Most Cyclists Under-Fuel Long Training Rides by 40–60%. The Bonk That Follows Is Not a Sign of Low Fitness. It Is a Predictable Consequence of a Calculable Fuelling Deficit.

Cycling folklore frames the bonk as a rite of passage — something that happens to riders who haven't suffered enough, who are not yet adapted, whose fitness is lacking. The actual mechanism has nothing to do with fitness. The bonk is a calculable event that occurs when the difference between carbohydrate expenditure and carbohydrate intake exceeds the available glycogen reserve. It is arithmetic, not character.

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The Energy Expenditure of a Long Ride

Carbohydrate is the primary fuel for cycling above approximately 65–70% of FTP. The carbohydrate expenditure rate depends on power output and duration:

A 70kg cyclist riding at Zone 3 tempo for 4 hours is utilising approximately 300–360g of carbohydrate from both endogenous and exogenous sources. Endogenous glycogen stores (muscle + liver combined) provide approximately 300–400g total. Starting a 4-hour ride at tempo intensity with no exogenous carbohydrate is physiologically marginal at best.

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The Standard Carry vs The Required Carry

Field observation at cycling sportives and club rides consistently reveals the same pattern: riders carrying two to three gels for 4+ hour rides. At 25g carbohydrate per gel, this represents 50–75g exogenous carbohydrate — roughly 15–25% of the total requirement.

The remainder is drawn from endogenous glycogen, which depletes progressively. The consequence is not linear — glycogen depletion does not produce proportional, gradual performance decline. The performance curve is relatively flat until glycogen falls below approximately 25% of starting stores, then declines rapidly. This non-linearity is why riders often report feeling strong for 3.5 hours and then abruptly unable to maintain pace — the depletion threshold has been crossed.

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What Adequate Fuelling Looks Like on a Long Ride

For a 3+ hour cycling session:

First intake: 15–20 minutes into the ride. Not at the start. Not after the first climb. 15–20 minutes, regardless of how full you feel from the pre-ride meal.

Rate: Match carbohydrate intake to carbohydrate expenditure as closely as possible:

• Zone 2 ride: target 40–50g/hr (approximately 1 gel per 30–40 minutes)
• Zone 3 tempo: target 60g/hr (1 gel per 25 minutes)
• Mixed intensity with threshold efforts: target 60–75g/hr

Format mix: For rides over 3 hours, gels alone become monotonous and may trigger palatability fatigue. A combination of gels, energy bars, and real food (bananas, rice cakes, dates) maintains palatability and variety. The bar or real food component can be consumed on flatter sections at lower intensity where the mechanics of chewing are less compromised.

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The Hydration Pairing on the Bike

Unlike running, cycling provides natural opportunities for fluid intake. Each gel should be paired with 150–200mL water — available from a water bottle carried on the bike. This is not optional; it is the osmolality management requirement for concentrated gel absorption.

Ride duration and temperature determine total fluid requirements. In temperate conditions (15–20°C), a 4-hour ride typically requires 0.5–0.8L/hr fluid — 2–3 bidons depending on bottle size. In hot conditions (25°C+), 0.8–1.2L/hr may be necessary. Plan bottle refills at petrol stations, cafes, or service stops accordingly.

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Training Ride Fuelling vs Race Ride Fuelling

A common and counterproductive practice: training rides completed under-fuelled to "train fat burning," then racing fully fuelled. The training adaptation argument for fasted or low-carbohydrate rides is valid in specific contexts (Zone 2 base training, metabolic flexibility development). It does not apply to threshold, tempo, or interval sessions, where glycogen is the primary substrate and under-fuelling compromises the quality of the training stimulus.

For interval and threshold training rides: fuel the session as you would the race. For Zone 2 base aerobic rides: modestly reduced carbohydrate intake is a legitimate training tool that is separate from the race fuelling conversation.

For cyclists calculating gel requirements for a long training ride or cycling event — based on duration, effort level, and gel carbohydrate content — the energy gel calculator at winsport.uk/tools/nutrition/energy-gel-calculator generates gel count, g/hr target, and a timed schedule showing exactly when to reach for each gel across the ride.

What is your standard gel carry for a 4-hour ride — and has the quantity ever been determined by calculation rather than habit?