Bonking is an incredibly unpleasant phenomenon brought on by the depletion of muscle glycogen during prolonged exercise. This is because the body needs carbohydrates to fuel the muscles. Without them, the body cannot continue to exercise at the same intensity.
So bonking happens during endurance events like marathons or cycling races when the body has no more glycogen to burn.
Glycogen is a carbohydrate stored in the liver and muscles that can be converted to glucose for energy. Glucose is the preferred source of energy for the brain and other parts of the body.
In reality, the process of bonking begins when the liver glycogen stores are depleted. The liver glycogen is broken down into glucose, circulating in the bloodstream, and provides energy for cells in the brain, heart, kidneys, and muscles. If there is not enough glucose in the bloodstream, it becomes difficult for cells to function correctly.
When glycogen stores are depleted, glucose can no longer be made for energy. Without sufficient power, brain function and physical performance decrease, leading to fatigue, dizziness, nausea, vomiting, and even hallucinations.
That nagging feeling of hitting the wall
The feeling of hitting the wall is often synonymous with bonking. The wall is a metaphor for hitting one’s limits, and it usually occurs when an athlete can no longer continue at their current intensity. It may also feel like extreme fatigue or sudden fatigue.
Athletes need to properly plan their nutrition before and during training so that they don’t run out of glycogen stores or suffer from fatigue.
Research commonly agrees that the body can store about 300-400 grams of glycogen in the muscles and about 120 grams in the liver. During your ride, this translates to about 1,200 calories of energy. If you don’t replenish this on the bike, you’ll deplete your reserves fairly quickly; at a moderate intensity, most cyclists will burn about 800 calories an hour.Science Daily
When possible, athletes should avoid intense training sessions two days in a row because this will cause them to run out of glycogen stores more quickly. A key factor in overtraining.
How glycogen stores deplete during exercise
Endurance athletes often experience bonking when they have depleted their glycogen stores and cannot replace it with food or drink. The body can only store a certain amount of glycogen in the liver, and once that’s gone, it needs to find a new source of energy.
Enter ketones as a secondary source of energy. Unfortunately, they’re not enough to keep up with the demands of a strenuous workout.
Avoid the dreaded bonk.
Athletes should also keep track of their blood glucose levels during exercise to know when they are running low and might benefit from an energy bar. This awareness allows them to take necessary steps like eating or drinking sports drinks before they bonk entirely.
Bonking causes serious physiological stress, which can put your immune system on high alert and result in inflammation.David C. Nieman et al
Athletes need to recognize the importance of preventative measures, like packing energy gel before a long ride, run, swim, or race. The early warning signs of bonking may happen too late to completely avoid or prevent it from happening.