g), giving a total storage capacity of about 400–800 g, or approximately 1600–3200 kcal of readily available energy. These stores are influenced by factors such as training status, diet (especially carbo-loading), recent exercise, muscle mass, and genetics.

During prolonged exercise, glycogen stores progressively decline, and once they become critically low, runners experience what is commonly known as “hitting the wall” or “bonking.” This is characterized by sudden fatigue, reduced performance, dizziness, elevated heart rate, “heavy” legs, and impaired concentration. As glycogen depletes, the body is forced to rely more heavily on less efficient energy pathways, which can increase physiological strain, contribute to dehydration and thermal stress, and even reduce blood flow to the gut, further compromising fuel tolerance. Consuming carbohydrates during a race helps maintain blood glucose levels, spare glycogen stores, and provide a continuous, rapid source of energy to working muscles, ultimately delaying fatigue and supporting sustained performance.

Why can most runners only tolerate 60-90g of carbs per hour:

Most runners hit a ceiling of around 60–90 g of carbohydrate per hour because of limitations in how the gut absorbs and transports different sugars. During digestion, carbohydrate sources like maltodextrin (glucose chains) and isomaltulose (glucose + fructose) are broken down into monosaccharides, primarily glucose, galactose, and fructose. These monosaccharides are then absorbed in the small intestine. Glucose and galactose rely on the SGLT-1 transporter, which is sodium-dependent and becomes saturated at roughly 60 g per hour. Fructose, on the other hand, uses a separate transporter (GLUT-5), which has a lower capacity of about 30 g per hour. Because these transport systems operate independently, combining carbohydrate sources, typically in a 2:1 glucose-to-fructose ratio it allows athletes to utilize both pathways and increase total carbohydrate absorption toward the upper limit of around 90 g per hour. Beyond this, unabsorbed carbohydrates remain in the gut, often leading to gastrointestinal discomfort, which is why intake above these levels is poorly tolerated for most runners.

What can we do to increase the amount of carbs we can tolerate per hour:

The advice to “never try anything new on race day” is especially relevant when it comes to carbohydrate intake, because the gut is highly trainable.

A key factor is gastric emptying: the process that delivers ingested carbohydrates and fluids from the stomach to the small intestine for absorption. When gastric emptying is delayed, often due to high exercise intensity, heat, or dehydration, it can lead to bloating, nausea, and vomiting. However, regularly practising your race-day fuelling strategy in training can improve this process.

By consistently consuming higher carbohydrate intakes during training sessions, the gut adapts not only by increasing the rate of gastric emptying, but also by upregulating intestinal transporters such as SGLT-1 (for glucose) and GLUT-5 (for fructose). This means you now have more of these transports to absorb the carbohydrates more efficiently with less gastro-intestinal discomfort.

Research shows that even short-term increases in carbohydrate intake prior to your race stimulate these adaptations, highlighting how carbo-loading may also enhance gut tolerance.

In addition to carbohydrate training, proper hydration and electrolyte intake play a crucial role. Fluids help facilitate gastric emptying and maintain blood flow to the gut, while sodium supports both fluid absorption and the function of glucose transport via the sodium-dependent SGLT-1 pathway. Dehydration, on the other hand, can slow gastric emptying and exacerbate gastrointestinal distress. This means athletes should aim to match their carbohydrate intake with adequate fluid (often ~400–800 ml per hour, depending on conditions) and include sodium in their fuelling strategy.

Starting with moderate carbohydrate intakes (e.g. 30–60 g/hour) and progressively increasing toward higher targets (up to ~90 g/hour using multiple transportable carbohydrates) during training allows the gut to adapt safely, improving both comfort and performance on race day.

For individualised carbohydrate, sodium & fluid intakes, it is always best to consult with a registered dietitian who specializes in sports nutrition.

---