Enduro mountain biking races are exhilarating, high-intensity events that challenge riders with a mix of sustained aerobic endurance and bursts of explosive power. You navigate technical terrain, steep descents, and long, demanding climbs, often over several hours. In such an environment, proper nutrition is not just a performance enhancer; it’s a necessity for optimal physical function, endurance, and recovery.

Nutrition directly influences how your body responds to training, racing, and recovery, playing a critical role in maintaining energy, preventing fatigue, improving focus, and aiding muscle repair. For enduro riders, who face a combination of aerobic efforts (like climbing) and anaerobic sprints (such as navigating technical downhill sections), understanding how to fuel and hydrate appropriately can make the difference between finishing strong or hitting the wall or a tree.

In this first article, we explore how intensity affects the type of fuel your body relies on during a race, using insights from events like the Pokhara Enduro as a guide.

Brief Outline of the 3 Main Fuel Sources the Body Uses During Exercise

Glycogen (Carbohydrates)
What It Is: Glycogen is the stored form of glucose (sugar) in the muscles and liver. It is the body’s primary and most readily available source of energy for high-intensity exercise and endurance activities.
How It’s Used: During exercise, glycogen is broken down into glucose and converted into ATP (adenosine triphosphate) to fuel muscle contractions.
Storage Capacity:
Muscles: ~300-500 grams (about 1200-2000 kcal, depending on body size and training level).
Liver: ~80-100 grams (about 300-400 kcal).
Total: 400-600 grams of glycogen, which provides roughly 1600-2400 kcal.
Key Points: Glycogen stores deplete quickly during prolonged or intense exercise (within 90-120 minutes of high-intensity activity), and must be replenished via carbohydrate intake.

Fats (Fatty Acids)
What It Is: Fats are a concentrated source of energy stored in adipose tissue (fat cells) and muscle cells. They are used predominantly during lower-intensity, longer-duration exercise.
How It’s Used: Fats are broken down into fatty acids, which are converted into ATP via aerobic metabolism. Fats provide a slower, but more sustained source of energy.
Storage Capacity:
Adipose Tissue: The average person has tens of thousands of grams of fat stored in the body, potentially providing tens of thousands of kcal (e.g., ~30,000-100,000 kcal in well-trained individuals).
Muscle Tissue: Small amounts of fat are stored directly in muscle cells for use during prolonged exercise.
Key Points: Fats are the primary fuel source for endurance activities, especially when glycogen stores are depleted or during low- to moderate-intensity exercise.
 
ATP (Adenosine Triphosphate)
What It Is: ATP is the immediate source of energy for all cellular functions, including muscle contractions. It is not stored in large quantities but is constantly regenerated.
How It’s Used: ATP is broken down into ADP (adenosine diphosphate) to release energy during muscle activity. ATP is produced via three main systems:
Phosphagen system (for short bursts of power, e.g., sprinting or lifting): Quickly replenishes ATP through stored creatine phosphate in muscles.
Glycolysis (for moderate efforts, e.g., 1-2 minutes of high-intensity work): Breaks down glucose or glycogen to form ATP.
Oxidative phosphorylation (for endurance, e.g., long-duration efforts): Uses oxygen to generate ATP from fats and carbs.
Storage Capacity:
ATP itself is stored in very small amounts in the muscles, enough to sustain a few seconds of maximal activity.
For sustained activity, the body constantly regenerates ATP from glycogen, fat, and phosphocreatine stores.
Key Points: ATP is always being regenerated, but the initial burst of energy (e.g., in a sprint) comes from stored ATP and phosphocreatine, which depletes quickly. For longer activity, glycogen and fat provide the fuel to regenerate ATP.

Summary:
Glycogen: Quick, primary fuel for moderate to high-intensity activity. Stored in muscles (300-500g) and liver (80-100g).
Fats: Slow, sustained energy for low to moderate intensity; stored in adipose tissue (~30,000-100,000 kcal).
ATP: Immediate energy used for muscle contractions; stored in small amounts, but constantly replenished from glycogen and fat stores.

The balance between these fuel sources depends on the intensity and duration of exercise.

Optimal Intensity for Glycogen and Fat Utilisation
Lower Intensity (55–65% of Max HR):
At around 55–65% of max HR, fat utilisation is maximised because the body relies more on aerobic metabolism, using lipids as a primary fuel source.
Glycogen is used sparingly in this range but still plays a role, particularly as exercise time extends.
Moderate Intensity (65–75% of Max HR):
Moving up to 65–75% of max HR, both glycogen and fat are utilised together, with glycogen use starting to increase.
This zone is common in endurance rides where both fat and carbohydrate (glycogen) sources are needed for sustained energy.
High Intensity (75–85% of Max HR):
In the 75–85% of max HR range, glycogen becomes the primary fuel source as the body relies more on anaerobic pathways, especially in the higher end of this range.
This zone is ideal for glycogen usage but still uses some fats, particularly if the intensity fluctuates or during longer efforts.
Threshold and Beyond (85%+ of Max HR):
Above 85% of max HR, glycogen is the dominant fuel due to high demand for quick energy.
Fat utilisation is minimal at these intensities, as anaerobic metabolism, which does not efficiently use fat, becomes predominant.

Summary
60–75% of max HR: Mix of glycogen and fats, leaning more on fat as the duration extends.
75–85% of max HR: Higher glycogen use with moderate fat contribution, suitable for glycogen targeting without excessive anaerobic work.
85%+ of max HR: Primarily glycogen, with little fat used.
For a balanced workout that targets both glycogen and fats, staying in the 65–75% of max HR range is effective for moderate efforts, while the 75–85% range is ideal for tapping into stored glycogen, especially in higher-demand rides.