Bonking: what is actually running out, and what to do about it

There are few experiences in endurance sport as complete as a proper bonk. It is not ordinary tiredness — it is the sudden, total conviction that you cannot continue, accompanied by shaking hands, a curious emotional flatness, and an inability to work out what to do about any of it.

It is also almost entirely preventable, and the reason riders keep experiencing it is that the mistake happens two hours before the symptoms do.

What is actually running out

Your body stores carbohydrate as glycogen in two places: in the muscles, where it fuels the working legs, and in the liver, where it maintains blood glucose. Fat is available in effectively unlimited quantities, but it burns slowly — you can ride on fat, just not very fast.

Muscle glycogen running low makes your legs heavy. Liver glycogen running low is what produces the dramatic version, because blood glucose falls and your brain notices. That is why a bonk feels like a mental event rather than a muscular one, and why "just push through it" is uniquely bad advice here: the organ making that decision is the one that is short of fuel.

It is not the same as dehydration or overheating

These three get used interchangeably and they need different responses. A rough guide:

• Bonk: comes on over minutes, legs empty, hands shaky, mood flat, thinking foggy. Fixed by sugar.
• Dehydration: comes on gradually, heart rate high for the effort, headache, dark urine, mouth dry. Fixed by fluid and electrolytes — cycling hydration strategy covers it properly.
• Overheating: skin hot, effort feels impossible at low power, no shakiness. Fixed by stopping, shade and cooling, not by eating — see racing in the heat.

They frequently arrive together on a long hot day, which is exactly why riders end up treating a bonk with water or heat exhaustion with a gel.

What to do when it has already happened

Preventing it, which is the only part that really matters

The rule is simple and almost universally ignored: start eating in the first hour, before you want to. By the time hunger arrives you are already behind, and you cannot absorb carbohydrate fast enough to catch up while riding hard.

A reasonable default for rides over about 90 minutes is 60–90 grams of carbohydrate per hour, started early and taken in small regular amounts rather than in one large stop. Fuelling long rides: carbs per hour has the detail, and gut training covers how to get to the top of that range without stomach trouble.

Two situations deserve extra care. Riding fasted deliberately — which has its uses and its risks, covered in riding fasted — should be kept short and easy. And a rider who is chronically under-eating across the whole week will bonk earlier and more often than their fuelling on the day explains; that pattern is the subject of low energy availability and REDs.

The indoor version is different and sneakier

Riders bonk on the trainer more often than they expect, for two reasons. Indoor sessions tend to be higher intensity for their duration, which burns carbohydrate faster. And there is no café, no shop, and no ride home — so the bonk arrives while you are in ERG mode being asked for another 20 minutes at threshold.

Treat a hard 90-minute indoor session as a ride that needs fuelling, not as a workout you can do on a coffee. A bottle with carbohydrate in it is usually enough, and it is the difference between finishing the session and abandoning it in the last block. Moveee Indoor saves the full power stream afterwards, which is a fairly reliable way of finding out whether the last interval sagged because the session was too hard or because you never ate anything.

One reframe worth keeping

A bonk is not a sign that you are unfit. Fitter riders can ride harder for longer, which means they can empty the tank faster — plenty of very strong cyclists have bonked spectacularly. It is a planning failure, not a fitness one, and the fix costs about 200 calories an hour and a small amount of discipline in the first sixty minutes.