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The first rule is the dullest: never try anything new on the day of an event.
The second: two hours before exercise, favour carbohydrates you can digest without thinking about it. For most people, that means bread, rice or a banana. Not an elaborate smoothie.
The third: drink before you are thirsty, but without overdoing it. Five hundred millilitres in the two hours beforehand is almost always enough.
The rest — gels, electrolytes, caffeine — only makes sense beyond ninety minutes of effort. Below that, it is marketing.
