Bonking is caused by depleted glycogen and low blood sugar, not weak legs. Prevent it by fueling on a schedule from early in the ride — not once you feel it coming — and by not going out harder than your fueling plan can keep up with.
What's actually happening
Your body stores a limited amount of carbohydrate as glycogen. Ride long or hard enough without replacing it, and both muscle glycogen and blood glucose run low at the same time — your legs and your brain both start running short, which is why a bonk feels like more than just fatigue.
Why it sneaks up on people
The drop can be sudden because your body compensates reasonably well right up until it can't — you can feel fine, then rapidly not fine, within a matter of minutes. That's part of why waiting to feel hungry or low before eating is a bad strategy: by the time you notice, you're already behind.
The fix: fuel on a schedule
Set a target from carbs per hour cycling and eat toward it from early in the ride on a timer — every 20–30 minutes is common — rather than waiting for a feeling. This is the single biggest lever against bonking: staying ahead of the deficit instead of trying to fix it after it's already there.
Pacing matters as much as fueling
Going out harder than planned burns through glycogen faster than your fueling can replace it, which can cause a bonk even with a normally-solid plan. Sticking closer to your intended effort, especially in the first half of a long ride, gives your fueling plan a fair chance to keep up.
If it happens anyway
Stop pushing, take in fast carbohydrate (a gel or regular soda works), and ease the pace for 10–15 minutes while it takes effect. Trying to power through usually just extends how bad it gets before it improves.
Common mistakes
- Waiting to eat until you feel low. That's already too late — fuel on a schedule instead.
- Starting too hard for the distance. An overly ambitious opening pace is a common, avoidable cause.
- Confusing it with dehydration. They can overlap, but the fixes are different — fuel for one, fluids and electrolytes for the other.
- Under-fueling on purpose to "train the body to burn fat." That's a specific, deliberate training tool for certain sessions, not a general strategy for rides where you want to perform well.