The bonk — that hollow, leg-emptying crash when you run out of fuel — is almost always avoidable. It's not bad luck or bad legs. It's a maths problem you didn't solve before the ride started.
Carbohydrate is the currency
Your body stores only 90–120 minutes of hard riding as glycogen. Beyond that, performance depends entirely on what you take in. The current guidance:
- 1–2 hours: 30–60g of carbohydrate per hour.
- 2–3 hours: 60–90g per hour.
- 3+ hours / racing: up to 90–120g per hour, using a glucose : fructose mix (around 2:1) so your gut can absorb more than glucose alone allows.
Don't forget the salt
Carbs get the headlines, but on long, hot rides sodium is what keeps cramps and fading concentration at bay. Generic numbers are a starting point, but sweat rate and salt loss vary enormously between people — which is exactly what a sweat test measures.
The most common fuelling mistake isn't the wrong product — it's starting too late. Begin eating in the first 30 minutes, before you feel you need it.
Train your gut
Taking on 90g of carbs an hour while your heart rate is high is a skill. The gut adapts like any other system:
- Practise race fuelling on your long rides, not just on race day.
- Build the volume gradually over weeks.
- Lock in the exact products and timing you'll use when it counts.
A simple working plan
For a four-hour ride:
- Every 20 minutes: a gel, a few chews or a bite of a bar — roughly 25–30g carbs.
- Every bottle: 500ml with carbohydrate and electrolytes.
- From the gun: start fuelling early and keep the drip-feed steady.
Solve the maths before you roll out, rehearse it in training, and the bonk stops being something that happens to you.
