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Combining energy and fluid

Especially during prolonged exercise when carbohydrates reserves become depleted, carbohydrate intake in addition to fluid intake is important. However, it is known that with increasing carbohydrate intake (increasing carbohydrate concentration), the absorption of fluid may be impaired. Hence sports drinks are always a compromise between delivering energy and delivering fluid. Most sports drinks are in the 4 - 8 % carbohydrate range where the impairment in fluid absorption is still acceptable. However, in the > 10% range it is generally thought that both gastric emptying and absorption of fluid are hampered even though it may result in a greater delivery of carbohydrate. Interestingly we recently advised very high carbohydrate intakes for prolonged exercise in the region of 90 g/h (1,5g/min). When such large amounts of carbohydrate were ingested in the form of glucose+fructose the delivery of carbohydrate to the working muscle was improved and performance was increased 8% more than with a traditional sports drink containing one type of carbohydrate.

Such large amounts of carbohydrate can only be delivered in concentrated carbohydrate solutions unless very large volumes of fluid would be consumed. For example, to ingest 90 g of carbohydrate per hour, one would have to drink 750 ml of a 12% carbohydrate solution or 1,5L of a 6% carbohydrate solution. Ingesting 1,5L/h is not always practical or even possible and therefore one would have to resort to drinking a more concentrated solution. However according to existing information this would reduce fluid delivery. In studies where stable isotopes were used to label water and study fluid delivery it was demonstrated however that mixtures of glucose and fructose result in a faster rate of gastric emptying and a superior fluid delivery compared with a single carbohydrate. Therefore in situations where both carbohydrate and fluid delivery are important and the excercise duration is >2h it would be advised to take solutions with multiple transportable carbohydrates and ensure an intake of 60-90g/h.