【Sports Medicine】Analyzing Mountain Biking (MTB) and Carbohydrate Loading Strategies: A 72-Hour Pre-Race Meal Plan at 10g of Carbs per Kilogram of Body Weight and the Latest Practical Strategies

Chapter 1: Introduction: The Dominant Metabolic Role of Carbohydrates in High-Intensity Endurance Mountain Biking (MTB)

Cross-country mountain bike racing (XC) and ultra-long mountain marathon events (XCM) are among the most demanding endurance disciplines in terms of the body’s energy reserves. Unlike road cycling, where riders can draft in the peloton for extended periods to conserve energy, mountain bikers must, from the moment the starting gun fires, independently tackle rugged singletrack, slippery tree roots, steep rock gardens, and frequent short, punchy climbs.

From a metabolic standpoint, this repeated-sprint, highly fluctuating power-output pattern relies heavily on the body’s supply of carbohydrates. Although fat is an important fuel at low-intensity riding, when a rider climbs steep slopes and power exceeds the anaerobic threshold (upper Zone 4 through Zone 6), the anaerobic glycolytic system in the muscle cytoplasm and the aerobic glycolytic system in the mitochondria become the absolute dominant energy suppliers. At this point, for every liter of oxygen consumed, the amount of ATP generated from carbohydrates is significantly higher than from fat:

$$\text{ATP yield per unit } O_2 \text{ (Carbohydrate)} > \text{ATP yield per unit } O_2 \text{ (Fat)}$$

This means that during high-intensity exercise, carbohydrates are the most efficient “high-octane fuel.” However, the body’s carbohydrate stores (primarily in the form of muscle and liver glycogen) are extremely limited, typically supporting only 90 to 120 minutes of high-intensity exercise without special loading. Once muscle glycogen is depleted, riders face severe physical collapse, manifesting as weak thigh muscles, an inability to maintain cadence, and central nervous system fatigue (commonly known as “bonking” or “hitting the wall”). Therefore, how to elevate glycogen stores to their physical limit before race day is the key physiological determinant of a mountain biker’s race success.

Chapter 2: Physiological Mechanisms: Storage Limits of Skeletal Muscle Glycogen and the Principle of Supercompensation

The body’s primary storage depots for carbohydrates are skeletal muscle and the liver. Generally, under a normal balanced diet, muscle glycogen concentration is approximately 100–120 mmol per kilogram of wet muscle weight. The physiological goal of “carbohydrate loading” is to use specific dietary strategies to push muscle glycogen concentration up to 150–200 mmol per kilogram, or even higher. This phenomenon is physiologically termed “glycogen supercompensation.”

Molecular Physiological Mechanisms of Glycogen Supercompensation

When skeletal muscle undergoes high-intensity training, glycogen stores are heavily depleted. This triggers a cascade of intracellular signaling:

1. Surge in Glycogen Synthase Activity: Glycogen depletion itself is the most potent activator of this enzyme. At this point, muscle cells are highly sensitive to glucose.
2. GLUT4 Transporter Translocation: The number of glucose transporter proteins (GLUT4) on the skeletal muscle cell membrane increases, accelerating the transmembrane transport of blood glucose into muscle cells.
3. Enhanced Insulin Sensitivity: Cells can efficiently take up glucose even at lower insulin levels.

If a sufficient high proportion of carbohydrates is consumed at this time (or in the days before the race), glycogen synthase synthesizes glucose into glycogen chains at a rate exceeding normal, thereby breaking through the original storage ceiling and achieving “supercompensation.”

Traditional loading strategies (such as the classic Astrand protocol from the 1960s) required athletes to first undergo a 3-day “depletion phase” (consuming no carbohydrates while performing high-intensity training), followed by a 3-day “loading phase.” However, this approach causes extreme physical and mental fatigue, immune suppression, and even increased injury risk during the depletion phase.
Modern sports physiology research has confirmed that riders do not need to endure a painful depletion phase. By significantly reducing training volume (entering a tapering phase) over the 3 to 4 days (72–96 hours) before a race while simultaneously consuming a very high proportion of carbohydrates, athletes can achieve the same or even better glycogen supercompensation without metabolic disruption.

Chapter 3: Why “10g of Carbs per Kilogram of Body Weight”? Scientific Research and Quantitative Basis of Metabolic Rates

In the consensus of modern sports nutrition, to achieve maximal glycogen supercompensation, the carbohydrate target intake for the 72 hours before a race is set at 10 grams per kilogram of body weight per day (10g/kg/day).

For a 70 kg rider, this means consuming 700 grams of pure carbohydrates daily. This is an enormous figure. Why is such a high dose necessary?

1. Quantitative Research on Metabolic Saturation and Storage Rates

Clinical studies indicate that when carbohydrate intake reaches 10g per kilogram of body weight per day, the glycogen synthesis rate in skeletal muscle reaches a saturation point. Below this dose (e.g., 6–8g/kg/day), muscle glycogen does rise, but it cannot reach the maximal “supercompensated” saturated state; above 12g/kg/day, intake exceeds the gastrointestinal digestive and absorptive limits, causing excess carbohydrates to ferment in the gut, leading to severe diarrhea or conversion to fat storage.

2. Energy Ratio and Caloric Deficit Compensation

During the pre-race tapering period, the rider’s exercise volume drops significantly, with total daily energy expenditure around 2,200–2,800 kcal. At this point, 700 grams of carbohydrates provides 2,800 kcal (1 gram of carbohydrate = 4 kcal). This means that carbohydrates will account for over 85% of total caloric intake. This configuration matches the molecular environment required for glycogen supercompensation: high carbohydrate, low fat, moderate-to-low protein.

The table below shows a comparison of glycogen supercompensation efficiency under different dietary ratios:

From this, it is clear that 10g of carbohydrate per kilogram of body weight per day is not an arbitrary figure, but rather a scientifically calculated sweet spot based on the maximum absorption limits of the intestinal transport proteins (SGLT1 and GLUT5) and the maximum saturation rate of skeletal muscle glycogen synthase.

Chapter 4: Precise Meal Planning and Food Selection for the 72-Hour Pre-Race Carbohydrate Load (Avoiding Gastrointestinal Crisis)

To consume 700 grams of pure carbohydrates in a single day while avoiding severe digestive distress, the “selection” and “form” of food are critical. Riders must completely set aside the intuition of “everyday healthy eating” and instead choose low-fiber, low-fat, easily digestible refined carbohydrates.

Core Principles of Food Selection:

1. Extremely Low Dietary Fiber: Strictly avoid oats, brown rice, whole wheat bread, or large amounts of vegetables. High fiber creates a strong feeling of fullness in the stomach, preventing you from consuming enough carbohydrates and potentially triggering gastrointestinal cramping on race day.
2. Extremely Low Fat: Fat slows the gastric emptying rate, reducing the absorption rate of glucose in the small intestine.
3. Make Good Use of “Liquid Carbohydrates”: When you cannot eat solid food, you must rely on sports drinks, fruit juice, or carbohydrate energy drink powders (Maltodextrin) to replenish sugar.

Sample 700g Carbohydrate Loading Menu for a 70kg Rider (Single Day):

• Breakfast (approx. 180g carbs):
  • 4 slices of refined white toast (with strawberry jam, no butter) —— 90g carbs
  • 1 large banana —— 30g carbs
  • 400ml apple juice —— 50g carbs
  • 1 tablespoon of honey —— 10g carbs
• Morning Snack (approx. 80g carbs):
  • Maltodextrin energy drink powder (mixed at high concentration) —— 80g carbs
• Lunch (approx. 180g carbs):
  • 2 large bowls of white rice (approx. 350g cooked rice) —— 110g carbs
  • Teriyaki chicken tenderloin (skinless, cooked with minimal oil, 120g) —— 0g carbs
  • 3 slices of canned peaches in syrup —— 30g carbs
  • 500ml sports drink —— 40g carbs
• Afternoon Snack (approx. 80g carbs):
  • 1 plain refined bagel (with honey) —— 55g carbs
  • 1 pack of gummy candies or gumdrops (approx. 30g) —— 25g carbs
• Dinner (approx. 180g carbs):
  • White pasta (with only tomato marinara sauce, oil-free, 2 bowls) —— 120g carbs
  • 1 medium baked sweet potato (peeled) —— 30g carbs
  • 400ml coconut water —— 30g carbs

Chapter 5: Practical Barriers and Common Misconceptions: Strategies for Managing Fiber Control, Fat Restriction, and Water Retention

When executing a high-carbohydrate loading protocol of 10g/kg/day, riders typically encounter three major physiological and psychological barriers.

1. Water Retention and “Water Weight”

After successful glycogen supercompensation, riders will notice their body weight increase by 1.5 to 2.0 kg within 3 days. This often triggers anxiety among climbers, who worry about its impact on their power-to-weight ratio (W/kg).

• Physiological Mechanism: For every 1 gram of glycogen stored in skeletal muscle, the body must simultaneously bind approximately 3 grams of water.
• Coping Strategy: Riders must understand that this extra 1.5 - 2.0 kg is not fat, but a “water reservoir.” During long, hot mountain races, this water is released into the bloodstream as glycogen is broken down, converting into valuable bodily fluids that effectively delay the onset of dehydration. For MTB cross-country stages where frequent hydration is difficult, this is a significant physiological advantage. Do not limit your loading because of this.

2. Neglecting Fat Restriction

Many riders, while increasing carbohydrates, inadvertently consume large amounts of fat (e.g., eating pasta with a creamy cheese sauce, or spreading peanut butter on white toast).

• Consequences: Fat significantly slows gastric emptying, hinders the rapid transport of glucose via GLUT4, and can lead to adipose tissue accumulation, causing a genuine increase in body weight.
• Coping Strategy: During the loading period, fat intake per meal should be kept below 5-10 grams. Prioritize steaming, boiling, and baking for cooking methods.

3. Race-Day Gastrointestinal Crisis Caused by Poor Fiber Control

Consuming too many high-fiber foods during the loading period (such as whole grains, sweet potato skins, broccoli) can leave food residue trapped in the colon. On high-impact, violently bumpy MTB sections, the intense jarring of the gastrointestinal tract can lead to abdominal pain, bloating, frequent bowel urgency, and even ischemic gastroenteritis.

• Coping Strategy: Strictly follow a “Low-Residue Diet” for the 48 hours before the race. All starchy foods should be peeled and refined, and vegetable intake should be minimized.

Chapter 6: Dynamic Transition of Nutrition on Race Morning and During the Event: How to Maximize the Loading Effect

The 72-hour carbohydrate loading before the race fills muscle glycogen stores to the brim, but a night of sleep before the race will deplete most of the liver glycogen. Liver glycogen is responsible for maintaining stable blood sugar levels and supplying energy to the brain. Therefore, nutrition on race morning and the dynamic transition during the event are equally crucial.

1. The Golden Meal 3 Hours Before the Race

• Purpose: To replenish liver glycogen stores without triggering reactive hypoglycemia during the race.
• Meal Plan: Consume approximately 2 grams of easily digestible carbohydrates per kilogram of body weight (e.g., white rice with pork floss, peeled banana with honey).
• Caution: Avoid consuming high-GI sweets alone within 60 minutes before the start, to prevent excessive insulin secretion which could cause a temporary drop in blood sugar after the race begins.

2. Dynamic Transition of In-Race Nutrition

Even with perfectly supercompensated muscle glycogen, continuous supplementation of exogenous carbohydrates is still essential in MTB races lasting over 2 hours. This helps slow the depletion rate of endogenous muscle glycogen.

• Target Intake Rate: 60 - 90 grams of carbohydrates per hour.
• Scientific Ratio (Glucose : Fructose = 2 : 1 or 1 : 0.8): The saturation limit of the glucose transport channel (SGLT1) in the human small intestine is approximately 60 grams per hour. To reach 90 grams per hour, you must also utilize fructose, which is absorbed via the GLUT5 channel. This significantly reduces the rate of gastrointestinal discomfort.
• Addressing MTB Feeding Challenges: Mountain courses are rugged, making it difficult for riders to grab food. It is recommended to mix high-concentration maltodextrin and fructose into a “liquid energy water,” placed in a hydration pack. Use the drinking tube to take small sips on smooth sections or descents, ensuring a steady and uninterrupted energy supply.

In summary, the carbohydrate loading strategy is not about blindly eating more, but rather a rigorous physiological engineering process. Through a meticulously planned menu of 10 grams of carbohydrates per kilogram of body weight per day for the 72 hours before the race, combined with the low-fat, low-fiber principles of a low-residue diet, mountain bikers can arrive at the starting line with maximally saturated muscle “glycogen stores.” This scientific strategy will enable riders to maintain powerful cadence output and clear decision-making on the relentless climbs in the latter half of the race, allowing them to stand out in highly competitive mountain cross-country events.

Related Reading

• 【Nutrition Recovery】The Ultimate Guide to Carbohydrate Loading for Endurance Athletes: A Comprehensive Practical Guide to Meal Planning and Recovery Scheduling with 10g of Carbs per kg of Body Weight in the 72 Hours Before a Race
• Carb-Loading Three Days Before a Race: Complete Plan and Practical Example
• The Carbohydrate Loading Method: What to Eat Before a Race
• Pre-Marathon Carbohydrate Supercompensation (Carbo-loading): A Practical 3-Day High-Carbohydrate Meal Plan