We realise that to run a marathon in 3 hours (or 2 or 4 for that matter) will cost us a certain amount of carbohydrate stored in glycogen and a certain amount stored in fats. We top this up with carbohydrates during the run and we have more or less success (in a nutritional sense)depending on how good we are at getting the right quantity into our system at the right time. But the complicating factor is that we sweat. On a hot day we sweat a lot, on a cool day we sweat a little and on a cold day we only sweat icecubes. But our carbohydrate intake is often linked directly to our re-hydration schedule. This is because for many of us we rely on carbohydrate drinks of various forms to provide our water, salts and carbohydrates in fixed quantities. We use the same formulas for a deadly hot day when our highest priority (and most impossible task) is to replace our salts and water, as we do for a freezing cold day ,when all we really need is carbohydrates (because we’ve lost little due to sweating).
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