Clubhouse #11 | The Role of Carbohydrates vs. Fat in Endurance Performance 🥑

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In endurance sports, nutrition is often the deciding factor between peak performance and hitting the wall. Athletes constantly debate whether carbohydrates or fat should be the primary fuel source for long-duration efforts. While both macronutrients play a role in energy metabolism, understanding how and when to optimize their use can dramatically impact endurance performance, recovery, and training adaptations.

In today’s Clubhouse, we’ll explore the science behind carbohydrates and fat metabolism, their role in different types of endurance training, and how to fuel effectively for peak performance. Whether you're training for a marathon, an Ironman, or simply improving endurance capacity, understanding fuel optimization is key to sustaining energy and avoiding fatigue.

So lets dive in to learn how nutrition can enhance your performance.

Read the past Clubhouses here:

• Clubhouse #10 | The Science of Periodization: Structuring Training for Maximum Gains 🏋️

• Clubhouse #9 | Mastering Sleep: The Athlete's Guide to Leveraging Rest for Peak Performance 💤

• Clubhouse #8 | Lactate Threshold Training: Unlocking Peak Endurance Performance ⚡️

• Clubhouse #7 | AI in Fitness: How Technology is Shaping Personalized Health Plans 🔧

• Clubhouse #6 | Biohacking Sleep: Techniques for Optimal Rest and Recovery 💤

• Clubhouse #5 | The Connection Between Gut Health and Athletic Performance 🍎

• Clubhouse #4 | The Science-Backed Power of Visualization for Achieving Your 2025 Goals 🌟

• Clubhouse #3 | The science-backed reasons why sugar is good for athletes 🔋

• Clubhouse #2 | Why you should invest in a health tracking wearable like WHOOP

• Clubhouse #1 | How to actually train for your first Ironman 70.3.

The Science of Energy Metabolism: How the Body Uses Carbs and Fat

Carbohydrates: The Preferred Fuel for High-Intensity Performance

Carbohydrates serve as the primary energy source for endurance performance, particularly during moderate to high-intensity efforts exceeding 65% VO2 max. The body efficiently stores carbohydrates in the form of glycogen within muscles and the liver, where they can be rapidly converted into glucose for energy production. This readily available fuel source allows athletes to sustain high work rates and execute explosive movements during races, interval training, and tempo efforts.

One of the greatest advantages of carbohydrates is their ability to be quickly broken down through glycolysis, producing adenosine triphosphate (ATP)—the body’s immediate energy currency. Unlike fat metabolism, which requires more oxygen and a slower breakdown process, carbohydrate oxidation allows for efficient ATP production, making it the dominant fuel for short bursts of intense activity. This rapid energy availability is essential for high-speed efforts such as sprints, surges, and hill climbs.

Despite their efficiency, carbohydrates have a limited storage capacity. The body can only store approximately 400-600 grams of glycogen between the muscles and liver, equating to about 1,600-2,400 kcal of available energy. During prolonged exercise, particularly sessions lasting over 90-120 minutes, glycogen stores become depleted. This depletion leads to the dreaded "bonk" or hitting the wall, where the body lacks sufficient energy to maintain pace and performance drops sharply. Athletes who fail to replenish carbohydrates during long-duration efforts experience muscular fatigue, impaired cognitive function, and reduced power output.

To mitigate glycogen depletion, endurance athletes must adopt strategic fueling strategies. Carbohydrate intake before and during prolonged exercise replenishes glycogen stores and sustains energy levels. Research suggests that consuming 30-90g of carbohydrates per hour during workouts longer than 60 minutes optimizes endurance performance and prevents fatigue. This intake can be achieved through easily digestible sources such as sports drinks, energy gels, fruit, or simple starches. Additionally, post-exercise glycogen replenishment is crucial for recovery, as consuming 1.2g of carbohydrates per kg of body weight within 30-60 minutes post-workout enhances muscle glycogen resynthesis and prepares the body for subsequent training sessions.

Carbohydrates provide 4 kcal per gram, making them the most efficient energy source for sustained, high-intensity activity. Their role in endurance training is indispensable, particularly for athletes engaging in frequent speed work, threshold training, or competition where rapid energy availability is critical. Without sufficient carbohydrate intake, performance deteriorates, recovery slows, and long-term endurance adaptations become compromised.

Fat: The Slow-Burning, Endurance Fuel

Fat serves as a nearly unlimited energy source, with even lean athletes carrying over 50,000 kcal of stored fat within adipose tissue and muscle. Unlike carbohydrates, which have a finite storage capacity, fat reserves can sustain endurance athletes for prolonged durations, making it the preferred fuel source at lower intensities (below 65% VO2 max), where oxygen supply is sufficient to break it down for energy.

One of the key advantages of fat metabolism is its ability to provide sustained energy for long-duration efforts. Endurance events exceeding two hours rely heavily on fat oxidation to delay glycogen depletion and maintain energy output. Since glycogen stores are limited, training the body to efficiently utilize fat as a fuel source can significantly improve endurance capacity. Athletes who develop strong fat oxidation capabilities can spare glycogen, ensuring that carbohydrate reserves are available for more critical, high-intensity segments of a race—such as surges, climbs, or finishing sprints.

However, fat oxidation is more oxygen dependent than carbohydrate metabolism. The process of breaking down fat molecules to generate adenosine triphosphate (ATP) requires a greater oxygen supply, making it inherently less efficient for fast-paced or anaerobic efforts. This is why fat is predominantly used during low-to-moderate-intensity exercise, where oxygen availability is not a limiting factor. Despite its slower breakdown rate, fat provides a high energy yield of 9 kcal per gram, making it an essential fuel source for ultra-endurance athletes and long-distance competitors.

Fat utilization is adaptable to training, meaning athletes can enhance their fat oxidation rates through consistent endurance training. By incorporating fasted-state workouts, low-carb endurance sessions, and extended aerobic training, the body gradually improves its ability to rely on fat as a primary energy source. This metabolic adaptation increases efficiency, allowing athletes to perform longer without needing excessive carbohydrate intake.

While fat is not ideal for high-intensity performance, its role in endurance sports is crucial. Training the body to balance fat oxidation and glycogen sparing can enhance overall endurance, delay fatigue, and improve an athlete’s ability to sustain energy output over extended periods. The key lies in developing metabolic flexibility, enabling athletes to efficiently shift between fat and carbohydrate metabolism based on exercise intensity and duration.

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Optimizing Fuel Use for Different Types of Endurance Training

Endurance training varies in intensity and duration, making it crucial to understand which fuel source best supports different types of workouts. The body's ability to utilize carbohydrates and fat depends largely on exercise intensity, with higher-intensity efforts relying predominantly on carbohydrates, while low-intensity, long-duration sessions favor fat oxidation. By strategically training both systems, athletes can develop metabolic flexibility, allowing them to efficiently switch between these energy sources based on demand.

High-Intensity Workouts: The Role of Carbohydrates

High-intensity workouts, including tempo runs, interval training, and race efforts, require a fuel source that can be rapidly mobilized to meet energy demands. Carbohydrates, stored as muscle and liver glycogen, serve as the primary energy source when exercise intensity exceeds 70% VO2 max. At these intensities, fat oxidation is too slow to provide sufficient ATP production, making carbohydrate availability critical to sustaining performance.

To optimize fueling for high-intensity sessions, athletes should consume 30-90 grams of carbohydrates per hour during workouts that exceed 60 minutes. This intake helps prevent glycogen depletion, which can lead to fatigue and performance decline. Carbohydrate-rich foods such as energy gels, sports drinks, bananas, or easily digestible starches ensure quick absorption and maintain blood glucose levels during prolonged efforts.

Workouts that typically fall into this category include race-pace efforts, speed intervals, tempo runs, and hill repeats. These sessions push the body to operate at near-threshold intensities, demanding efficient glycogen utilization for sustained power output. Without adequate carbohydrate intake, athletes risk premature fatigue, muscle breakdown, and impaired neuromuscular coordination, reducing overall performance.

Low-Intensity, Long-Duration Training: Tapping Into Fat Stores

Conversely, low-intensity, long-duration training allows the body to rely more on fat oxidation, preserving glycogen stores for later use. Efforts conducted below 65% VO2 max, such as Zone 2 endurance runs, long slow distance (LSD) sessions, and recovery workouts, encourage the body to metabolize fat for energy.

To enhance fat adaptation, athletes can incorporate fasted training sessions or workouts with lower carbohydrate availability. This approach forces the body to become more efficient at oxidizing fat, delaying glycogen depletion and improving overall endurance. While these adaptations are beneficial for ultra-endurance events, they should be balanced with periods of higher carbohydrate availability to support high-intensity training needs.

Hybrid Approach: Developing Metabolic Flexibility

Elite endurance athletes exhibit metabolic flexibility, meaning they can seamlessly switch between carbohydrate and fat metabolism depending on exercise intensity. This adaptability is developed through structured training strategies that balance low-carb, fat-oxidation-focused sessions with high-carb, glycogen-dependent workouts.

A “Train Low, Race High” strategy is commonly used to build metabolic efficiency. This involves conducting some training sessions with reduced carbohydrate intake to stimulate fat oxidation while ensuring that pre-race and high-intensity training blocks are supported by ample carbohydrate availability. Additionally, periodizing carbohydrate intake based on training demands—such as increasing carbs before a key interval session while maintaining lower carbohydrate intake for recovery runs—can maximize endurance adaptations.

By integrating both low-carb and high-carb fueling approaches, athletes can train their bodies to effectively utilize both energy systems, leading to greater endurance capacity, improved fuel efficiency, and enhanced long-distance performance. The key lies in balancing carbohydrate timing, training specificity, and individual fueling needs to optimize overall performance outcomes.

Conclusion

Understanding the balance between carbohydrate and fat metabolism is crucial for endurance athletes looking to maximize performance. Carbohydrates remain the dominant fuel source for high-intensity training, offering rapid energy availability and efficient ATP production. However, fat oxidation plays an equally vital role, particularly in long-duration, lower-intensity efforts, where it helps conserve glycogen and sustain energy output over extended periods.

By incorporating both fuel sources strategically, athletes can train their metabolic flexibility, allowing them to adapt to various intensities and durations. Carbohydrate timing, fat adaptation strategies, and individualized fueling plans should all be tailored to specific training goals. Endurance athletes can benefit from a hybrid approach—leveraging high-carb fueling for speed and race-day intensity while enhancing fat oxidation for endurance efficiency.

Ultimately, a well-structured fueling strategy helps athletes avoid energy crashes, premature fatigue, and poor recovery, ensuring that every workout and competition is optimized for peak endurance performance. By mastering the balance between carbohydrates and fat, athletes can go further, train smarter, and race stronger.