What really happens when you bonk on the bike

Bonking is the sudden, dramatic drop in power and mood that turns a steady ride into a slow crawl. It isn’t just “getting tired.” Bonking is a physiological event driven by low liver glycogen, falling blood glucose, and depleted muscle glycogen that disrupts both muscle and brain function. The good news: with the right training and fueling, it’s preventable.

The physiology of bonking

Your body stores carbohydrate as glycogen in two main places: muscles (the on-site fuel for contracting fibers) and the liver (maintains blood glucose for the brain and working muscle). During moderate-to-hard riding, carbohydrate supplies a large share of your energy. When these stores run low, several things happen:

• Blood glucose drops as liver glycogen empties. The brain relies on glucose, so perception of effort spikes and coordination suffers.
• Muscle glycogen falls, limiting glycolysis and impairing calcium handling in the muscle. Power output at a given cadence feels impossible.
• Catecholamines and stress hormones rise, increasing RPE and sometimes causing lightheadedness, cold sweats, and irritability.

On the bike this looks like holding 230–260 watts comfortably and then, within minutes, struggling to turn the pedals at endurance pace. Heart rate may drift down despite maximal effort, and your ability to respond to surges disappears.

Bonking is primarily a carbohydrate availability problem, amplified by intensity choices and poor fueling timing.

Intensity, pacing, and why surges drain the tank

Carbohydrate use rises disproportionately as you cross key thresholds. Below your first ventilatory threshold (often called endurance or Zone 2), fat contributes more. As you move through tempo and threshold (around 80–100% of FTP), carbohydrate dominates. Repeated surges above FTP can burn through glycogen quickly, even on a ride that isn’t very long.

• Endurance (Z2): sustainable, higher fat oxidation, slower glycogen use.
• Tempo/threshold: carbohydrate-heavy; glycogen falls faster.
• Above threshold: very high carbohydrate burn; limited by glycogen and lactate tolerance.

Practical takeaway: if your goal is to finish strong, cap efforts near the top of your planned zone and avoid unnecessary spikes. Smooth pacing spares glycogen, even if average watts are the same.

Train your body to delay the bonk

Two levers reduce bonking risk: improving how you use fuel and improving how you take in fuel.

• Build endurance capacity: 2–4 Zone 2 rides per week increase mitochondrial density and fat oxidation, sparing glycogen at a given power.
• Keep the hard stuff hard: interval sessions (sweet spot to VO2 max) raise FTP and your economy, so you can hold more watts at a lower fractional cost.
• Practice your gut: regularly take 60–90 g of carbohydrate per hour in training. Progress toward 90–120 g/h with mixed carbs (glucose + fructose) if you race or do hard group rides.
• Use carbohydrate periodization: fuel the key sessions generously; keep some easy rides lower-carb if desired, but avoid doing many long or intense sessions with low glycogen.

Low-glycogen sessions can be a tool for experienced riders at very low intensity, but use sparingly. They reduce quality and increase illness risk if overused. Never do high-intensity work in a low-fueled state.

Fueling to avoid bonking

Before the ride

• 24–48 hours: for big days, emphasize carbs across meals; target 7–10 g/kg/day for events or training camps.
• 1–4 hours pre-ride: 1–4 g/kg carbohydrate, low fiber/fat. Examples: rice and eggs; oatmeal with banana and honey; rice cakes and yogurt.
• Caffeine optional: 2–3 mg/kg 45–60 minutes before can lower perceived effort.

During the ride

Choose multiple transportable carbohydrates (glucose/maltodextrin + fructose) to raise absorption and oxidation rates.

Set a timer every 15 minutes and alternate sips of bottle and bites of food. Front-load slightly in the first hour to stay ahead of depletion.

After the ride (recovery)

• If another session is within 24 hours: 1.0–1.2 g/kg/hour carbohydrate for 3–4 hours, plus 20–30 g protein soon after finishing.
• Rehydrate with ~150% of fluid lost and include sodium to restore plasma volume.
• For normal day-to-day recovery: prioritize total daily carbs and balanced meals over exact timing.

Warning signs and what to do mid-ride

• Signs: sudden weakness, dizziness, nausea, irritability, chills, tunnel vision, RPE spikes at usual watts, inability to respond to wheels.
• Immediate actions: reduce to Zone 1, take 30–60 g fast carbs (gels/chews), sip fluid with electrolytes, and give it 10–15 minutes. If symptoms are severe (confusion, near-fainting), stop and seek help.

Simple fueling plans you can copy

3-hour rolling group ride (tempo with surges)

• Breakfast 2–3 g/kg carbs 2–3 hours before.
• On-bike: aim for 75–90 g/h using a mix of bottles and solids. Example per hour: 500 ml bottle with 30 g carbs + one gel (25 g) + half bar (20–30 g).
• Pacing: cap long pulls at 85–90% FTP; sit out sprints unless they’re the point of the day.

2 x 20 min @ 95–100% FTP within a 2-hour ride

• Pre-fuel 60–90 g carbs within the hour before.
• On-bike: 60–75 g/h; start fueling in the warm-up. One gel 5 minutes before each interval helps.
• Cool down and take 20–30 g protein plus 1–1.2 g/kg carbs if you train again tomorrow.

Make bonking rare

Bonking is not a badge of honor; it’s a preventable limit. Train your aerobic engine, pace within your training zones, and match carbohydrate intake to duration and intensity. Monitor how different fueling strategies feel at the same watts, and practice them long before event day.