Why You're Still Bonking at the End of Your Ride

The bonk is usually diagnosed as a fitness problem. It isn't.

Most athletes who bonk in the final hour of a long ride are fit enough to finish. Their legs have the aerobic capacity. What they don't have is fuel. And the two feel identical when it's happening, which is why the misdiagnosis is so common.

Understanding why the bonk happens — and what's actually running out — changes how you approach the problem.

What's Actually Depleting

Your body stores carbohydrates as glycogen, primarily in your liver and muscles. Those stores are finite. A trained athlete holds roughly 400–600g of glycogen — approximately 1,600–2,400 calories — enough to sustain moderate to high-intensity exercise for about 90 to 120 minutes before levels become critically low (Jeukendrup, 2011).

At that point, your body doesn't run out of energy. It shifts its strategy. Fat oxidation increases, intensity drops, and your brain — which runs almost exclusively on glucose — starts to feel the deficit. Focus narrows. Decision-making slows. Legs that were turning over fine now feel like they belong to someone else.

That's the bonk. It's not a fitness failure. It's a fuelling miscalculation.

Where Most Athletes Go Wrong

The mistake usually happens in one of three places.

The first is starting too late. Many athletes don't begin fuelling until they feel hungry or fatigued — by which point glycogen levels have already dropped substantially. The window for preventive fuelling has closed. You're now playing catch-up against a deficit your body can't fully reverse mid-ride.

The second is not fuelling enough. The research on carbohydrate oxidation during moderate to high-intensity exercise consistently shows that athletes need 30–90g of carbohydrates per hour to maintain performance (Burke et al., 2011). Most athletes consume significantly less — often because they underestimate the demand, or because the cost and palatability of their products makes consistent intake difficult.

The third is a mismatch between what they're consuming and what their body can actually absorb. Not all carbohydrates are created equal in this context. The intestine uses specific transporter pathways to move carbohydrates into the bloodstream, and those pathways have limits. A formula optimized for how those pathways work — rather than simply adding more carbohydrates — delivers more usable energy to working muscles without the gut distress that pushes athletes off their nutrition plan entirely.

The Fix Is Simpler Than It Sounds

Start fuelling within the first 20 minutes, before you feel like you need it. Set a consistent cadence — a sip every 15–20 minutes rather than waiting until you're thirsty or depleted. Target 60–90g of carbohydrates per hour depending on intensity and duration. And train with the same nutrition you plan to use in events, giving your gut time to adapt to consistent intake.

The final hour of a long ride shouldn't be a survival exercise. It usually becomes one because of decisions made in the first hour — or the lack of them.

Fuelling early and consistently is the discipline that separates athletes who finish strong from those who just finish.