What is a bonk, and how do you avoid it?
A bonk is the moment your legs go heavy, your head goes foggy and every pedal stroke feels like work. Most of the time it is a fuel problem, and one you can plan around.
What actually happens
Your muscles run on stored carbohydrate, called glycogen. At a hard effort that store can be largely used up after about an hour. When it is low and you are not taking in enough carbohydrate, power drops and the effort feels much harder than it should. That is a bonk, also called hitting the wall.
Warning signs
It usually builds up, so catch it early.
- Heavy legs and a sudden drop in power
- Feeling shaky, dizzy or very hungry
- A foggy head and trouble concentrating
- A mood that turns sour
The usual causes
In practice it is almost always one of these.
- You started empty. No time for a real meal before an early session, so the first hour already runs on low stores.
- You fell behind on carbs per hour. Starting late or skipping gels is much harder to fix than starting early and keeping steady.
- You pushed a long, hard session on low stores. Training low is a coach level strategy, to be used rarely.
How to avoid it
Before, during and in the weeks leading up to it.
- Before: eat a carbohydrate rich meal 1 to 4 hours before a long or hard session. For events longer than 90 minutes, consider carb loading in the last 36 to 48 hours.
- During: aim for your carbs per hour and start early. Our calculator gives you a number for your session.
- In training: practice your plan, and train your gut so it can take in more.
If it happens anyway
Slow down, take carbohydrates that are easy to absorb, such as a carbohydrate drink or a gel, and keep taking small amounts steadily. Do not stop eating because you feel nauseous: a race is usually much harder when you fall behind on carbohydrate per hour. Easily absorbed carbohydrate drinks are the best option if solid food will not go down.
General information for healthy adults, not medical advice. If you have diabetes, a gut condition, an eating disorder, or take medication, talk to your doctor or a sports dietitian first.
Common questions
How many carbs per hour prevent a bonk?
It depends on duration and intensity, from about 30 g per hour in sessions of one to two hours to 90 g per hour or more in long, hard efforts. Use the carbs per hour calculator.
Can I bonk in a short or easy session?
Rarely. A session of an hour or less usually needs no fuel during the effort. A bonk is mostly a long or hard session problem.
Should I skip breakfast to burn more fat?
For most athletes, no. Training low is a strategy for coached, specific cases and used sparingly, because training quality can drop more than the benefit is worth.
Want this planned around your own week?
The 2fuel app turns every training session into kcal, carbs per hour and protein, then into meals. With an AI coach or a sports dietitian. In beta now.