What actually happens when you bonk
Most riders describe a bonk as "suddenly feeling tired," which massively undersells it. It's closer to the moment a phone screen dies mid-call — not fading, just off. Muscle contraction becomes laboured, your legs turn into something resembling wet concrete, and your brain slows to a crawl. Coordination wobbles. Nausea is common. Some riders have pulled over and sat on the kerb for twenty minutes, unable to explain to passing cars why they can't move. This is glycogen depletion doing what it does: withdrawing the fuel your working muscles depend on at the worst possible moment.
Your body stores carbohydrate as glycogen, split between your liver (roughly 80–100g) and your muscles (400–500g). During moderate to hard cycling, glycogen is your primary fuel. Fat provides background energy, but it can't cover the demands of anything above a steady endurance pace on its own — the biochemistry simply doesn't run fast enough. Once the glycogen supply drops below a critical threshold — and interestingly, that threshold is not full depletion; usually 10–30% of the original stores still remain — the wheels come off. Getting your daily nutrition for cyclists right is the first line of defence, because how you eat day-to-day directly determines how much glycogen you start a ride with.
The science behind the shutdown
For a long time the standard explanation was simple: glycogen runs out, legs stop. That's still broadly true, but recent research has added important nuance. A major review published in Endocrine Reviews now points to exercise-induced hypoglycaemia — a drop in blood glucose rather than muscle glycogen depletion alone — as the central trigger for performance collapse. The liver is the key player: it normally releases glucose into the bloodstream to keep your brain and muscles fuelled, but during prolonged hard effort that supply fails to keep pace with demand. Blood glucose falls. Your brain, which runs almost entirely on glucose, starts to panic, and it expresses that panic by shutting down non-essential processes — like the ones keeping you upright on a bike.
This reframe matters for practical fuelling. It explains why eating and drinking during a ride can reverse a near-bonk even when muscle glycogen is already quite depleted. The goal of on-bike nutrition is not to reload muscle glycogen mid-ride — that is mostly impossible during exercise — but to stabilise blood glucose and reduce the liver's burden. Once you understand that distinction, the advice to "eat early and often" stops being a vague platitude and starts to make physiological sense. You're not filling a tank; you're maintaining a supply line. The two feel similar in practice but have meaningfully different implications for when, how much, and what type of carbohydrate you eat.
It also explains the cognitive impairment that comes with a bonk. The brain consumes around 20% of the body's glucose at rest and has no meaningful glycogen storage of its own — it depends entirely on moment-to-moment blood glucose. When that supply drops, decision-making slows, spatial awareness deteriorates, and risk perception dulls. Riding in that state, particularly on descents or in traffic, is not just uncomfortable. It's genuinely dangerous. A bonk is a physiological emergency, not a fitness problem.
The numbers that actually matter
Let's be clear: most cyclists who bonk do so not because they didn't know they needed to eat, but because they underestimated how much. The gut has a ceiling on how much carbohydrate it can absorb per hour — roughly 60g when relying on glucose alone, and up to 90g when you mix glucose and fructose, which use different intestinal transport channels and don't compete. These are not arbitrary targets; they reflect the hard limits of intestinal physiology. For rides under 75 minutes at moderate intensity, stored glycogen is typically enough and on-bike fuelling is optional. From 75 minutes to around two hours, 30–60g per hour is sensible. Beyond two hours, especially at race intensity or in heat, 60–90g per hour is where most serious riders operate. Understanding your exact carb targets for cyclists by ride duration and intensity will sharpen these numbers for your specific training load.
Timing matters as much as quantity. The most common mistake is waiting until you feel flat before eating. By the time you notice you're flagging, blood glucose is already declining and it takes 20–30 minutes for ingested carbohydrate to reach the bloodstream in useful form. Eat before you're hungry. Many coaches advise starting to fuel 20 minutes into any ride longer than 90 minutes, and eating every 20–30 minutes after that, irrespective of how you feel. There is no performance benefit to waiting. There is a significant cost to waiting too long. And if you're planning a particularly hard or long block of training, reading up on the full framework of cycling fuelling and hydration will help you see how on-bike carbs fit into the bigger picture.
Pre-ride nutrition is where a lot of energy goes wrong before the ride has even started. Overnight, the liver uses most of its glycogen supply to maintain blood glucose while you sleep — you wake up with liver stores near zero. A solid meal two to three hours before any effort over 90 minutes is not optional. That meal should be carbohydrate-dominant and reasonably low in fibre and fat, both of which slow gastric emptying. 100–150g of carbohydrate in that pre-ride window is a sensible target for most riders. It's not about being overly regimented; it's about not starting a long effort already in deficit.
The mistakes that cause most bonks
Riding too hard too early is underrated as a cause of bonking. At zone 4 and above, carbohydrate oxidation accelerates sharply while fat oxidation contributes progressively less. A rider who goes out hard on a long ride burns through glycogen at a rate that outpaces what the gut can absorb, even with good fuelling. Experienced riders learn to respect this by starting long efforts conservatively, even when they feel strong — burning matches in the first hour is a reliable route to a bonk at kilometre 80, regardless of how well you've eaten. Pacing and nutrition are not separate decisions; they interact directly.
Heat amplifies everything. Higher core temperatures increase carbohydrate oxidation rate, and sweating reduces plasma volume, which makes glucose delivery to working muscles less efficient. Dehydration is not just a cramp risk — it degrades the whole fuel delivery system. Even a 2% drop in body weight from fluid loss measurably impairs performance. On a hot day, your fuelling requirements go up and your gut's tolerance for concentrated carbohydrate goes down, which creates a genuine logistical challenge. Lighter carbohydrate solutions and higher fluid volume tend to work better in heat than gels taken with minimal water.
One final mistake worth flagging: assuming that because you've ridden the distance before, you know how to fuel it. Conditions shift. A week of poor sleep reduces glycogen synthesis efficiency. High work stress elevates cortisol, which increases fuel burn even at rest. A viral illness two weeks back may have left your gut more sensitive than usual. Bonking is not a beginner problem. It happens to experienced riders who assume their usual pattern is sufficient and skip the basics — eating enough, starting early, staying consistent throughout the ride. The basics are basic for a reason: they work every time, without exception.
Related reads
Sources
- Carbohydrate Ingestion on Exercise Metabolism and Physical Performance. Endocrine Reviews, 2025. https://academic.oup.com/edrv/advance-article/doi/10.1210/endrev/bnaf038/8432248
- Muscle Glycogen Depletion Following 75-km of Cycling. Frontiers in Physiology / PMC, 2016. https://pmc.ncbi.nlm.nih.gov/articles/PMC5037214/