Runners Bonk: Causes, Symptoms, and Prevention Strategies

Why do runners bonk?

Runners "bonk," or "hit the wall," primarily due to the severe depletion of glycogen stores in their muscles and liver, leading to an abrupt and profound loss of energy and performance during endurance activities.

What is "Bonking"?

In endurance sports, particularly running, "bonking" (also known as "hitting the wall" or experiencing "the wall") refers to a sudden, overwhelming feeling of fatigue, weakness, and loss of power that can bring a runner to a near standstill. It's more than just being tired; it's a profound physiological shutdown, often accompanied by mental fogginess, dizziness, nausea, and an inability to maintain pace or effort. This phenomenon is a stark reminder of the intricate relationship between physiology, nutrition, and athletic performance.

The Primary Culprit: Glycogen Depletion

The most significant factor contributing to bonking is the exhaustion of the body's stored carbohydrate reserves, known as glycogen.

• Carbohydrate as Fuel: During moderate to high-intensity exercise, carbohydrates are the body's preferred and most efficient fuel source. They are broken down into glucose, which is then used to produce adenosine triphosphate (ATP), the primary energy currency for muscle contraction.
• Glycogen Stores: The body stores carbohydrates primarily as glycogen in two locations:
  • Muscle Glycogen: Stored directly within the muscle fibers, this glycogen is readily available for local muscle contraction. It's the primary fuel for sustained running.
  • Liver Glycogen: Stored in the liver, this glycogen is crucial for maintaining stable blood glucose levels. When blood glucose drops, the liver releases glucose into the bloodstream to supply vital organs, especially the brain.
• The Depletion Process: An average runner can store enough glycogen to fuel approximately 90 to 120 minutes of continuous moderate-to-high intensity exercise. Once these stores are significantly depleted, the body must rely more heavily on fat for fuel. While fat stores are vast, converting fat into usable energy (ATP) is a slower and less efficient process, especially at higher intensities. This shift results in a dramatic reduction in the rate of energy production, making it impossible to sustain previous efforts.

Beyond Glycogen: Contributing Factors

While glycogen depletion is the central mechanism, several other physiological factors can exacerbate or directly contribute to the sensation of bonking.

• Dehydration and Electrolyte Imbalance:
  • Fluid Loss: Sweating leads to fluid loss, which decreases blood volume, strains the cardiovascular system, and impairs the body's ability to cool itself. Even a 2% loss of body weight due to dehydration can significantly impair performance.
  • Electrolyte Depletion: Sweat also contains essential electrolytes like sodium, potassium, and chloride. These minerals are vital for nerve impulse transmission, muscle contraction, and maintaining fluid balance. Significant depletion can lead to muscle cramps, weakness, and confusion.
• Hypoglycemia (Low Blood Sugar):
  • As liver glycogen stores dwindle, the liver's ability to release glucose into the bloodstream diminishes. This can lead to a drop in blood glucose levels (hypoglycemia). Since the brain relies almost exclusively on glucose for fuel, hypoglycemia can cause symptoms like dizziness, confusion, irritability, and an inability to concentrate, further contributing to the bonk.
• Central Fatigue:
  • Beyond peripheral muscle fatigue, the central nervous system (CNS) also plays a critical role. Prolonged exercise can alter neurotransmitter balance in the brain, such as an increase in serotonin, which is associated with feelings of fatigue and a reduced drive to continue. Ammonia accumulation, a byproduct of muscle metabolism, can also contribute to central fatigue.
• Heat Stress:
  • Running in hot and humid conditions significantly increases the body's core temperature. The body diverts blood flow to the skin for cooling, reducing blood flow to working muscles and the digestive system. This not only increases cardiovascular strain but also accelerates glycogen depletion and exacerbates dehydration, making a bonk more likely and severe.
• Inadequate Pacing:
  • Starting a race or long run too fast is a common mistake. An overly aggressive initial pace rapidly burns through precious glycogen stores, leading to premature depletion and an early bonk. Proper pacing ensures that energy reserves are managed efficiently throughout the duration of the effort.

Preventing the Bonk: Strategies for Runners

Understanding the causes of bonking provides a clear roadmap for prevention. Strategic fueling, hydration, and training are key.

• Fueling Strategies:
  • Carbohydrate Loading (for longer events): For events lasting longer than 90 minutes, increasing carbohydrate intake in the 24-48 hours prior to the event can maximize muscle and liver glycogen stores.
  • Pre-Run Meal: Consume a carbohydrate-rich, low-fiber, moderate-protein meal 2-3 hours before your run to top off glycogen stores.
  • During-Run Nutrition: For runs over 60-75 minutes, consume 30-60 grams of carbohydrates per hour (e.g., gels, sports drinks, chews). For ultra-endurance events, this can increase to 60-90+ grams per hour, depending on individual tolerance and training.
  • Post-Run Recovery: Replenish glycogen stores within 30-60 minutes post-run with carbohydrates and protein to aid recovery and prepare for the next session.
• Hydration Plan:
  • Pre-Hydration: Begin hydrating adequately in the days leading up to a long run or race.
  • During-Run Hydration: Drink fluids regularly, aiming for 400-800 ml per hour, depending on sweat rate and environmental conditions. Use sports drinks that contain electrolytes for runs longer than 60 minutes or in hot conditions.
  • Post-Run Rehydration: Continue to rehydrate after your run, consuming fluids and electrolytes to replace losses.
• Pacing Discipline:
  • Start Conservatively: Resist the urge to go out too fast. Adhere to a planned pace that is sustainable for the entire duration.
  • Negative Splits: Aim to run the second half of your race or run slightly faster than the first, a strategy that often prevents early energy crashes.
• Training Adaptations:
  • Long Runs: Incorporate progressively longer runs into your training to teach your body to become more efficient at utilizing fat for fuel (sparing glycogen) and to tolerate sustained efforts.
  • Metabolic Efficiency Training: Specific training strategies, such as fasted easy runs (under strict supervision and only for well-adapted athletes), can enhance the body's ability to burn fat.
• Electrolyte Management:
  • For longer efforts or heavy sweaters, consider supplementing with electrolyte tablets or consuming sports drinks with adequate sodium and potassium to prevent imbalances.

When to Seek Professional Advice

While bonking is common in endurance sports, persistent or severe symptoms, especially if accompanied by extreme dizziness, disorientation, or loss of consciousness, warrant medical attention. These could indicate more serious underlying conditions or severe exertional heat illness.

Conclusion

Bonking is a complex physiological event primarily driven by the depletion of glycogen stores. However, it's often compounded by dehydration, electrolyte imbalances, hypoglycemia, central fatigue, and poor pacing. By understanding these mechanisms and implementing strategic fueling, hydration, and training practices, runners can significantly reduce their risk of hitting the wall, allowing them to perform optimally and enjoy their runs to the fullest.