Carbohydrates and Endurance Running: What the Science Really Says
- Rebecca
- Feb 10
- 4 min read
Few topics in endurance nutrition generate as much discussion as carbohydrate intake. For decades, high-carbohydrate diets and in-race fuelling strategies have been the dominant recommendation for endurance runners. More recently, this approach has been challenged by advocates of low-carbohydrate, high-fat (LCHF) nutrition, reigniting debate among athletes and practitioners alike.
This article examines what the scientific evidence actually tells us about carbohydrates and endurance performance, drawing on laboratory research, applied studies, and real-world elite practice.
The Role of Carbohydrates in Endurance Running Performance
High-carbohydrate strategies are grounded in decades of exercise physiology research. Carbohydrates play a critical role in human performance, particularly during moderate- to high-intensity endurance exercise.
Muscle contraction and the central nervous system function rely heavily on carbohydrate availability. While the body has substantial fat stores, these are metabolised more slowly and require more oxygen to convert into usable energy. In contrast, carbohydrates can be accessed and utilised rapidly, making them especially important when exercise intensity increases.
This is why, as exercise intensity rises, the body progressively shifts towards carbohydrate as its primary fuel source. When carbohydrate availability becomes limited, either due to depleted muscle glycogen or insufficient intake during prolonged exercise, the brain acts protectively by reducing pace and power output.
Why High-Carbohydrate Fuelling Became the Standard Approach
Elite endurance athletes are increasingly adopting aggressive carbohydrate fuelling strategies. For example:
Joe Klecker, an elite marathon runner who finished in the top 10 at the NYC Marathon last year, reported intakes of 150–175 g of carbohydrate per hour during competition.
Professional cyclists in multi-day stage races, such as Tadej Pogacar, frequently consume 100–120 g per hour as part of an integrated fuelling strategy.
These practices reflect advances in sports nutrition, particularly the use of multiple transportable carbohydrates (such as glucose and fructose), which increase intestinal absorption capacity. However, elite practice should not be mistaken for universal guidance. These athletes operate at exceptionally high intensities, have highly-trained gastrointestinal systems, and receive extensive professional support.
Low-Carbohydrate and High-Fat Diets: What the Evidence Shows
Proponents of LCHF nutrition, including Prof. Tim Noakes, argue that training with low carbohydrate availability enhances fat oxidation and reduces dependence on exogenous carbohydrate intake. A recent narrative review by Noakes and colleagues suggested that as little as 15–30 g of carbohydrate per hour may be sufficient to maintain blood glucose during endurance exercise.
It is important to understand what this type of evidence represents. Narrative reviews summarise existing research but do not generate new experimental data. The physiological principle underpinning this argument - improved fat metabolism with carbohydrate restriction - is well-established. However, enhanced fat oxidation does not necessarily translate to improved performance, particularly at higher intensities.
Carbohydrate Intake During Running: How Much Is Actually Useful?
Controlled laboratory and field studies consistently demonstrate that carbohydrate ingestion during endurance exercise improves performance, especially when intensity is moderate to high or duration is prolonged. The current evidence suggests:
The body can oxidise up to 120g of carbohydrate per hour when multiple carbohydrate sources are used.
Performance benefits plateau at about 80g per hour.
A recent study in elite marathon runners (published November 2025) showed improved running economy (the amount of oxygen used to run) up to 120g per hour, although actual performance outcomes were not measured. Notably, gastrointestinal symptoms were common at the highest intake levels. This highlights an important point: our ability to take on high levels of carbohydrates does not automatically equal practical benefit.
Training Status and Intensity: Why Carbohydrate Needs Differ
There is no single “optimal” carbohydrate intake that applies to all endurance athletes. Fuelling strategies should be individualised, taking into account:
Training status and performance level
Event duration and intensity
Gastrointestinal tolerance
Previous fuelling experience
Just as it would be inappropriate for a recreational runner to follow an elite training programme, it is equally inappropriate to adopt an elite athlete’s fuelling strategy without adaptation.
Individualised Fuelling Strategies for Endurance Runners
Carbohydrate needs vary substantially depending on exercise intensity. Elite marathon runners often operate close to their maximal aerobic capacity for prolonged periods, which markedly increases their need for carbohydrates. In contrast, runners whose goal is completion rather than competition - and those training or racing at lower relative intensities, such as most ultramarathon runners - can rely more heavily on fat oxidation and may not require high carbohydrate intakes during exercise.
This is why fuelling should be matched not just to distance, but to intensity.
Gastrointestinal Tolerance and the Importance of Gut Training
Higher carbohydrate intakes are consistently associated with an increased risk of gastrointestinal symptoms. These symptoms can impair performance and, in some cases, end races.
Gut training - systematically practising race-day fuelling during training - can significantly improve tolerance. Elite athletes invest substantial time in this process. For example, successful ultra-endurance runners like Ruth Croft who won the Ultra Trail de Mont Blanc last year have been open about the months of structured practice to consistently tolerate intakes of 120g per hour under race conditions. Without adequate gut training, attempting high carbohydrate intakes on race day is risky.
Practical Takeaways for Endurance Running Nutrition
The current body of scientific evidence continues to support carbohydrate availability as a benefit in endurance sports, particularly for longer and higher-intensity efforts.
Low-carbohydrate training approaches may have a place within specific training contexts, but they do not eliminate the need for carbohydrates when performance matters most.
The most effective fuelling strategy is one that is:
Evidence-informed
Individualised
Practised thoroughly in training
If you want help translating the science into a fuelling strategy that actually works for your training, racing goals, and gut tolerance, consider working with a qualified sports nutrition professional. An individualised approach can help you train harder, race smarter, and avoid costly nutrition mistakes on race day.
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