The main symptoms of bonking

A bonk hits both body and brain. The most common signs are a sudden power drop with heavy legs, dizziness or shakiness, and brain fog that makes it hard to focus or make decisions. You may also feel weak and uncoordinated as your body switches to slower fat-burning, and many riders notice a racing heart as the body tries to compensate. Finally, bonking can mess with temperature control, causing cold sweats, clammy skin, chills, or trembling.

The science behind bonking (simple explanation)

Glycogen is your body’s fast fuel for riding, especially when intensity rises - climbs, surges, headwinds, or pushing the pace. When glycogen stores drop too low, your body shifts to burning more fat, but fat oxidation can’t meet the immediate energy demand of harder cycling. Meanwhile, blood glucose can fall significantly, and because the brain runs largely on glucose, mental symptoms appear: confusion, irritability, and poor decision-making.

Your body also releases stress hormones like cortisol and adrenaline to try to mobilize energy from fat and protein stores. But that response often isn’t enough to sustain high output, and it can actually intensify the “crash” feeling like racing heart, sweating, shakes, while your muscles still don’t have the fuel they need.

How to prevent bonking

Bonking is best prevented by fueling properly from the start. Begin eating within the first 30–45 minutes of your ride and continue consistently, aiming for 30–90 grams of carbohydrates per hour, depending on the ride.

As a guide, take around 30g per hour for moderate endurance rides of 1.5–2 hours at a steady pace. For harder group rides, or interval training under 2 hours, aim for 60g per hour, and increase to around 90g per hour for longer rides (3+ hours).

Use a mix of gels, bars, bananas, or carbohydrate sports drinks rather than relying on water alone. Stay hydrated and replace electrolytes, especially on long or hot rides. Before the ride, eat a carbohydrate-rich meal 2–3 hours beforehand (such as oats, toast with banana, or rice) to top up glycogen stores. The key is simple: start fueled, eat early, and eat regularly.

How to recover from a bonk

If you bonk, your goal is to stabilize quickly and avoid digging an even deeper hole.

Stop and rest. As soon as you notice symptoms like extreme fatigue, dizziness, or mental fog, stop riding. Trying to push through usually makes things worse and can be unsafe.

Refuel quickly. Your body needs fast carbohydrates immediately to raise blood sugar. Choose quick-digesting options like energy gels, a banana, or a sports drink, and aim for 20–40 grams of carbs right away.

Hydrate. Bonking often comes with dehydration. Drink water, or ideally a sports drink with electrolytes to replace fluids and minerals like sodium and potassium.

Eat a balanced post-ride meal. Once you’re stable, have a proper meal with carbs, protein, and healthy fats. Carbs replenish glycogen, protein supports muscle repair, and fats help provide longer-lasting energy. A simple example: rice or pasta with lean protein and vegetables.

Rest and recover. A full bonk can take time to recover from. Give yourself 24–48 hours before attempting another intense session and prioritize sleep to speed up recovery.

Can you bonk while training?

Yes. Bonking can happen during training just as easily as on event day. It’s most common in longer rides or hard sessions where energy expenditure outpaces what you’ve eaten. The risk is higher if you start training with depleted glycogen (missed meals, low intake earlier in the day) or don’t refuel during the workout.

The best prevention is simple: prioritize nutrition and hydration, and don’t wait until you’re hungry to start eating. Fuel early, fuel consistently, and treat it as part of the session, because it is.