According to recent research, here’s what causes you to bonk: When you begin exercising, your body will use glycogen in your bloodstream for energy. When that runs out, it will turn to your liver and muscles’ glycogen stores to keep you going. Once your muscles are depleted of energy, your brain begins to limit muscle function to prevent damage, leaving you unable to continue. This is the sensation known as bonking.

Bonking doesn’t just feel awful; it can also have lasting effects on your training efforts and physical health. Even just bonking once can harm your body. The adverse side effects include: Decreased immune function: Bonking and overtraining can suppress your immune system and increase your risk of getting sick.4 Muscle loss: When your body doesn’t have enough fuel from carbs and other glucose sources, it will start taking energy from fats and proteins. Dehydration: If you’re running or cycling in a long event, you might not have adequate time or opportunity to hydrate. Cognitive impairment: When your brain doesn’t have enough glycogen, it won’t function properly.

A runner on the side of the path sitting down because of exhaustion.

What Is Bonking?

Q: What does bonking feel like?

The most common symptoms of bonking include: Nausea Extreme weakness Poor coordination Dizziness Shakiness Cognitive problems An overall miserable feeling

Table Of Contents

What does it mean to bonk when exercising?
What does bonking feel like?
What causes bonking?
How long does bonking last?
Why is bonking bad?
How to prevent bonking
How do you treat bonking?
Key Takeaways:

The dreaded bonk is something most endurance athletes have experienced at least once before. And once you experience it, you never want it to happen again. This article will look closely at bonking, what it is, and how to prevent it. That way, you can have the best experience possible while completing endurance workouts and avoid the dreaded consequences of bonking.

What does it mean to bonk when exercising?

A female runner on a gravel path hunched over on the ground trying to catch her breath.

“Bonking” is a term used by runners, cyclists, and other athletes to describe a condition where your muscles run out of fuel. Essentially, it’s when you’ve hit the wall and depleted your body’s glycogen (or energy). As a result, you can’t keep going and must stop.¹

Bonking tends to occur during endurance activities, like running or cycling, and although it’s not a scientific term, it’s a very real condition. And you might experience different types of bonking, depending on the situation.

For example, you might have the muscle-glycogen bonk (described above), where you feel fine mentally, but your legs feel extremely heavy and refuse to work. Or, you may experience a blood-glucose bonk, where your body feels fine, but you’re extremely mentally fatigued, hallucinating, and feel like you can’t go on.

What does bonking feel like?

If you’re working as hard as possible while exercising, you’re bound to feel tired, but that feeling of exhaustion differs from bonking. True bonking is being physically unable to continue exercising. It’s true and total exhaustion, with symptoms worsening as the minutes tick on.

The most common symptoms of bonking include:

Nausea
Extreme weakness
Poor coordination
Dizziness
Shakiness
Cognitive problems
An overall miserable feeling

What causes bonking?

A female cyclists on the side of the road catching her breath holding her should looking ahead.

Bonking typically occurs during endurance exercises, like running a marathon or cycling a very long route, and it’s a side effect of your body exhausting its energy supply. You can even bonk while running or cycling on a treadmill with Vingo if you don’t eat enough or hydrate properly.

According to recent research, here’s what causes you to bonk:^2,3

When you begin exercising, your body will use glycogen in your bloodstream for energy.
When that runs out, it will turn to your liver and muscles’ glycogen stores to keep you going.
Once your muscles are depleted of energy, your brain begins to limit muscle function to prevent damage, leaving you unable to continue. This is the sensation known as bonking.

How long does bonking last?

A male runner in the middle of field looking at his phone.

Fully recovering from a bonk usually takes a few days, but it depends on how depleted your body becomes. Typically, consuming some food and liquids will take the edge off the immediate nausea and weakness you feel when you bonk. However, it will take a few minutes for your body to adjust.

Unfortunately, even after the acute effects like nausea, shakiness, and weakness have gone away, your body may still need more time to recover from the stress of bonking. Often, it takes a few days to rest and replace lost calories and energy stores.

Why is bonking bad?

Bonking doesn’t just feel awful; it can also have lasting effects on your training efforts and physical health. Even just bonking once can harm your body. The adverse side effects include:

Decreased immune function: Bonking and overtraining can suppress your immune system and increase your risk of getting sick.^4,5
Muscle loss: When your body doesn’t have enough fuel from carbs and other glucose sources, it will start taking energy from fats and proteins. As a result, you can lose muscle mass.
Dehydration: If you’re running or cycling in a long event, you might not have adequate time or opportunity to hydrate. The water loss can contribute to bonking and cause severe fatigue and painful cramping. Continually being dehydrated can also cause cognitive problems.
Cognitive impairment: When your brain doesn’t have enough glycogen, it won’t function properly. As a result, you might experience cognitive issues, such as having trouble focusing or making decisions. These side effects can be hazardous in a race while cycling or running.

How to prevent bonking

A man sprawled out on a track field tired after a hard training session.

Bonking is very unpleasant and something most athletes actively try to avoid. So how do you prevent bonking?

Simply put, you have to give your body enough fuel to power your workout by eating enough of the right foods and staying hydrated.

It sounds simple, but it may take more effort than you anticipate, and it’s a very individualized process.

Learn how to properly fuel your body for exercise.

A running taking a break eating an apple on a running trail.

This isn’t something you want to figure out on race day, as it can take some time to experiment and work it all out. Ideally, you’ll want to determine your consumption rate, which is just a fancy term for your individual carbohydrate and calorie requirements.

All athletes have different consumption rates based on the type of workout they’re doing, the intensity of their training, and their fueling schedule. You’ll need to be in tune with your body to identify your individual fueling needs.

Start by counting and tracking your macros and adjust your carbohydrate intake based on your activity level. You may need to work with a nutritionist to get professional help with this. (If you need some ideas, we have guides on the best diet for runners and the best diet for cyclists.)

Many serious athletes also use a continuous glucose monitor (CGM) to track how their glucose levels respond to various factors, including different macronutrients and types of workout sessions. This is a more precise way to tailor your diet to boost your performance and learn more about your body to optimize your training.

To help you arrange your schedule, we’ll answer the commonly asked question: “How long should you wait to exercise after eating?”

Stay hydrated.

A close up of a male runner drinking out of his water bottle.

Since dehydration can contribute to bonking, drinking plenty of water should be a priority. You can also drink low-sugar electrolyte drinks and eat specific foods like tomatoes, melons, and other hydrating fruits and vegetables before a race.

Make sure you have a plan.

Two people looking at a paper with "Workout Plan" with days of the week on it.

You’re more likely to bonk if you complete an endurance event or activity without planning for it.

Be prepared by timing your meals to support your workout sessions and improve your performance. Also, consider how much food and the types of food you should be eating before you exercise. By ensuring you eat enough of the right foods and eat frequently throughout the day, you’ll ensure your body has enough fuel to get through your workouts.

To enhance your physical performance, it’s also a good idea to get adequate rest before a workout and take enough time to recover afterward.

If you’re participating in an endurance event…

A group of cyclists in a race taking a sharp turn.

Make sure you’re refueling your body throughout the event. Depending on your preferences, having a sports drink or energy gel every so often can help.
Go into the event with a plan. Know when and how often you’ll refuel and what foods or products you’ll carry with you to do it.
While you train for the event, practice refueling like you would during the race. This will help you determine how many carbs your body needs and can handle and the appropriate pace for refueling.⁶

How do you treat bonking?

A older runner bent over on the side of the road taking a breath.

If you do bonk, there are several ways to speed up your recovery and cope:

Refuel with high-quality energy sources like healthy carbs, fats, and proteins.
Drink liquids with your food. You might feel nauseous when you bonk, but drinking liquid while eating can help counteract nausea.
Have a friend or family member come pick you up. If you’re out in the middle of a trail running or you have miles to cycle before you can get back home, it’s safest to have someone pick you up instead of trying to do it while your body is completely depleted of energy.
Take time to rest and recover after a marathon or other endurance event. Don’t jump right back into your training. Instead, take a day or two off to give your body time to recover and restore its glycogen stores.

Key Takeaways:

If you’re an endurance runner or cyclist, you may experience bonking, which can be very unpleasant. Fortunately, by learning how to fuel and hydrate your body well, you can prevent bonking and improve your overall performance.

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Sources:

Club, K. C. (2020, July 12). What is Bonking? Causes, Dangers, and Prevention — Cycling Club in Kent – K20 Cycling. Cycling Club in Kent – K20 Cycling. https://k20cyclingclub.org/k20cycling-club-news-blog/what-is-bonking
Scott, P. (2022, March 2). The Science Behind Bonking. Runner’s World. https://www.runnersworld.com/nutrition-weight-loss/a20851510/the-science-behind-bonking/
Noakes, T. D., Peltonen, J., & Rusko, H. (2001). Evidence that a central governor regulates exercise performance during acute hypoxia and hyperoxia. The Journal of Experimental Biology, 204(18), 3225–3234. https://doi.org/10.1242/jeb.204.18.3225
Abbasi, A., Fehrenbach, E., Hauth, M., Walter, M., Hudemann, J., Wank, V., Niess, A. M., & Northoff, H. (2013). Changes in spontaneous and LPS-induced ex vivo cytokine production and mRNA expression in male and female athletes following prolonged exhaustive exercise. Exercise immunology review, 19, 8–28.
Nieman, D. C., & Wentz, L. M. (2019). The compelling link between physical activity and the body’s defense system. Journal of Sport and Health Science, 8(3), 201–217. https://doi.org/10.1016/j.jshs.2018.09.009
Reporter, G. S. (2014, September 25). Do you bonk when you run? The Guardian. https://www.theguardian.com/lifeandstyle/the-running-blog/2014/sep/25/bonk-run-long-distance-runner-athlete-beat