Bonking is a dreaded experience and occurs when the body becomes functionally depleted of glycogen. This exhaustion of fuel shuts down the body’s ability to exert itself. The term gets thrown around a lot, but if you’ve ever felt it, you know how bad it can be and how hard it is to recover from. So just what is a bonk, why is it a bad thing, and how can you avoid it?

For more information on bonking, check out Ask a Cycling Coach Ep. 261.

Key Takeaways:

  • Bonking is caused by a functional depletion of Glycogen
  • Bonking can cause muscle loss, weaken the immune system, lead to disruptions in training, and put you in physical danger
  • Glycogen depletion can be a beneficial training technique, but should be used carefully and in moderation
  • Use training rides to fine-tune your nutritional strategies
  • Develop a fueling plan before you get on the bike, and use reminders to follow this plan during your workout or event.

What is Bonking?

Many cyclists use the term bonking to lightly describe a shortage of energy or a late-race moment when the legs cramp, and the engine seems to fail. While these are all symptoms of bonking, and it’s not a scientific term, it does have a specific meaning. Excessive fatigue resulting from going too hard during a ride is usually a symptom of outpacing your ability. On the other hand, Bonking tends to occur only on longer rides when you fully exhaust your energy supplies. Once you experience it, you never forget it, and you never want it to happen again.

What Does Bonking Feel Like?

A true bonk is not just a flat feeling or tired legs. It’s a total inability to continue, marked by nausea, extreme physical weakness, poor coordination, and a profoundly awful feeling. Essentially, bonking is exercise-induced hypoglycemia or low blood sugar. Typically, bonking is preceded by waves of progressively worsening symptoms.

At first, you may feel hungry or exhibit negative self-talk that eventually leads to an increase in your rate of perceived exertion. As a bonk gets worse, you may develop a headache, nausea, shaking hands, loss of focus, or dizziness. If the bonking continues, so does the likelihood of confusion and the inability to function physically.

It’s essential to recognize what bonking feels like, not only for your health and safety, because the sooner you realize the problem, the quicker you apply the solution.

Bonking Causes

Bonking occurs when the muscles become functionally depleted of glycogen, the carbohydrate energy stores by which the body fuels itself. Even in the worst bonk, the muscles are not completely empty of glycogen, with somewhere between 10% and 30% of the original supply remaining. However, the shortage of available energy renders the muscles unable to continue working effectively. 

Exactly why this occurs is the subject of some debate and research. A traditional theory, known as the “catastrophe model,” says that bonking and exhaustion occur as muscle function reaches its physiological limit and begins to fail. Newer research suggests the brain acts as a “central governor” to limit the muscles preemptively, shutting their function down as energy supplies run low to prevent damage. Whatever the cause, the effect is profound.

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Why Bonking is Bad

Bonking is particularly vicious. It feels terrible and affects your general health and wellbeing. Not only can bonking ruin a ride, workout, or race, but it can derail your training and future performance.

Muscle Loss

During exercise, the body is in a catabolic state of stress. An increase in cortisol causes blood glucose levels to rise, making more of the body’s carbohydrate fuel available to enable hard work. An absence of carbohydrates shifts the metabolism to other fuel sources, increasingly burning fat and eventually protein. This is a state in which the body is essentially eating itself, consuming its own muscle as an energy supply. Since we need our muscles to turn pedals, it should be obvious why this is less than ideal.

Immune Suppression

The emerging science of immuno-metabolism suggests a connection between carbohydrates and the immune system’s ability to fight infection. It’s well known that exercise has a beneficial long-term effect on immunity, but overtraining and bonking can cause the opposite. Just like muscles, immune cells contain glycogen and use it as a fuel source. Studies show that depleting this glycogen supply renders the immune cells less effective in doing their job- essentially, your immune cells can bonk!

This is one reason why recovery is so important in training, and why pushing the body to the point of failure in bonking can be dangerous. Suppressed immune function can last from 24-72 hours after the ride, depending on how deeply you run yourself down.

Training Disruption and Physical Danger

Training is cumulative, and each individual workout in a training plan is designed with the following workout in mind. Bonking disrupts this rhythm by creating a massive amount of unproductive fatigue and stress. In many cases, bonking goes hand-in-hand with dehydration, as riders who are not eating enough are often not drinking enough, either. Glycogen itself is water-dense, and running out of it reduces the amount of available hydration within the muscles. Dehydration is dangerous and compounds the fatigue and negative effects bonking leaves behind. 

The psychological aspect of bonking can be a powerful deterrent to training, too. As cyclists, we regularly push ourselves further than we think we can. The extremely uncomfortable experience of a bonk might be just the discouraging association your subconscious needs to impose extra limits. Worse, it often leads to situations that are just plain dangerous. Cognitive function declines in tandem with muscular ability, creating temporary mental and physical impairment. Judgement, balance, and vision can all be negatively affected. 

Depletion and Adaptation

The purpose of any training is to disrupt homeostasis and force the body to adapt. Riding in a glycogen-depleted state can stimulate more efficient storage and use of glycogen in the future. In addition, riding with low glycogen and glucose levels in the bloodstream causes more fat to be metabolized as fuel. However, both of these adaptations can be targeted through safer and more productive means than an outright bonk. Long/slow distance rides, careful post-ride carbohydrate restriction, and low-intensity rides without extra sugar can work towards this end without significant negative consequences. 

How to Avoid Bonking

The best way to avoid bonking is pretty simple—give your body enough food and drink to stay fueled during your workout! This sounds easy, but it’s often more challenging than you might think. Here are some helpful tips to keep yourself fueled.

Figure Out Your Consumption Rate

Each of us have our own unique carbohydrate and caloric intake requirement, and individual tolerance for what we can stomach during a workout or race. Don’t wait until race day to learn what works for you. Training rides are a great time to experiment with different fueling schedules, and to try out solid vs. liquid foods or a combination of the two. Also, keep hydration in mind and try different electrolyte drinks and mixes. Sweet Spot workouts are ideal for this experimentation; proper nutrition makes these workouts noticeably easier, and they are just hard enough to make a decent approximation of a race’s demands.

Develop a Plan Before Your Ride

Decide before your ride begins how much you’ll need to eat and drink, and set benchmarks for yourself to follow. This may mean knowing to eat a certain number of gels by a particular point in the ride, or you might even set yourself a timer to remember to eat something every half hour. If you’re riding with a friend or teammate, look out for each other and offer reminders to maintain good nutrition together. Whatever strategy you choose, plan ahead- once you’re on the bike, it’s too late to reliably make smart fueling choices.

Know the Signs and the Solution

If you do find yourself beginning to bonk, remember that food and drink are the solution. Nausea frequently makes eating difficult when bonking, despite the body’s desperate need for fuel. To counter this effect, ingest some liquids with your food. Once your body gets a restorative taste of carbohydrates, the brain usually responds favorably. Finally, don’t be afraid to call for a ride home. Bonking is a serious and counterproductive event, and your priority afterward should be a safe recovery.

For more cycling training knowledge, listen to Ask a Cycling Coach Podcast— the only podcast dedicated to making you a faster cyclist. New episodes are released weekly.


  1. Noakes, T., Peltonen, J., Rusko, H. (2001). Evidence that a central governor regulates exercise performance during acute hypoxia and hyperoxia
  2. Murray, B., Rosenbloom, C. ( 2018). Fundamentals of glycogen metabolism for coaches and athletes
  3. Neiman, D., Wentz, L. (2018) The compelling link between physical activity and the body’s defense system

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Sean Hurley

Sean Hurley is a bike racer, baker of sourdough bread, and former art professor. He is a connoisseur of cycling socks and a certified USAC level 3 coach. Rumor has it he also runs a famous cycling instagram account, but don't tell anyone about that.