Why an Athlete Needs to Consume Glucose During a Marathon

December 24, 2025 •

4 min read

During a marathon, an athlete's body requires a continuous and immediate energy source, and this is why an athlete needs to consume glucose at certain intervals. The human body has limited carbohydrate reserves, and without regular fuel top-ups, performance would significantly drop, a feeling known as 'hitting the wall'.

Quick Summary

Athletes consume glucose regularly during a marathon to replenish depleting energy stores and maintain blood sugar levels. This strategy provides a quick energy source to muscles, delaying fatigue and preventing performance decline over the long distance. It's a critical fueling strategy for endurance sports.

Key Points

Replenishes Energy Stores: During a marathon, an athlete's stored energy (glycogen) is used up, and consuming glucose replenishes this fuel.
Provides Quick Energy: Glucose is a simple sugar, so the body can absorb it quickly and use it for immediate energy during exercise.
Delays Fatigue: By providing a constant fuel supply, regular glucose intake prevents energy crashes and delays the onset of fatigue.
Prevents 'Hitting the Wall': Consuming glucose helps to prevent the severe fatigue and drop in performance that occurs when the body's glycogen stores are depleted.
Maintains Blood Glucose: Regular intake helps keep blood sugar levels stable, which is crucial for maintaining both physical performance and concentration.
Fuels the Brain: The brain relies on a steady supply of glucose for proper function, and a lack of it can lead to impaired focus.
Aids Recovery: Consuming glucose during and after a race helps kickstart the recovery process and restore glycogen levels.

The Body's Race Fuel: Glucose and Glycogen

To understand why an athlete needs to consume glucose at intervals, we must first look at how the body produces energy. The human body is a complex machine that uses different fuel sources for different activities. For a high-intensity, long-duration event like a marathon, the primary fuel is carbohydrates, which are broken down into glucose. This glucose is either used immediately or stored as glycogen in the liver and muscles for later use.

Why Marathoners Run on a Timer

During the initial phase of a marathon, the body uses glucose readily available in the bloodstream. Once this is used up, it turns to the stored glycogen. However, these glycogen stores are not limitless. For most well-trained athletes, they can provide energy for about 90 to 120 minutes of moderate-to-high-intensity running. After this point, if new carbohydrates are not introduced, the body's primary fuel source becomes depleted, leading to severe fatigue and a sharp decrease in performance. This is the moment a runner often feels like they have 'hit the wall'.

By consuming glucose (usually through sports drinks, gels, or chews) at regular intervals—often every 30 to 45 minutes—the athlete continuously replenishes their blood glucose levels. This provides the muscles with a steady supply of easy-to-use fuel, allowing them to continue performing at a high level without tapping out their limited glycogen reserves too quickly. This strategic fueling maintains energy and helps prevent the dramatic drop in speed and energy that comes from glycogen depletion.

The Science Behind Fueling

The science is relatively simple: glucose is a simple sugar, meaning it can be absorbed by the body very quickly without a lot of digestion. This makes it an ideal energy source to be consumed during a race when the body's resources are already stretched thin. Complex carbohydrates, like those in bread or pasta, would take too long to break down into usable glucose, and the digestive process itself would divert energy away from the muscles. This is why marathon runners don't snack on sandwiches mid-race but opt for easy-to-digest gels or drinks.

Additionally, consuming a mix of carbohydrates, such as glucose and fructose, can be even more effective. The body has different transporter proteins for different sugars, so using a combination can maximize the rate at which carbohydrates are absorbed into the bloodstream, potentially allowing for even higher hourly intake and improved performance. This is an advanced strategy, but the core principle for young students remains the same: regular glucose intake is essential for sustained energy.

Comparison of Energy Sources During a Marathon


Energy Source	Function	Availability	Consumption Timing
Stored Glycogen	Primary fuel source for endurance.	Limited; typically lasts 90-120 minutes of moderate-high intensity exercise.	Pre-race loading and internal reserves.
Consumed Glucose	Immediately available fuel source.	Replenished regularly during the race.	Every 30-45 minutes throughout the race.
Stored Fats	Backup fuel source.	Abundant, even in lean athletes.	Primarily used at low intensity, and only after glycogen is depleted.
Protein	Not an ideal fuel source for exercise.	Can be broken down for energy but also used for muscle repair.	Minimal, generally avoided during the race to prevent muscle breakdown.

Training Your Gut for Race Day

Just like athletes train their muscles and cardiovascular system, they also train their digestive system. A sudden, high intake of glucose on race day can cause gastrointestinal distress, as the body's blood flow is diverted away from the gut to the working muscles. To avoid this, runners practice their fueling strategy during long training runs to teach their stomach to tolerate the carbohydrate intake. This 'gut training' is crucial for maximizing carbohydrate absorption without discomfort and is a key part of any serious marathon training plan.

Conclusion: Fueling the Long Run

In summary, consuming glucose at certain intervals is a fundamental strategy for any athlete competing in a marathon. It's not about providing a short-term 'sugar rush' but about strategically managing the body's fuel sources to prevent fatigue. By understanding that stored glycogen is limited and that external glucose can provide a rapid, steady source of energy, young students can grasp the essential science behind endurance sports nutrition. This approach helps athletes maintain their pace, sustain their energy levels, and cross the finish line feeling strong.

Glossary: Key Terms

ATP (Adenosine Triphosphate)

The main energy-carrying molecule used by cells to power all bodily functions, including muscle contraction.

Glycogen

Stored form of glucose in the liver and muscles, serving as the body's primary carbohydrate reserve for energy.

Carbohydrate Loading

A nutritional strategy of increasing carbohydrate intake and reducing exercise before a race to maximize glycogen stores.

The Wall

Extreme fatigue and lack of energy experienced by endurance athletes when their glycogen stores are depleted.

Gut Training

The practice of consuming carbohydrates during training runs to accustom the digestive system to processing fuel during exercise.

Hypoglycemia

Abnormally low blood sugar levels, which can lead to fatigue and impaired performance.

Frequently Asked Questions

Glucose is a simple sugar that serves as the main source of energy for the body's cells and muscles, especially during intense, prolonged exercise. It is essential for athletes to fuel their performance and prevent fatigue.

If a marathon runner doesn't consume glucose, their body will eventually deplete its stored energy (glycogen). This leads to a severe energy drop and fatigue, commonly known as 'hitting the wall'.

Athletes typically consume glucose at regular intervals, often every 30 to 45 minutes, during a marathon. The exact amount and frequency can vary based on the athlete's body, intensity, and hydration strategy.

No, the body also uses fat for energy, especially at lower intensities. However, fat is a less efficient fuel source for high-intensity efforts, and carbohydrates (glucose) are crucial for maintaining speed and power.

Excess glucose is stored in the liver and muscles in the form of glycogen, ready to be converted back into glucose when the body needs more energy during exercise.

For endurance athletes during a race, consuming quickly digestible sugar like glucose is beneficial, not harmful. It is a necessary fuel source to maintain energy and performance. This is different from consuming large amounts of sugar in daily life.

A regular meal contains complex carbohydrates, fats, and protein, which take a long time to digest. During a race, an athlete needs a quick energy source, so they consume simple sugars like glucose, which is absorbed almost instantly and doesn't require a lot of digestive effort.