.png)
Many of us have seen videos of an athlete crawling across the finish line. Like baby deer learning to walk, their legs wobble beneath them as they struggle to stay upright, but their bodies fail them. The determination to finish is, no doubt, impressive. Endurance sports reward those who can persevere. Proof of grit, leaving it all out there. But what is that footage actually showing? It is a person in a state of exercise-induced hypoglycemia, with blood glucose low enough to impair muscle function and cognition, sometimes to the point of losing coordination or consciousness. Reaching that state doesn't reflect a higher level of effort or commitment. It is a fueling failure, and in most cases a preventable one
What is Bonking?
Bonking describes a sharp, often sudden decline in exercise capacity that occurs during prolonged endurance activity. In running, it’s called “Hitting the Wall”. The classic presentation includes fatigue, an inability to maintain power or pace, and cognitive symptoms such as difficulty concentrating, irritability, and confusion. The underlying cause is a depletion of the body's carbohydrate reserves, both in the muscles doing the work and in the liver, to the point where the body can no longer meet the fuel demands of continued exercise.
There are two main glycogen pools the body relies on during exercise: liver glycogen and muscle glycogen. Muscle glycogen is stored within the muscle fibers. Muscle cells lack the enzyme needed to release glucose back into the bloodstream; muscle glycogen is destined to be used in the muscle it is stored in. Liver glycogen works differently. The liver breaks down its glycogen into glucose and releases it into circulation, where it becomes available to the brain and other tissues that need it, so liver glycogen helps maintain blood glucose levels.
As exercise continues, muscle takes up glucose from the bloodstream. Early on, the liver keeps pace through glycogenolysis, but liver glycogen is a comparatively small reserve, and as the exercise bout lengthens, especially past the one to two hour mark, that reserve becomes depleted. The balance between glucose uptake by the muscle and glucose release by the liver begins to tip in the wrong direction.
Glucagon rises and stimulates the liver to release more glucose. This also promotes lipolysis, the release of fatty acids from adipose tissue. These fatty acids help offset the growing energy shortfall. This compensatory shift toward fat oxidation proceeds at a substantially slower rate than carbohydrate oxidation, so it cannot fully substitute for the drop in available glucose at the intensities typical of racing.
As plasma glucose continues to fall despite these compensatory efforts, the muscles' fuel supply becomes constrained, and this decline is linked to fatigue. Some researchers hypothesize that the fatigue is mediated by the central nervous system mediates fatigue, but it is driven primarily by depletion of the fuel reserves the muscle depends on.
Symptoms may not all appear at once, but the physical symptoms of shakiness, dizziness, or nausea appear, and cognitive effects such as difficulty concentrating or irritability tend to emerge only once glucose has fallen severely.
Why an Athlete Might Bonk
Bonking is usually the downstream result of a fueling deficit somewhere along the way, and that deficit can begin before the event even starts or during it.
Insufficient carbohydrate intake or recovery leading into the event.
An athlete who trains hard in the days before an event without adequately restoring carbohydrate intake enters race day with already compromised glycogen stores. This may be worsened if the athlete fails to eat enough to match their expenditure. A few days of insufficient fueling relative to training load can measurably reduce muscle glycogen content and impair carbohydrate oxidation during exercise (Kojima et al., 2022). Chronic under-fueling in the training block leading into a race, sometimes described as relative energy deficiency in sport, leaves an athlete starting the event in a fuel deficit, shortening the runway before hypoglycemia sets in.
Inadequate carbohydrate intake during the event itself.
Even an athlete who arrives at the start line well glycogen loaded can bonk if race fueling does not keep pace with the rate at which carbohydrate is being used. This is the most common and most correctable cause of bonking, and it typically stems from athletes underestimating how much carbohydrate prolonged exercise actually requires. It could also be caused by gastrointestinal issues that lead athletes to under-fuel, or simply from a fueling plan that was never tested in training and falls apart under race conditions.
Other factors may contribute as well. Underestimating the duration or intensity of an event, or hot conditions that increase carbohydrate utilization and fluid losses, can make bonking more likely.
Why Carbohydrates Matter So Much for Endurance Athletes
Carbohydrate holds a unique position among fuel substrates because it is the only one that can be oxidized quickly enough to meet the energy demands of moderate to high intensity endurance exercise, and because, unlike fat, the body's carbohydrate stores are genuinely limited. A comprehensive review on carbohydrate research for endurance athletes notes that high carbohydrate availability remains a priority for both prolonged intense exercise and competition performance, and that dietary carbohydrate intake guidelines exist specifically because glycogen availability so directly affects how long high intensity effort can be sustained (Podlogar & Wallis, 2022). Beyond its role as fuel, carbohydrate presence in the mouth and gut appears to send signals that influence perceived effort and central nervous system function, which is part of why even modest carbohydrate intake during shorter, harder efforts can improve performance before any meaningful amount has been absorbed and metabolized.
How to Prevent a Bonk
High carbohydrate availability remains a priority for both prolonged intense exercise and competition performance. Prevention operates on two timelines: the days before the event and during the race itself.
Before the event.
Carbohydrate loading, also known as glycogen supercompensation, is an established strategy for maximizing glycogen stores ahead of events. Carb loading calls for daily carbohydrate intakes in the range of seven to twelve grams per kilogram of body mass in the two days before competition, paired with a reduction in training volume. It was once thought that glycogen depletion was necessary before loading could occur. However, this has been shown to be false. Athletes do not need an initial depletion phase to achieve glycogen supercompensation; increasing carbohydrate intake while tapering training is enough.
During the event.
Carbohydrate intake during the event is one of the greatest tools athletes can use to prevent a bonk. The amount of carbs an athlete needs depends on the relative and absolute intensity, and duration of the event. Fuelin calculates the numbers for each athlete based on their individual needs. For those aiming for intakes over 90g carb per hours, using multiple transportable sugars such as glucose and fructose together allow for higher absorption rates. Because the gut can be trained to tolerate carbohydrate more efficiently, athletes who plan to fuel at the higher end of these ranges benefit from practicing that intake repeatedly in training so the fueling plan is familiar.
Heat: A Related but Distinct Risk
Heat and bonking often show up in the same races. An athlete slows dramatically, loses coordination, feels nauseated, and sometimes struggles to think clearly. This overlap in symptoms makes the two easy to confuse, but heat illness and bonking are separate concerns, even if they are closely connected.
Exercising in hot conditions substantially increases the rate at which the body draws on its glycogen stores. Muscle glycogen use can be significantly greater during prolonged exercise in hot conditions compared to temperate weather. Heat speeds up the clock toward glycogen and glucose depletion; if fuel is not consumed, that produces bonking, so athletes racing in hot conditions can run low on carbohydrate earlier than the same event would on a cooler day.
Despite their correlation, heat illness is not simply a cause of bonking. The elevated core temperature and tissue damage related to heat injury are symptoms of bonking. An athlete can bonk in cool conditions purely from inadequate carbohydrate intake, and an athlete can develop heat illness with glycogen stores reasonably intact if the problem is inadequate heat dissipation rather than fueling. Because the two conditions can present with overlapping symptoms: fatigue, dizziness, and impaired coordination, a sudden, severe decline in hot conditions should be treated as a possible heat-related event requiring cooling and medical treatment, not assumed to be a fueling problem that carbohydrate will fix.
Other Considerations
Fueling needs are highly individual; body size, training status, gut tolerance, and event intensity should be considered. Bonking is preventable, but only when fueling stops being a guess and starts being a plan. Fuelin calculates individualized carbohydrate targets for each athlete based on their real training data, then adjusts those recommendations as their tolerance and training load changes, so race-day fueling reflects what the athlete's body needs rather than a one-size-fits-all number. Bonking is a fueling deficit, not a display of grit. Fuelin exists to make sure those moments- the wobble, the shutdown, the fight to stay upright never have to happen, and athletes can race with confidence that their bodies will stay strong through the finish.