Reviewed for accuracy: This article describes the physiology of glycogen depletion and exercise-induced hypoglycaemia, drawing on the carbohydrate metabolism research of Tim Podlogar, Sam Impey and Prof. Asker Jeukendrup. It is an explanation of normal exercise physiology, not medical advice. Riders who experience repeated hypoglycaemia, or symptoms at rest, should see a doctor.
You know the moment. Three hours in, the group lifts the pace on a drag you've ridden a hundred times, and instead of responding your legs just... aren't there. Not heavy — hollow. The road goes slightly grey at the edges. You feel irritable, then strangely emotional, then desperate for anything sweet. That's the bonk. And here's the thing most people get wrong about it: it's not your muscles running out of fuel. It's your brain pulling the plug to protect your blood sugar.
Once you understand what's actually happening — which organ runs dry, why the brain reacts the way it does, and what role fat plays — the prevention becomes obvious and the bonk becomes a thing that happens to other people.
The short answer
Bonking is acute hypoglycaemia: low blood sugar. It's caused by your liver running out of glycogen, not your muscles. The liver is the only organ that can release glucose back into the bloodstream to keep blood sugar stable, and it stores only about 80-100g of glycogen — enough for roughly 90 to 120 minutes of hard riding if you eat nothing. When the liver empties, blood glucose falls. Your brain, which runs almost entirely on glucose and has no meaningful fuel store of its own, senses the threat and slams the brakes on your effort to protect itself. That's the grey-out, the despair, the collapse in power. You prevent it by eating carbohydrate from the start of long rides, pacing the early hours to spare glycogen, and training your body to burn more fat so it leans on its limited sugar stores less.
Two glycogen stores, two completely different problems
This is the part that clears everything up. You carry carbohydrate in two places, and they do two different jobs.
Muscle glycogen sits inside the muscle fibres themselves. It's the local store — the muscle burns it directly and cannot share it with anywhere else in the body. There's no enzyme to release muscle glucose back into the blood. When muscle glycogen runs low, the muscle gets weak and the legs feel like concrete. That's local fatigue. Unpleasant, but your head stays clear.
Liver glycogen is the body's blood-sugar buffer. The liver — and only the liver — can break its glycogen down into glucose and release it into the bloodstream to keep blood sugar stable. Your brain, your nervous system and your red blood cells all depend on that steady supply. The liver holds far less than your muscles do: roughly 80-100g versus the 400-500g a trained rider can store in muscle.
The bonk is a liver event. When hepatic glycogen runs out and you're not eating enough to replace the glucose leaving your blood, blood sugar drops. And because the brain is the most glucose-dependent organ you own, it reacts hard and fast.
Researchers like Sam Impey, whose work sits right in the muscle-glycogen and fuel-availability space, have spent careers mapping how these stores deplete and refill. The practical headline is simple: heavy legs and the full-blown bonk are two different failures with two different causes. You can have one without the other.
Why your brain hits the brakes
When blood glucose falls, you don't just feel a bit tired. You get a specific, recognisable cluster of symptoms: dizziness, confusion, difficulty making decisions, irritability, anxiety, cold sweats, shaking, sometimes a wave of emotion that feels wildly out of proportion to riding a bike. That's neuroglycopenia — the brain not getting enough sugar.
The drop in power that comes with it isn't only your muscles failing. Your nervous system actively reduces the drive to your muscles. Tim Noakes' "central governor" idea — that the brain regulates effort to protect the body from catastrophic harm — fits the bonk almost perfectly. Faced with falling blood sugar, the brain would rather make you slow to a crawl than let glucose drop to a level that threatens it. The hollow-legged feeling is partly the brain refusing to let you push, not just the legs refusing to respond.
That's also why you can't think straight when you bonk badly. Riders make terrible decisions in that state — abandoning, taking wrong turns, getting upset with people. It's not weakness of character. It's an under-fuelled brain.
Where fat fits in
Here's the thing that separates riders who bonk from riders who don't: fat oxidation.
At any given intensity, your body fuels itself from a blend of fat and carbohydrate. The lower the intensity, the higher the proportion of fat. Even a lean rider carries tens of thousands of calories of fat — effectively unlimited for cycling purposes. The problem is that fat burns slowly and can't be relied on at high intensity. Carbohydrate is the fast-burning fuel, and it's the one in short supply.
So the more energy you can draw from fat at a given pace, the less you draw on your limited glycogen — and the longer that glycogen lasts. This is precisely what Zone 2 base training builds: the ability to burn more fat at a given effort. A well-trained endurance rider burning a higher fraction of fat at 200 watts spares glycogen with every pedal stroke compared to an untrained rider grinding through their sugar stores at the same power.
This is the real argument against chronic fasted riding as a fuelling strategy, by the way. Yes, training fat oxidation matters — but you build it through smart Zone 2 volume, not by riding long and hard on empty until you bonk 60km from home and hate your life. The bonk doesn't teach the gut or the body anything useful. It just digs a hole.
How long until you hit the wall
Rough numbers, because they make the prevention concrete:
- Liver glycogen, unfuelled: about 90-120 minutes of hard riding before blood sugar starts to fall — less at high intensity, more if you're going easy. That drop is what triggers the classic bonk.
- Muscle glycogen: a far bigger tank (400-500g) that powers the legs directly and lasts longer — but it can't top up your blood sugar, which is why you can bonk with fuel still left in the legs.
This is exactly why rides under 90 minutes rarely need on-bike fuelling, and why everything longer does. It's also why the cardinal rule of fuelling is to start eating before you feel empty. By the time the first symptoms arrive, the liver is already running dry and ingested food takes 15-30 minutes to lift blood sugar. You're always fuelling for where you'll be in half an hour, not where you are now.
Preventing the bonk
Four things, in order of impact:
1. Eat from the first 30 minutes. On any ride over 90 minutes, start fuelling early and keep it steady: 60-90g of carbohydrate per hour depending on intensity. Don't wait for hunger or hollow legs. The in-ride fuelling calculator will set your per-hour target for a given ride, and the carbohydrate-per-hour guide breaks down how to match the number to duration and intensity.
2. Pace the early hours. Going too hard in the first third of a long ride burns through glycogen faster — the higher the intensity, the bigger the share of carbohydrate you're spending. Banking effort early to "get ahead" is how riders bonk late. Ride the first two hours like you've got four to do, because you have.
3. Start full. A proper pre-ride breakfast tops up liver glycogen, which depletes overnight while you sleep. For big events, carb loading in the days before maximises muscle stores so you start with the biggest possible tank.
4. Train the gut to keep up. None of the above works if you can't actually absorb the fuel. Gut training is what lets you take in 90g per hour comfortably on a long, hard day instead of gagging on the third gel. It's also worth getting your hydration and sodium right alongside the carbohydrate, because the fluid you drink carries the fuel across the gut wall — under-drink and the eating plan stalls with it.
If you've already bonked
There's no clever rescue. Ease right off, take in fast carbohydrate — a gel, a sugary drink, a Coke at the petrol station — and accept that blood sugar takes 15-30 minutes to recover. Keep eating small amounts. Ride gently once you stabilise. You won't get full power back on that ride; the day's already written. But you'll get home. A day that empties you that completely also leaves a deep hole to refill, so be deliberate about post-ride recovery nutrition once you're back — the glycogen you burned through doesn't come back on its own. Then you log it, look at what went wrong with the fuelling or the pacing, and fix it on the next ride.
That's the whole point. The bonk is one of the most preventable failures in cycling — it's a fuelling and pacing problem with a known cause and a known fix. Get the nutrition system right and it simply stops happening.
Got a specific question — your own per-hour target for a hilly sportive, why you keep fading at the same point in long rides, or how to build fat oxidation without bonking your way there? Come talk training with us — it's where riders work this out together, building on the conversations with Tim Podlogar, Sam Impey and the rest of the nutrition and physiology guests on the podcast.