The role of carbohydrates in endurance training and performance

Part 1: The Science of Carbohydrates

An article written by Sophie Herzog, Øyvind Sandbakk, Trond Nystad and Rune Talsnes, with expert inputs by Dr. Tim Podlogar

Introduction

According to initial results of our screening, over 70% of athletes described themselves as either "very conscious" or having a "normal" level of nutritional awareness and knowledge. However, a significant proportion reported consuming little to no fuel, beyond water, during both low-intensity (LIT) and high-intensity (HIT) endurance training sessions. Specifically, 24% of athletes reported taking in no carbohydrates during HIT sessions and 40% during LIT sessions. This indicates a potential disconnect between nutritional awareness and actual application of fueling strategies, which is also reported in the research literature (Sampson et al., 2024) and raises an important question: how many athletes are unknowingly underperforming (or at least limiting their performance) due to suboptimal carbohydrate intake?

Historically, our understanding of carbohydrates’ role in endurance performance has evolved significantly. However, it's worth noting that not all researchers and nutritionists agree on the necessity of carbohydrate intake during endurance training. Initial research primarily focused on the role of carbohydrates in enhancing endurance performance, establishing their critical importance during moderate to high-intensity efforts (ref). Later, periodized carbohydrate intake strategies gained popularity through the famous slogan “fuel for the work required” (Impey et al., 2018). The idea of this approach is to go low in carbohydrates when the intensity/workload is low (a very extreme example was Chris Froome’s breakfast of 4 eggs, avocado and salmon on a rest day during the Tour de France), and to go very high in carbohydrate intake when the intensity/workload is very high. Furthermore, trends of low carbohydrate diets or commencing training in a low glycogen state were rooted in the belief that it could lead to increased mitochondrial adaptations, higher fat oxidation rates, preservation of carbohydrate stores and potentially improving endurance performance (Hetlelid et al., 2018, Noakes et al., 2017). Although low carbohydrate approaches and related literature should be acknowledged and might represent a relevant approach in certain scenarios, current literature does not support that these dietary strategies are beneficial for competitive endurance athletes (Burke et al., 2024) and practical observations indicate that such approaches are less used in world-class endurance athletes today, and contemporary practices among elite athletes shifted more towards higher carbohydrate intake during training and competition.

Why do we need carbohydrates?

Undertaking intense exercise sessions – such as competitions and high-intensity training – places large demands on the body, particularly on glycogen stores, the body’s primary way to store carbohydrates. Especially for world-class athletes, who sustain noticeably higher power and speed outputs, this results in a substantially greater energy turnover and places large demands on their internal carbohydrate storages – even during their low intensity training.

Without adequate carbohydrate intake, athletes risk a state of relative energy deficiency (REDs), which may not only be triggered by low-energy availability but could most likely also be caused by low-carbohydrate availability alone (Stellingwerff et al., 2021). Chronic REDs can compromise training, performance and even career longevity. However, the extent to which underperformance states in endurance athletes are caused by knowledge gaps regarding carbohydrate intake or other factors remains unclear.

How much carbohydrate is available?

Carbohydrates are stored as glycogen, primarily within the muscles and the liver. However, these glycogen stores are limited to 1000-3500 kilocalories of energy - although the ability to store muscle glycogen can be increased to some extent with improved training status and manipulated further using training and dietary manipulations (Areta et al., 2018). Liver glycogen stores also play a crucial role in maintaining energy levels, both at rest and during exercise. As liver glycogen is broken down into glucose and released into the bloodstream, it helps to stabilize blood glucose levels, supplying energy to vital organs, including the brain. When these stores become depleted – especially during prolonged exercise – blood glucose levels drop, which can lead to a light-headed, low energy feeling, and potentially contribute to fatigue (Elghobashy et al., 2024).

Muscle glycogen depletion is also implicated in fatigue during prolonged endurance exercise and recent studies suggest that when muscle glycogen stores are depleted, the mechanisms driving muscle contraction are impaired. It is now well established that carbohydrate intake during competitions improves endurance performance (Stellingwerff et al., 2014). But why and when is that the case?

Ingested (i.e., exogenous) carbohydrates during exercise help to 1) maintain stable blood glucose levels over long exercise durations (Jeukendrup et al., 1985) and 2) provide an additional fuel source for ATP formation during exercise. In other words, exogenous carbohydrates substitute endogenous carbohydrates (i.e., the stored glycogen) such that the oxidation rate of carbohydrates can be maintained despite declining endogenous glycogen levels (Coyle et al., 1985; Coyle et al., 1986). In addition to fueling during competition, strategies like pre-competition glycogen loading and post-exercise recovery nutrition have been shown to enhance endurance performance. These approaches are particularly important in long-duration sports, where energy demands far exceed glycogen storage capacity (Plasqui et al., 2019; Sjödin et al., 1994).

The role of carbohydrate in preventing underperformance?

High training volume is a defining characteristic of world-class endurance athletes (Sandbakk et al., 2023). To maximize exercise capacity and performance, optimal carbohydrate availability before and during training is essential, whereas chronic low carbohydrate availability can compromise exercise capacity (training volume and intensity), quality of training and performance.

While strategically implementing training with low carbohydrate availability (“train low”) can arguably be a strategy for time-restricted amateur athletes or for elite athletes who have already exhausted all other possibilities for improvement, it is a high-risk strategy if not done right and is therefore not advisable when aiming for long-term success (Gejl et al., 2021).  Furthermore, elite endurance athletes with high training volumes generally have a well-developed capacity to utilize fat as a fuel source (Maunder et al., 2021). Therefore, increasing fat oxidation rates as such should not be prioritized ahead of the goal to improve one’s aerobic capacity, especially in the light of evidence demonstrating that training with low carbohydrate availability can result in a reduced ability to exercise at high intensities. In addition, based on our own experience, insufficient carbohydrate intake can be a contributor to excessive fatigue and underperformance states in endurance athletes.

In the cycling world in particular, the concept of “fuel for the work required” (Impey et al., 2018) was widely used and accepted, but did not prove to be superior (Gejl et al., 2021). More recent evidence argues against “train low” strategies and favours continuous sufficient energy intake for optimal performance and subsequent recovery (Podlogar et al., 2022).

The calculation of energy intake is often informed by wearable data (e.g., through power meters and rarely continuous glucose monitors) or even isotope tracing to quantify energy turnover and has been instrumental in helping athletes adjust their carbohydrate intake to meet performance goals. However, as our understanding of athlete health and performance evolves, so too does the way we approach fuelling strategies.

A Modern Framework for Carbohydrate Use

The last years in endurance sports has also been coined “Carbolution”[1], and many have suggested that higher carbohydrate-intake is better – not just for performance, but also for recovery and adaptation.

If carbohydrate utilization efficiency during exercise is and remains high, this – on one hand - may enhance energy availability and performance. On the other hand, going super high in carbohydrates during exercise may also mean that you utilize carbohydrates excessively and therefore end the sessions depleted. This may cause a problem as carbohydrate intake during exercise depletes the daily energy budget, leaving insufficient carbohydrates for post-exercise muscle and liver glycogen replenishment or athletes have to go into an energy surplus, which is also not desired.

On the contrary, if carbohydrate utilization decreases during exercise and carbohydrates accumulate in the gut, this can cause gut issues. Furthermore, it is important to understand that the main limitation is not how many carbohydrates you can tolerate, but how much you can absorb and utilize (Podlogar et al., 2022).

Despite the growing use of high carbohydrate fuelling strategies, we still know very little how this affects our metabolic health and gut microbiome in the long-term. Therefore, the next step in carbohydrate research is to better understand how the body processes extremely high carbohydrate intakes acutely and chronically. Moving forward, it is to be expected that fuelling strategies will become increasingly personalized, with athletes having their carbohydrate absorption and utilization capacity measured using laboratory-grade equipment.

With that in mind, a modern carbohydrate framework should balance immediate performance needs with long-term health, integrating fuelling into a holistic nutrition strategy guided by a few key principles:

• Health First: Sustainable performance depends on overall health, including gut health and energy availability.

• Energy Budget: High-level performance requires substantial training and energy expenditure, making well-timed, sufficient and context-dependent (e.g., heat, altitude) carbohydrate intake essential.

• Nutritional Balance: Fuelling for performance should still leave space for high-quality, nutrient-dense foods - such as vegetables, fibres, and fermented foods – that support gut health, immune function and long-term well-being.

Ultimately, fuelling during exercise isn’t just about optimizing short-term performance but also about sustaining training consistency, promoting effective recovery and building a balanced nutritional foundation for long-term athletic development.

What’s Next?

This article sets the stage by explaining why carbohydrates are indispensable for endurance training and performance. In the follow-up, we’ll provide actionable recommendations, personal insights, and strategies for effective fueling before, during, and after exercise.

Stay tuned for practical tools to help you optimize your performance and recovery!

References

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[1] https://velo.outsideonline.com/road/road-training/pushing-the-carbohydrate-revolution-could-reshape-pro-cycling/?utm_medium=organic-social&utm_source=Velo%2Cvelovelovelo_%2Cvelovelovelo__-facebook%2Cthreads%2Ctwitter