Bonking explained, why it happens and how to avoid it

Every endurance athlete has a bonk story. Mine was a 5-hour ride in the Alps where I thought I could get away with a single gel an hour. By the last climb I was off the bike, pushing it up a 6% gradient with my head spinning. A walker offered me half a sandwich. I have never accepted food faster in my life.

If you have ridden or run long enough, you have bonked. The good news is it is preventable. The slightly less good news is that most amateur athletes do not actually understand what is happening to their body, which is why they keep doing it.

Here is what bonking is, why it happens, and the plan to avoid it.

What bonking actually is

Bonking is not just being tired. Being tired is a slow, manageable build-up of fatigue. Bonking is sudden, total, and unmistakable. One minute you are pedalling along. The next minute your legs feel like wet cement, your vision narrows, and you cannot generate any power. Some athletes get dizzy, some get cold sweats, some find themselves unable to think clearly.

The mechanism is simple. Your muscles run on a mix of fat and carbohydrate during exercise. Fat is the big tank, but it burns slowly. Carbohydrate (glucose, mostly stored as glycogen in your muscles and liver) is the fast fuel, but the tank is small. At endurance pace you are burning both. At higher intensity you lean harder on carbs.

Your liver holds roughly 80 to 100 g of glycogen. Your muscles hold another 300 to 500 g depending on your size and training. That is your total fast-fuel tank. When it runs out, your body cannot deliver glucose to your working muscles or your brain fast enough, and everything falls apart at once.

That moment is the bonk.

How long until you bonk?

Rough back-of-the-envelope: a fit 70 kg endurance athlete burns 500 to 800 kcal per hour at moderate effort. About 60% of that comes from carbs at endurance pace, more at higher intensities. So you are burning maybe 75 to 100 g of carbs an hour.

If you start the session with full glycogen stores (around 400 to 600 g total) and you do not eat anything, you have roughly 4 to 5 hours before you bonk hard. Push the intensity up, and that drops to 2 to 3 hours. Skip breakfast, and you might bonk in 90 minutes.

This is why long rides demand fuelling, and why short hard rides on an empty stomach are surprisingly risky.

Warning signs before the bonk

Most bonks do not arrive out of nowhere. There are signs, you just have to be listening for them:

• Sudden hunger that feels urgent. You go from "I am fine" to "I need food right now" in 10 minutes.

• Foggy thinking. You forget what gear you should be in. You miss a turn you have done a hundred times.

• Legs that feel hollow on small climbs. Not heavy, hollow. Like someone took the strength out of them.

• Mood crash. You start finding everything annoying. The wind. Your bike. The route. This is your brain noticing it does not have enough glucose.

• Cold sweats or shivering in conditions that should not be cold.

If you spot any of these, you have maybe 15 to 30 minutes before the proper bonk arrives. Eat now. A gel works. A banana works. Even a sugary fizzy drink works in an emergency.

How to never bonk again

The plan is not complicated. It just needs discipline.

Start the session with full glycogen. Eat a proper meal 2 to 3 hours before any session over 2 hours. Carbs, moderate protein, light on fat and fibre.

Eat early in the session, not late. The single biggest mistake amateurs make. By the time you feel like you need food, you are already behind. Start eating within the first 30 minutes of any ride or run over 90 minutes.

Hit the hourly target. For most endurance athletes that is 60 to 90 g of carbs per hour during sessions over 2 hours. Set a timer if you have to. Eat to the timer, not to hunger.

Train your gut. If you cannot stomach high carb rates, you cannot fuel for them either. Practice on long sessions. Build up from 40 g/h to 60 g/h to 80 g/h over weeks.

Have a bail-out plan. Always carry one more gel than you think you need. Petrol stations, village shops, and feed zones exist. Use them.

Why generic nutrition apps make this worse

Most nutrition apps treat every day the same. A flat 2,500 kcal daily target, regardless of whether you did a recovery spin or a 4-hour ride with intervals. That makes it easy to under-fuel a hard day without realising.

M0VE Nutrition reads your training data from Strava, Intervals.icu, or Apple Health and bumps your daily targets up on hard days, drops them on easy days, and gives you a per-session carb target you can actually use. Free 14-day trial.

You should never have to accept a stranger's sandwich on a mountain pass again.