Your muscles carry about 90 minutes of glycogen. Ride longer than that without eating and you will hit the wall, hard. I know because I ended up begging a stranger for a punnet of strawberries on the side of the road outside Skerries.
Key Takeaways
Under 90 minutes, you don't need to bring anything. Your muscles have enough glycogen stored to cover it. Eat normally before, eat normally after, and you're fine. Once you go over that threshold, you need to fuel on the bike, and the mistake most people make is waiting until they're hungry. Sean Kelly's line covers it: if you're hungry, it's too late. Set an alarm for every 30 minutes and eat something small and sugary each time. Your body absorbs small amounts of carbohydrates better when they're spread out than when you dump a large amount in one go at the hour mark.
On longer rides, four hours or more, think of it as a sliding scale. Real food at the start: wraps cut into sections, croissants chopped into three pieces, cereal bars. Towards the end, when you're tired and digestion is harder work, move to simpler stuff. I make energy balls in batches once a week, dates, honey, oats, natural ingredients you can actually read on the label. That's what bridges the gap between real food and the sports-specific stuff you might need at the very back end of a hard race. Gels and commercial bars with 40-letter ingredients are a last resort, not a strategy.
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If you want the full picture on nutrition and training, the Barry Murray episode goes deep on how World Tour riders actually fuel. The fasted training episode is worth reading alongside this one if you're trying to work out when to ride without eating at all.