What is bonking: what it feels like and how to prevent it

Here’s how to sustain energy levels and prevent bonking.

There’s few worse feelings than bonking on the bike or hitting the wall when running.

Your training session or race is going well, and then suddenly, you run out of energy, hit a metaphorical wall and struggle to continue. It’s like you’ve run out of gas. There’s nothing left in the tank.

This is known as bonking.

It’s when you run out of glycogen, your body's preferred energy source for endurance activities. 

To help you avoid the dreaded bonk, this article explains what it feels like and how to prevent it, with tips from a registered sports dietitian.

What you will learn:

• What bonking is and what it feels like 

• What to do if you bonk or hit the wall when running or cycling 

• How to prevent bonking and how to recover from it (if you unfortunately hit the wall)

What is bonking?

“Bonking is when glycogen stores (a.k.a carb stores) in the liver and muscles become depleted, leading to sudden fatigue and poor energy. It really feels like "hitting a wall" in the middle of a workout,” says Grace Kelley, a Sports Dietitian and owner of Grace Kelley Nutrition. 

If you’ve been running or cycling for quite some time, you’ve likely experienced this.

It’s an easy mistake to make, especially if you usually run or cycle short distances. But once you increase the duration of your long run or ride, you are more likely to bonk if you do not have a proper nutrition strategy.

You should eat before and during endurance activity for sessions longer than 75 minutes to reduce the likelihood of bonking or hitting the wall. Read our post on how long to wait after eating to work out for more advice on how to fuel for your training sessions.

What does bonking feel like?

Bonking feels like you’re running on empty. Common short-term symptoms include:

• Extreme fatigue and lethargy 

• Nausea

• Hunger

• Heavy legs

• Cramping 

• A slowed pace or feeling like you can’t keep going 

• Confusion

• Lightheadedness or dizziness

You may not experience all short-term symptoms, but a mixed bag. 

You’ll know when it happens because you will feel like you can’t continue, no matter how hard you try. 

You might be able to walk, jog, or cycle very slowly, but your performance will take a big hit.

Is hitting the wall the same as bonking?

Hitting the wall and bonking are essentially the same thing.

Bonking is the more commonly used phrase in cycling culture, and hitting the wall is used more in the running community. 

The phrase “hitting the wall” is often used among the marathon and ultra-marathon communities.

These phrases both describe the same phenomenon: a sudden and severe drop in energy due to depleted glycogen stores.

What to do if you bonk

If you bonk during a ride or hit the wall on a run, whether a training session or a race, stop or slow down — this will be your natural instinct.

Next, consume 30-60g of fast-acting carbs like an energy gel or sports drink. GEL30 contains 30g of carbs and supplies maximum carbohydrates for maximum energy.

Hydrate and replace fluids, too.

While carbohydrates should be your main priority, if it’s a hot or humid day or it’s a long event, consider adding electrolytes. SLT07 ensures rapid rehydration and prevents muscle cramping. It also helps increase water intake and replaces lost sodium from sweat.

If possible, you should take a short rest of anywhere from 10 to 15 minutes.

You want to give time for fast-acting sugars to enter your bloodstream and give you energy to keep going.

And once you are ready to go, keep fueling to prevent further bonking in your run or ride.

Aim for a minimum of 30-60g of carbohydrates/hour. Options include energy bars, like BAR50 or energy gels, chews, sweets, or carbohydrate drink mixes. 

A proper nutrition plan and constant feeding during long endurance exercise will help sustain energy levels and reduce the risk of bonking.

What happens to your body when you bonk

When you bonk or hit the wall, your body runs out of glycogen, its main energy source for endurance and aerobic activity.

Your muscles and body use glycogen, so when your glycogen stores are depleted, your body relies on fat as fuel — this is a much slower energy source.

When glycogen is low, you may feel confused or irritable. Your legs may feel heavy, shaky, or uncoordinated. Your body is low on its main fuel source and is starved of much-needed energy.

Think of bonking like a full-system crash — your muscles, brain, and nervous system suffer because your fuel tank is empty.

You need to restock your glycogen levels to support the required exercise intensity. 

How to prevent bonking

“When our glycogen stores hit zero, the only way to replenish them is by ingesting carbohydrates. We have a maximum glycogen storage that lasts about 1.5-2 hours of exercise, which means the stores need to be continuously topped off with carbohydrates during exercise,” says Kelley.

To prevent bonking, she emphasises the importance of maximising glycogen stores and continuing to replenish throughout exercise. 

Prioritise foods high in carbohydrates before training and take at least 30g of carbs when exercising for more than 60 minutes. The longer and more intense the exercise, the more carbs you need!”

Below are a few extra tips to help you prevent the dreaded bonk.

1. Plan your nutrition 

If you don’t have a nutrition plan, you’re much more likely to bonk.

Map out what you will eat before and during exercise to maintain energy levels, to stop you from hitting the wall and bonking.

For a run or ride longer than 75 minutes, aim for a minimum of 30-60g of carbohydrates per hour. 

Some cyclists, in particular, eat upwards of 90-120g of carbohydrates per hour. But this is something you will need to experiment with, as we are all different.

Popular carb sources during exercise include:

• Energy gels

• Chews

• Energy bars

• Carbohydrate drinks

• Rice cakes

• Bananas 

• Sports drinks

2. Eat before a run or ride 

Some endurance athletes prefer to work out fasted (without food, usually in the morning).

However, if you are running or riding for longer than 75 minutes, it’s important to have a pre-workout meal or snack to top up your glycogen stores.

The International Society of Sports Nutrition says nutrient timing is essential for optimal fueling and training adaptations.

The longer you have before a workout, the more balanced a meal you can have.

The shorter the time you have, the more carbohydrates you should prioritise, but the smaller the portion you should have.

If you have 15 to 30 minutes before a workout, try and get 15-30g of carbohydrates. 

If you have 1 hour, aim for 0.5g of carbs per pound of body weight. For a 170lbs athlete, that’s 85g.

And if you have upwards of 2-3 hours, you can get 0.9-1.4g per pound of bodyweight, which is approximately 153-255g of carbs for a 170lbs athlete.

Again, experiment with this and find what works best for you. There’s no rule saying you can’t work out fasted, but when trying to reduce the risk of bonking, it’s best to fuel before endurance exercise.

3. Experiment before race day

Use your training sessions, especially long runs and rides, to experiment with your nutrition.

Find what pre-workout meal works best for you and how many carbs (g) — and what carb sources — work best for you. 

Although you’d no doubt prefer not to bonk at all, it’s better to do it in a training session and learn from it than for it to happen during a race.

Learn from your mistakes and adjust your nutrition as needed to give you the energy you need to achieve the results you know you can achieve.

How to recover from bonking

“It's easier to prevent bonking than it is to recover from it, but the solution is still the same: replenish glycogen stores with carbohydrates,” says Kelley.

“If you experience bonking consistently, you might not be eating enough throughout the day and have persistent low blood sugar. Chronic under-fueling can lead to fatigue, injury, hormone imbalances, and poor performance, with the only treatment being increased dietary intake.”

You can have the best nutrition during exercise, but if you don’t eat properly the remaining hours of the day, you may negatively impact recovery and training adaptations (as well as possibly bonking). 

Key takeaways:

• Bonking, also known as hitting the wall, is when your glycogen levels are depleted and your body’s ability to continue aerobic exercise is impaired

• Common symptoms include tired legs, a slowed pace, cramping, feeling lightheaded or dizzy, and negative self-talk 

• Eat enough carbohydrates before and during exercise to reduce the risk of bonking (30-60g/hour for exercise longer than 75 minutes)

• If you bonk, eat quick-to-digest carbohydrates to increase glucose levels (energy gels, chews, bars, and sports drinks are good options)

• Consider taking electrolytes on a hot or humid day or when running or cycling long distances 

• Prevention is better than recovery so plan your nutrition ahead of time to avoid bonking

FAQs

What does it mean when you bonk?

Bonking, also called “hitting the wall,” is when your body runs out of glycogen (energy). Depleted energy stores cause an energy crash and extreme fatigue, mental fog, and the inability to continue at a normal pace.

How to tell if glycogen is depleted?

Signs of muscle glycogen depletion include sudden fatigue, heavy legs or weak muscles, dizziness or lightheadness, confusion, or intense hunger or shaking, to name a few. You will know when your glycogen stores are empty because it will be next to impossible to continue.

How do you treat bonking? 

Prevention is better than treatment. But if you do hit the wall or bonk when running or cycling, take fast-digesting carbs like energy gels or sports drinks to quickly increase available muscle glycogen. Hydrate and rest for 10 to 15 minutes or until you feel good to go again.

What is an example of bonking?

An example of bonking would be running a marathon and not eating before or during the race. By mile 15 to 18, you would likely run out of energy, feel sudden fatigue, and have the need to stop. This is because your glycogen stores are deplete, and there’s nothing left in the tank.