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What Is “Bonking” (No, Not That), And How Can You Avoid It?

The bonk: once felt, never forgotten.

Laura Simmons - Editor and Staff Writer

Laura Simmons

Laura Simmons - Editor and Staff Writer

Laura Simmons

Editor and Staff Writer

Laura is an editor and staff writer at IFLScience. She obtained her Master's in Experimental Neuroscience from Imperial College London.

Editor and Staff Writer

cyclist resting at the side of the road

To make it through a long ride without bonking, preparation is key.

Image credit: LovetheLifeyouLive/

If you’re not an avid cyclist, there’s a chance you won’t have come across the term “bonking” before. Actually, you probably have, but not in this context. Bonking is the word cyclists and endurance athletes give to that feeling of hitting a wall, the moment during your workout when all your reserves have run dry and you’ve got nothing left to give. But what causes it, and more importantly, how can you prevent it?

What is bonking?

Bonking is a frivolous-sounding word for a phenomenon that, at best, can ruin an event that you’ve trained hard for, and at worst, can be dangerous. 


At its most basic level, a bonk generally happens when the body’s glycogen – the stored form of glucose in the muscles and liver – is totally depleted. It’s a problem that will be familiar to all sportspeople. Glucose from the bloodstream (blood sugar) is a vital source of energy for the brain and other bodily systems. As it gets used up, the pancreas produces glucagon to convert stored glycogen into more glucose; but, there’s a finite amount, so at some point you need to start replacing the glycogen reserves by eating carbohydrate-rich foods.

Speaking to Total Women’s Cycling, nutrition scientist Dr Stacy Sims pointed out that there is also a slightly different type of bonking that can occur: “One type of bonking is from a drop in blood volume (loss of body water) and is perceived as flat, dead legs, a drop in power with an elevated heart rate. Most people think this is a need for calories, but in fact, they are behind the 8-ball in hydration.”

Over the years, several other factors that could increase the risk of bonking have been proposed. One of these theories suggests that mindset may have a part to play. “It's a very interesting phenomenon that we're only now coming to grips with – that mental fatigue will lead to the perception of muscular fatigue,” emeritus professor of nutrition Dan Benardot told Runner’s World in 2004. “The brain is juggling all of this information and can eventually make the decision: 'Whoa, things are not good here, I'm going to shut it down.”

Some of this science is still being debated. Doing what you can to keep yourself in a positive headspace is likely a good idea in general, but for most people, nutrition – specifically carbohydrates – is key. And once you’ve experienced trying to carry on when the glycogen is gone, you’ll certainly know about it.

What does a bonk feel like?

The symptoms of a bonk can vary between people, but many report a sudden and extreme feeling of weakness, dizziness, hunger, and sometimes feelings of disorientation. Writing for Cycling Weekly, Anita Bean described the feeling of simply being unable to continue a ride: “I had nothing left – the road ahead was starting to blur and I was barely holding a straight line.” 

If this happens in the middle of a race, you can pretty much kiss goodbye to that PB you’ve been striving for. If it happens while you’re traveling along a busy road, in unfamiliar surroundings, or far from home, you can imagine how the situation can easily become dangerous.

This unpleasant experience can stay with you, and for some, it can be enough to put them off training altogether for a while. Some of the symptoms can also linger for a few days - after all, your body has just been put through an intensely stressful situation.

What should you do if you bonk?

The short answer to this is eat something. You’ll most likely have to stop and take a break, and then most experts would advise that you start by consuming something sugary – glucose gels, sweets, even a sugar cube if you have one to hand (perhaps your route goes by a field of horses?).


If the issue is dehydration, Dr Sims suggests drinking water with a tiny dash of salt (less than half a gram in 600 milliliters), but explains that this type of bonking is particularly hard to recover from: “This takes hours to come back from – no, guzzling water doesn’t help (water will just sit in the stomach), and typical sports drinks can exacerbate the dehydration.”

A rest and some sugary goodness should hopefully at least get you to a point where you can make it back home, but experts agree that the best way to beat the bonk is through prevention, not cure.

How to prevent bonking

Refueling as you go is the way to prevent bonking, but how you do this will look very different for different people.


Anyone who has ever trained for a long ride or run will know the importance of figuring out nutrition on the fly. It’s good to plan in advance, particularly if you’re taking a route with refueling stops along the way, and to carry water and the types of carbohydrate-rich snacks that you know work best for you. 

Learning to recognize the signs of bonking before it really hits can also help you to head it off at the pass. You may be surprised at how often you need to eat to keep those reserves up.

Preparation is also key. Ensuring that you begin your workout with a good store of glycogen will set you up for success, which is why many athletes talk about carb-loading.

“Starting a race with full muscle glycogen stores by eating sufficient carbohydrate has been shown to help endurance performance,” nutritional therapist Jo Scott-Dalgleish told the Guardian, recommending a modest increase in carbohydrate intake in the 24 hours before a big race or event. “Good sources are pasta, rice, or porridge oats.”


Sometimes, when a race or workout doesn’t go to plan, there’s no obvious explanation. But, if you take steps to prevent it, you can hopefully avoid the dreaded bonk, or at least be able to recognize it when it comes.

[H/T: Cycling Weekly]

All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current. 

The content of this article is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of qualified health providers with questions you may have regarding medical conditions.  


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