Why performance as an endurance athlete depends on carbohydrates...

Carbohydrates have increasingly become a target in nutrition culture, and are often demonised, with low-carb diets often promoted as being superior for health, body composition, and performance. However, this is just not the case when it comes to endurance sports - the science is consistent that carbohydrates are KEY to performance.

From marathon runners and cyclists to triathletes and ultra-endurance athletes, successful endurance performance depends heavily on the body's ability to access and utilise carbohydrates efficiently. The body needs carbohydrates to provide rapid and efficient energy to sustain pace, maintain power output, and delay fatigue. Let's take a look why this is by deep diving into our physiology.

Carbohydrates: The body's preferred fuel

When we eat carbohydrates, they are broken down into glucose which circulates in the bloodstream and is used by cells to produce energy. Any excess glucose is stored as glycogen in our muscles and liver. These glycogen stores act as a readily available fuel reserve during exercise.

The body's energy currency is ATP. During exercise ATP must be regenerated continuously to sustain muscle contraction. While both fat and carbohydrate can contribute to ATP production, carbohydrates can generate energy far more rapidly and fat alone will simply not produce energy at a rate our bodies need during endurance sport. Things such as tempo efforts, hill climbs, surges, threshold intervals, and race pace efforts will all require substantially greater carbohydrate utilisation.

Glycogen: The limiting factor in endurance performance

One of the defining characteristics of endurance exercise is the gradual depletion of glycogen stores.

The body can only store a limited amount of carbohydrate and although these stores are valuable, they are finite. During prolonged exercise, especially at moderate to high intensities, glycogen depletion becomes one of the primary causes of fatigue.

As glycogen stores decline:

• power output decreases

• perceived exertion rises

• pacing becomes more difficult

• neuromuscular function begins to deteriorate

• blood glucose levels fall

This is what endurance athletes often describe as 'hitting the wall' or 'bonking' - it's not just tiredness, it's a significant reduction in fuel availability.

In many endurance events, glycogen availability becomes the limiting factor between maintaining performance and slowing dramatically in the later stages of competition.

We can 'boost' the amount of glycogen stored by 'carb loading' before a race. This means ensuring our glycogen stores are full prior to racing so we are not starting low on energy straight away. Having full glycogen stores and adequate carbohydrate intake during racing - you are allowing your body to perform at their best.

Why fat alone cannot sustain high-level endurance performance

One of the most common misconceptions in endurance competition is that athletes can train their bodies to rely almost entirely on fat. While athletes can become more efficient at oxidising fat during training, fat metabolism does have its limitations.

The process of converting fat into usable energy is slower and more oxygen-intensive than carbohydrate metabolism. As exercise intensity increases, the body shifts towards greater carbohydrate dependence as carbohydrates can support faster rates of ATP production.

This is especially relevant during climbs, race surges, threshold efforts, and finishing sprints.

Even highly fat-adapted athletes increase carbohydrate oxidation when intensity rises. This explains why many low-carb athletes can perform adequately at steady aerobic intensities but struggle to sustain top end race efforts or repeated high-intensity surges.

Carbohydrate intake during endurance exercise

Sports nutrition has evolved significantly in recent years, and one of the biggest developments has been the growing emphasis on aggressive carbohydrate fuelling during endurance events. For any training/race over 60-90 minutes, you should always be having some form of fast acting carbohydrate during your activity.

Research now shows that endurance athletes can oxidise high amounts of carbohydrate during exercise when intake is properly trained and structured.

Advancements in sports nutrition research have shown the use of multiple transportable carbohydrates - particularly the need for glucose and fructose combinations when taking on carbohydrates during exercise. This allows for optimal carbohydrate uptake and utilisation, meaning performance is boosted.

Equally important is gut training. Just as our muscles adapt to training stress - our guts adapt to higher carbohydrate intake over time. Athletes who regularly practice race fuelling during training often experience:

• improved tolerance

• reduced gastrointestinal distress

• better carbohydrate absorption during competition

For endurance athletes, fuelling and practicing this prior to competition is essential.

Carbohydrates for recovery

Carbohydrates do not only influence performance during exercise - they also play a major role in recovery.

Following prolonged or intense training sessions, glycogen stores can be substantially depleted. If carbohydrate intake after exercise is inadequate, recovery can be poor - especially during periods of high training volume.

Insufficient carbohydrate intake between training sessions can result in:

• reduced training quality

• impaired adaptation

• increased fatigue

• poor performance

This is especially important for endurance athletes training multiple times per week or multiple times per day. Restoring glycogen efficiently allows athletes to maintain consistency across training blocks and recover more effectively between hard sessions.

Carbohydrates consumed alongside 20-30g protein is the optimal post-workout recipe for recovery.

Key points:

• Carbohydrates are the body's preferred fuel source and sufficient carbohydrate intake is key for all aspects of endurance performance - training, recovery, performance, health

• 'Carb loading' is essential for ensuring glycogen stores at 'full' prior to race day

• Fuelling during training is essential for anything over 60-90 minutes - this should be in the form of fast acting carbohydrates such as energy gels

• Carbohydrate intake between training sessions and after sessions is also essential for adequate recovery

For endurance athletes, understanding carbohydrates is not simply about nutrition - it is about performance. The ability to fuel effectively before, during, and after exercise can often determine not only how fast an athlete performs, but how well they recover and adapt long term.

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