Bonking: What It Is, How It Feels, & Expert Tips To Avoid It

We will discuss the meaning of "bonking" in running and other endurance sports, explore what causes it, and provide some expert tips to prevent it.

If you are an experienced runner, you have most likely experienced a famous “bonk” during one of your races, whether a half marathon, marathon, or ultra. If you haven’t, you’re one of the lucky ones.

If you’re more of a beginner runner, you have probably at least heard of bonking or “hitting the wall” and fear that one day it may be you. 

If you have ever started out a race absolutely bursting with energy, exploding off the start line much faster than your intended pace, and then halfway to three-quarters through have had to slow down, walk, or even stop because you can’t go on, you’ve probably bonked.

A runner who has bonked, lying on the road.

What Is Bonking?

The term “bonk” originates from slang used by endurance cyclists, but now it is commonly used in many different forms of long-distance mild-intensity exercise.1Bonk. (2024, March 8). Rehook. https://rehook.bike/blogs/saddle-slang-the-dictionary-of-cycling-lingo/bonk#:~:text=The%20term%20%E2%80%9Cbonk%E2%80%9D%20is%20thought

When you consume carbohydrates, some of them are digested and stored as glycogen in your muscles and organs. Then, when your body needs energy, these glycogen stores are converted into glucose and enter your bloodstream, which raises your blood sugar and provides your body with the energy it needs.

Bonking, or hitting the wall during a race, is when your glycogen stores have been depleted. Once this happens, your body needs to rely on other sources of energy, such as those that come from fats and proteins, which are not as efficient as fuel sources.2Academy, U. S. S. (2016, April 29). Strategies for Working with First Time Marathon Runners. The Sport Journal. https://thesportjournal.org/article/strategies-for-working-with-first-time-marathon-runners/#:~:text=Hitting%20the%20Wall%20(HTW)&text=According%20to%20Stevinson%20and%20Biddle

When glycogen stores are depleted, this inhibits a runner’s capacity to continue to perform. This means that when bonking, you feel mentally and physically exhausted, as if you have run out of gas.

Hence, glycogen depletion and the resultant low blood sugar (known as hypoglycemia) can make it difficult or sometimes impossible to keep on running. 

What Are the Symptoms of Bonking?

Some of the most common symptoms of bonking include physical and mental fatigue, disorientation and loss of focus, muscle fatigue such as heavy legs, unwanted muscle contraction or cramping, dizziness, lightheadedness, and exercise intolerance to the point where you don’t or can’t continue.3Kanungo, S., Wells, K., Tribett, T., & El-Gharbawy, A. (2018). Glycogen Metabolism and Glycogen Storage Disorders. Annals of Translational Medicine6(24), 474–474. https://doi.org/10.21037/atm.2018.10.59

No endurance athlete wants to experience these feelings during a race, a run, or at any time for that matter.

Additionally, bonking can even weaken the immune system.4Hurley, S. (2022, March 28). What is Bonking? Causes, Dangers, and Prevention – TrainerRoad Blog. Www.trainerroad.com. https://www.trainerroad.com/blog/what-is-bonking-causes-dangers-and-prevention/#:~:text=Bonking%20can%20cause%20muscle%20loss

A runner bonking.

How To Avoid Bonking During a Race

As runners, to achieve a target time or complete a race, we want to do everything in our power to avoid bonking.

Here are some of the top tips I give my athletes to help them avoid bonking during a race: 

#1: Fuel Properly 

Your fueling strategy can make or break your race, as you need a constant stream of energy to sustain your trained paces and avoid bonking or hitting the wall.

This includes what you consume leading up to the race as well as your race-day nutrition.

Pre-Race Carb Load

Your nutrition during the two to three days leading up to the race should be focused on foods that you are used to eating, can easily tolerate, and that contain enough carbohydrates to ensure your glycogen stores are topped off to provide you with the energy you need to perform.

Replace some of your meals’ usual protein or fat portions with carbohydrate-rich foods such as pasta, rice, and starchy vegetables.

Be sure to eat a carb-rich dinner the night before your race, such as a bowl of pasta. Also, don’t forget to hydrate.

All of this will help to increase your muscle glycogen stores before the race so you have optimal energy levels by the time you start your long-distance event, improving your body’s ability to keep going.

A bowl of pasta.

Pre-Race Breakfast

The morning of your race, fuel two to three hours beforehand so you have time to process your food and not feel like you are starting the race with a full stomach. 

Eat the same carbohydrate-dense breakfast you are used to eating before your long runs, such as toast and honey or a bagel and jelly.

Bring an extra energy gel or small snack to eat about 15-20 minutes beforehand if you want to eat something before the start.

Mid-Race Fueling

Your nutrition and hydration during the race are essential to avoid bonking. Keeping hydrated and consuming enough electrolytes are also crucial to muscle function.

Ensure you have enough gels, gummies, and/or carbohydrate-rich sports drinks to get you through the race and consume them and the practiced intervals to ensure you are always fueled. 

A runner opening an energy gel

If you need to, set an alarm on your watch to the intervals you need to fuel so you don’t get caught up in the excitement of the race and forget. If this happens, it could be too late to catch up. 

This constant stream of fuel will help keep your energy levels consistent throughout your race and help you avoid bonking. Without this, your body’s glycogen stores would continue to decline until you bonk.

As your body does not instantly convert carbohydrate intake into energy, don’t wait until your energy stores are already depleted before you refuel. Rather, maintain carb intake and keep your blood glucose levels high enough so you don’t end up eating before it’s already too late.5Murray, B., & Rosenbloom, C. (2018). Fundamentals of glycogen metabolism for coaches and athletes. Nutrition Reviews76(4), 243–259. https://doi.org/10.1093/nutrit/nuy001

Practice your fueling strategy during all of your long runs so you are like a well-oiled machine on race day. Your body should be used to receiving and utilizing fuel and avoid any unexpected tummy trouble.

To calculate an estimate of how many grams of carbs to take in per hour during a race, use the following equation:

Weight in kilos = grams of carbohydrates per hour. 

For example, if you weigh 154 pounds or 70 kilos, you would need about 70 grams of carbohydrates per hour to fuel correctly. Some people work with a bit more or less, but I have found this calculation to work very well among my athletes.

Runners in a marathon.

#2: Pacing: Run Negative Splits 

Running negative splits is the ideal race strategy to avoid bonking. 

Running negative splits is where you start the race at a specific pace and gradually speed up as you advance.

It can be easy to unintentionally do the opposite – starting way too fast by getting caught up in the excitement of the race and flying off the starting line.

This, however, is a recipe for disaster and a surefire way to bonk. Of course, we feel great at the beginning of the race as we are just starting out and are fresh as can be. 

I am no different. When I ran my very first marathon, I shot off the start line as if I were being chased.

Starting out much faster than my planned race pace, I felt on top of the world and that, of course, I would be able to hold my newly found pace! What were people talking about, holding back at the start?

A runner, bonking.

About halfway through, guess who was walking? That’s right, I bonked and could barely get a jog going for the entire second half!

For your next race, try and run negative splits

Break your race into two or three equal sections and determine a pacing strategy that will result in negative spits.

For example, in a marathon, you can run the first third of the race about 5 seconds slower than your planned race pace, the second third at your planned race pace, and the last third 5 seconds faster than your planned race pace. 

With just a few kilometers left, if you still have gas in the tank you could even try to speed up, but only if you are confident you can sustain a faster pace until the end.

If you worry that starting out slightly slower will affect your pacing strategy, you could start at your race pace and try to speed up slightly in the second half of the race or sustain your race pace for the entirety of the race and run equal splits.

The most important takeaway is that you don’t get caught up in race day excitement, start your race out too fast, and end up bonking.

A person running on a track.

#3: Train Properly

Being prepared for your race by taking the appropriate period of time to train, following a well-thought-out training plan for your specific experience and fitness level, and sticking to it will also contribute to a successful race and help you avoid bonking.

If you are underprepared, you risk physically or mentally breaking down, and it will not make for an enjoyable experience.

As a coach, I see many people signing up for races at the last minute without adequate time to train. Without sufficient training, your body will unlikely be able to adapt to the stressors of running in time for the event.

Be sure to follow your run training plan as completely as possible, strength train twice a week to help gain strength and avoid injury, respect your rest days to avoid overtraining, and listen to your body. 

Training sessions with a certified running coach or local running group can provide support and guidance to ensure you reach your goal race as prepared as possible.

So, whether it be cycling, running, or a triathlon, focus on your nutrition, practice your pacing, and train, train, train to help you avoid bonking during your next race. 

For our free marathon training plans database for every level, click here.

People running a marathon.


Photo of author
Katelyn is an experienced ultra-marathoner and outdoor enthusiast with a passion for the trails. In the running community, she is known for her ear-to-ear smile, even under the toughest racing conditions. She is a UESCA-certified running coach and loves sharing her knowledge and experience to help people reach their goals and become the best runners they can be. Her biggest passion is to motivate others to hit the trails or road alongside her, have a blast, and run for fun!

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