Bonking in cycling: what it is, symptoms, how to recover and how to avoid it

My most memorable bonk happened alone, in a remote part of central Portugal with 90km of a 160km ride left to tackle, most of which was against a headwind.

In just a few short minutes, I was overcome with full-body fatigue and an inability to concentrate that felt like the flu; a truly awful experience. Unfortunately, I couldn't shake it fully, even after eating and resting, and I suffered the whole way back.

In this guide, we will help you understand the causes of bonking and how to avoid it. We'll cover what it is, how to spot it, how to recover and provide some famous examples of bonking in professional cycling.

Glycogen depletion explained

Bonking is the popular name given to hypoglycaemia or low blood sugar. If you don't replace your limited liver glycogen – the stored form of glucose that feeds blood glucose – during exercise by eating or drinking carbs, your glycogen stores eventually run out, and your tightly regulated blood glucose begins to drop.

Because your brain relies mostly on glucose from the blood to function, when it detects a drop in blood glucose beyond a certain point, it sends signals to the body to reduce energy expenditure in order to preserve glucose for itself, and this is the sensation of bonking, where it feels impossible to continue at even a moderate intensity. Other symptoms include weakness, dizziness, difficulty concentrating and mental fog.

Again, bonking is mostly avoidable if you continue to eat while you ride. Typical recommendations for fuelling exercise range from 30–90g of carbs per hour, or up to 120g/h for professionals.

Exercise intensity and glycogen depletion

Research on exercise to exhaustion estimates that glycogen stores can fuel 90 to 120 minutes of high-intensity exercise.

Exercise intensity plays a big role in how quickly your glycogen stores are depleted: high-intensity exercise uses a lot of glycogen, with very little energy coming from fat. Low to moderate-intensity activity uses less glycogen overall, and a higher percentage of total energy comes from fat.

A 2020 research review by Spriet and Randell estimates that at <40% of your VO2max, fat supplies more energy than carbohydrate. As energy demands increase, glucose, the faster-acting energy source, provides an increasingly higher percentage of energy. At 40–65% of VO2max, it's around half and half, and at around 85% of VO2max, it's mostly glycogen.

What are common bonking symptoms?

Having experienced a few textbook bonks myself, I can tell you one thing: when you bonk, you know it; it's an incredibly unpleasant ordeal.

When you hit the metaphorical wall, your muscles feel extremely weak, and every pedal stroke becomes a struggle. It's as if your legs have turned into lead weights.

When you bonk, it can be hard to stay focused on the road ahead, keep up a conversation or even make sound decisions, which can get you into real trouble. This energy drop is often swift, too: one moment you feel fine, and the next, it's as if all your energy has vanished.

If you're unsure of whether or not you've bonked, you can check your experience against the following list.

Bonking symptoms checklist

The UK's National Health Service (NHS) lists the symptoms of hypoglycaemia as the following:

• Physical: heavy, weak legs, a sudden drop in power, trembling muscles, sweating, a racing heart, light-headedness and intense hunger
• Mental: brain fog, confusion, blurred vision and poor decisions
• Emotional: irritability, anxiety or sudden despair

If someone becomes confused or unresponsive, treat it as an emergency. Riders with diabetes should follow their hypo plan.

Bonking in cycling vs running

Hitting the wall is the same whether you're running or cycling, but the reasons they occur and the results can differ.

Firstly, it's easier to eat on the bike, so runners may eat less than cyclists while competing, meaning a higher likelihood of bonking.

Another important factor in cycling is the ability to freewheel or soft-pedal in a group or on a descent, saving energy and allowing you to keep moving even if you have bonked.

For triathletes, the run comes last, and it's harder to eat while running than while cycling, so it's the most likely place where you'll experience a bonk, especially on long-distance events like IRONMAN.

Famous bonks in cycling

If the symptoms checklist above sounds familiar, then we'd like to welcome you to the club. But don't feel bad; we've all experienced it, and it happens to the best of us (literally), as you'll see below.

One of the most famous examples of bonking among pro cyclists is the case of Lance Armstrong during the 2000 Tour de France. In stage 16 from Courchevel to Morzine, Armstrong bonked on the slopes of the Col de Joux-Plane, a challenging mountain pass, while wearing the yellow jersey. He struggled to maintain his pace and lost almost two minutes to Jan Ullrich. In a later interview, he put it down to his failure to eat enough.

Another famous case of hitting the wall occurred during the 2013 Tour de France, when race leader Chris Froome bonked 5km from the finish on Alpe d'Huez. "It's not the first time in my career I've run out of sugar," said Froome, who still won the Tour that year.

A third example of bonking by the best was when five-time Tour de France winner Miguel Indurain bonked during the 1996 edition. On the final climb on stage 7, he lost more than three minutes, with a 20-second penalty for taking drinks too late.

How to recover from bonking

If you recognise a bonk early and quickly consume a sufficient amount of easily digestible carbohydrates, with a little bit of rest (15–30 minutes), you may recover and be able to continue at a reasonable intensity.

In most cases of bonking, the recovery is much longer. It can take 20–24 hours for the body to fully replenish glycogen when it's fully depleted, especially if you don't consume enough carbohydrates during the recovery period.

What to do when you bonk – step by step

If you pass the point of no return and hit the wall hard, here's what to do:

1. Stop or ease off completely. Common symptoms of hypoglycaemia, like light-headedness, weakness and confusion, increase the likelihood of crashing, so it's best to stop for a few minutes as soon as it's safe and you can get food or continue very slowly until you find somewhere to buy food.
2. Eat simple, fast-acting carbs. Treat low blood sugar with orange juice, a can of full-sugar cola or a handful of jellies. Start with pure sugar and avoid fat, protein and fibre.
3. Wait at least 15 minutes. The severe symptoms of low blood sugar should fade after you consume sugar. Wait until you feel better, then eat something else like a banana before setting off. If you have a long ride home, wait longer.
4. Ride home easy. Take the shortest, flattest route home and ride easy. Expect to feel tired and unable to produce much power.
5. Refuel properly. Roughly 1g of carbohydrate per kg of bodyweight per hour is recommended for the first 4 hours after finishing, with plenty of fluids to accompany the food.

Low blood sugar after exercise

If you don't eat enough in the period after a long or high-intensity ride, you may experience low blood sugar. The symptoms include feeling shaky, irritable or very hungry.

After exercise, your body needs to replenish its liver stores, so the amount of glucose your muscles take up to rebuild glycogen stays elevated for up to 48 hours. That's why it's important to eat enough carbohydrate following exercise for proper recovery. In non-diabetic people, it's unlikely that you will experience true hypoglycaemia post-exercise. If you have diabetes, seek medical advice from your doctor for fuelling exercise.

To ensure you're well-fuelled, follow the guideline above: 1g/kg of bodyweight in the four hours post-ride.

6 tips on how to avoid bonking

Proper nutrition and hydration

Cyclists bonk due to the depletion of glycogen stores. To avoid this, it's crucial to maintain proper nutrition and hydration throughout your ride and fuel properly beforehand.

During your ride, fuel your body with easily digestible carbohydrates such as energy gels, sports drinks and energy bars. Stay hydrated by drinking fluids regularly, and add some electrolytes if you determine that your body needs them.

How many carbs per hour?

As we mentioned briefly above, riders need 30–90g per hour, depending on how long and how hard they ride:

• Under 90 minutes: 0–40g, depending on intensity
• 90 minutes to 2.5 hours: 30–60g
• Over 2.5 hours: 60–90g
• Above 90g: only once you've practised it

Our guide to carbs per hour for cycling explains how to hit those numbers with bottles, gels and bars.

Plan your pre-ride meal

The day of your ride, ideally 90 minutes to 4 hours before it starts, eat a meal consisting mostly of easily digestible carbohydrates, and keep fibre and slow-digesting whole-grain carbs, protein and fats to a minimum. Always try to eat familiar foods before exercise, especially if it's an event you've been training for.

Examples include porridge oats with honey and banana, white bread toast with jam, or a bagel with sliced banana, honey and a thin layer of peanut butter. White rice or pasta with a simple, low-fat sauce are also great options.

It usually surprises people when they learn how many carbohydrates are needed to fuel a long ride. The following example will give you an idea of how many carbs you can eat, with research recommending anywhere from 1–4g per kg of bodyweight.

For a 70kg cyclist aiming for 2g/kg, this would be:

• A large bowl of porridge with 80g of oats, one banana and one spoon of honey
• Two slices of white bread toast with jam

To many of us, that amount of food sounds like an awful lot for one sitting, but it's actually the lower-middle of the recommended range.

Pace yourself

Next to underfuelling, riding at high intensity for too long is the most likely reason you bonk. To put it simply, the harder you go, the more glycogen your body uses per minute of exercise, and it increases sharply, as this graph by INSCYD illustrates.

Pay attention to your heart rate or perceived exertion and adjust accordingly to slow glycogen depletion.

It's always important to know your limits, such as avoiding the front if you're with riders who are much stronger than you.

Take regular breaks

On longer rides, taking short breaks is beneficial for recovery. Use these breaks to refuel and rehydrate.

If you're riding in a group and stopping isn't your decision, this could mean sitting 3 or 4 wheels from the front to maximise the draft but minimise the sudden accelerations from the accordion effect at the back of the group.

If you're riding alone, listen to your body and rest when needed. Even a few minutes of rest with a snack can help recharge your energy levels and prevent the bonk.

Train your gut and practise fuelling

Just like your legs and lungs, your gut can be trained to digest carbohydrates better while you're riding, so practise eating on the bike, starting with small amounts and increasing gradually until you're comfortable eating 90g (or more) per hour.

For more information on how to train your gut to avoid GI distress while riding, check out our guide.

Practise your fuelling plan on ROUVY

A controlled environment, like when training indoors on ROUVY, is a great place to practise your fuelling plan for a long ride, get used to opening and eating your food with one hand, and train your body to digest carbohydrates while you exercise.

Firstly, you've got quick access to a toilet. You also don't have to deal with potholes, obstacles and traffic while you get used to the mechanical movement.

Practising on your indoor trainer means you can focus on how you feel at each gram-per-hour increment, and increase it little by little each training session, backing off and allowing time to adapt if you reach a point that feels uncomfortable.