How do carbohydrates fuel the body during exercise?

November 18, 2025 •

4 min read

Over a century of research has proven that carbohydrates are the most critical energy source for athletes and physically active individuals. This vital macronutrient powers muscle contraction and sustains performance, from short sprints to ultra-endurance events, by supplying working muscles with readily available fuel. Without sufficient carbohydrate stores, fatigue sets in rapidly, often leading to a significant drop in performance.

Quick Summary

This article details the metabolic processes that convert carbohydrates into immediate energy for muscles and the brain during exercise, focusing on glycogen stores and glucose absorption.

Key Points

Glycogen Stores: The body converts carbohydrates into glycogen, storing it in the muscles and liver to provide a ready supply of energy for muscle contraction during exercise.
Performance at Higher Intensity: At higher exercise intensities, carbohydrates are the most efficient fuel, providing energy faster than fat to meet the increased demand for ATP.
Delaying Fatigue: Strategic carbohydrate intake before and during exercise, especially in events over 60-90 minutes, is crucial for delaying fatigue caused by glycogen depletion.
Carbohydrate Source Variation: Combining different carbohydrate types, such as glucose and fructose, can increase absorption rates and fuel delivery during prolonged exercise sessions.
Optimizing Recovery: Consuming carbohydrates immediately after strenuous exercise helps replenish depleted glycogen stores, aiding in faster recovery and preparing the body for the next training session.
Mouth Rinse Effect: Even a carbohydrate mouth rinse can improve performance during short-duration, high-intensity exercise by stimulating the central nervous system, affecting perceived effort and motor output.
Training the Gut: Athletes can train their digestive system to tolerate and absorb higher amounts of carbohydrates during prolonged events, minimizing gastrointestinal issues and maximizing performance.

The Foundational Role of Carbohydrates in Exercise

Carbohydrates are the body's primary and most efficient fuel source for moderate to high-intensity exercise. Unlike fat, which requires more oxygen to metabolize, carbohydrates can produce ATP (adenosine triphosphate), the body's energy currency, both aerobically (with oxygen) and anaerobically (without oxygen). This dual-metabolism capability makes them indispensable for activities ranging from weightlifting and sprints to endurance events like marathons. The body stores carbohydrates in two main forms: muscle glycogen and liver glycogen. While liver glycogen helps maintain stable blood sugar levels to fuel the brain, muscle glycogen directly powers the contracting muscles. The availability and replenishment of these glycogen stores are fundamental to sustaining performance and delaying fatigue.

The Breakdown: From Carbohydrate to Energy

The journey of a carbohydrate from food to muscle fuel is a multi-step metabolic process. First, carbohydrates are digested and broken down into simple sugars, primarily glucose. This glucose enters the bloodstream, causing a rise in blood sugar levels. In response, the pancreas releases insulin, a hormone that facilitates the uptake of glucose into the body's cells, including muscle cells. Glucose can be used immediately for energy or converted into glycogen for storage in the liver and muscles.

During exercise, the body's energy demands increase dramatically. At rest, fat is the primary fuel, but as exercise intensity rises, the body shifts towards using more carbohydrates. This shift is crucial because carbohydrate oxidation provides energy faster than fat oxidation. Muscle glycogen is the first line of defense, rapidly being broken down into glucose-6-phosphate to produce ATP for muscle contraction. When muscle glycogen stores begin to deplete, the body increasingly relies on blood glucose, supplied by the liver. For endurance activities lasting over 60-90 minutes, exogenous carbohydrate intake—from sports drinks, gels, or food—is necessary to maintain blood glucose and continue fueling the working muscles.

Glycogen vs. Blood Glucose: The Dual Fuel System

Muscle Glycogen is a localized fuel source, meaning it can only be used by the specific muscle in which it is stored. It is the power source for high-intensity, anaerobic activities. For example, during a 400-meter sprint, an athlete relies almost entirely on existing muscle glycogen because the energy demand is too high for the body to rely on blood glucose or fat metabolism. However, these stores are finite and can be depleted in as little as 60-90 minutes of high-intensity activity, leading to what many endurance athletes call "hitting the wall".

Blood Glucose, on the other hand, is a systemic fuel source maintained by the liver. The liver breaks down its stored glycogen (glycogenolysis) and can also produce new glucose (gluconeogenesis) to release into the bloodstream. During prolonged exercise, the liver's role becomes critical, but its output can't always keep pace with the muscles' demand. This is why consuming carbohydrates during prolonged exercise is so important; it provides an external source of glucose to supplement liver production, maintaining blood sugar and delaying fatigue.

The Importance of Timing: Pre-, During, and Post-Exercise Nutrition

Strategic carbohydrate intake is key to maximizing performance. Proper timing can significantly impact an athlete's energy availability.

Pre-Exercise: Consuming a high-carbohydrate meal 2-4 hours before an event helps top up both liver and muscle glycogen stores. For exercise lasting less than 60 minutes, a small snack of high-glycemic carbs 30-60 minutes before can be beneficial.
During Exercise: For activities over 60 minutes, consuming 30-60 grams of carbohydrates per hour is recommended. For longer endurance events (over 2.5 hours), this can be increased to 90-120 grams per hour, using a combination of glucose and fructose to maximize absorption. This exogenous fuel source helps maintain blood glucose levels as internal glycogen stores diminish.
Post-Exercise: Replenishing glycogen stores immediately after exercise is critical for recovery. The body's ability to synthesize glycogen is highest in the first 1-2 hours post-exercise. Consuming 1.0-1.2 grams of high-glycemic carbohydrates per kilogram of body weight, often combined with protein, can accelerate this process.

Carbohydrates vs. Fats for Exercise Fuel


Feature	Carbohydrates	Fats
Energy Production Rate	Fast & Efficient	Slower & Less Efficient
ATP Yield Per Oxygen	Higher	Lower
Storage Capacity	Limited (muscle/liver glycogen)	Very Large (body fat)
Intensity Preference	Moderate to High	Low to Moderate
Anaerobic Metabolism	Yes (glycolysis)	No
Endurance Limit	Performance limited by storage capacity	Performance not limited by storage, but by energy production rate
Energy Density	4 kcal/gram	9 kcal/gram
Performance at High Intensity	Essential	Inefficient/Inadequate

Conclusion

Understanding how carbohydrates fuel the body is essential for optimizing athletic performance and enhancing overall fitness. Through a dynamic metabolic process, carbohydrates are stored as glycogen in the liver and muscles, providing both a systemic reservoir for the brain and a direct power source for muscle contraction. The body's reliance on carbohydrates increases with exercise intensity, making proper pre-, during, and post-exercise fueling strategies critical for maintaining performance, delaying fatigue, and accelerating recovery. By managing carbohydrate intake, athletes can effectively train their bodies to maximize this potent and efficient fuel source. For further reading on exercise metabolism, consider the review in the Nature Reviews Endocrinology journal.

Frequently Asked Questions

When you run out of glycogen during exercise, a state often called "bonking," you will experience severe fatigue and a significant drop in performance, as your body must switch to less efficient fat metabolism for energy.

For fully fueled, high-intensity exercise, muscle glycogen stores can be depleted in approximately 60 to 90 minutes. The exact duration depends on the intensity, individual fitness level, and initial glycogen reserves.

Muscle glycogen is used directly by the muscle where it is stored for fuel, whereas liver glycogen is released into the bloodstream as glucose to maintain stable blood sugar levels, which is vital for the brain.

For exercise lasting less than 60 minutes, carbohydrate intake is generally not necessary if you start well-fueled, as your existing glycogen stores are sufficient. However, a carbohydrate mouth rinse may provide a performance benefit during high-intensity efforts.

Complex carbohydrates are digested more slowly, providing sustained energy, and are ideal for pre-exercise meals. Simple carbohydrates are absorbed quickly, offering a rapid energy boost, and are beneficial during and immediately after exercise.

Neither fat nor protein is a better fuel source than carbohydrates for high-intensity exercise. Carbohydrates provide ATP much faster, and while fat is a fuel source for lower-intensity activity, fat metabolism cannot meet the demands of high-intensity work.

A carbohydrate mouth rinse improves performance by activating receptors in the mouth that stimulate the central nervous system, which enhances motor drive and reduces the perception of effort, without the need for swallowing.

Yes, some training strategies, such as training with low carbohydrate availability ("train low"), aim to increase the body's efficiency at fat oxidation. However, for elite endurance performance, maximizing carbohydrate availability and utilization is still considered the most effective strategy.