Any marathon runner knows that the diet before the race is as important as training, but drinking regularly during the race can make the difference between running a good time or not. In fact, dehydration is one of the most common reasons marathon runners end up in the medical tent. In very hot or humid environments, an athlete can lose nearly three litres of water per hour, or a total of six litres during a marathon. So it is no accident that the organisers set up “watering holes” for the runners every few kilometres.
Even mild dehydration results in suboptimal performance. For example, water loss equivalent to 1-2% bodyweight, (0.7-1.4 litres in a 70kg subject), may reduce performance, while a 3-4% water loss reduces performance by 5-20%. The decrease in performance parallels the rapidity with which dehydration develops. By the time the athlete feels thirsty, it is too late. Thirst occurs only when water loss reaches about 1% of total body weight loss, i.e., when performance has already decreased.
While proper planning by the athlete is important in preventing dehydration, the taste and sodium content of a beverage can impact both the voluntary drive and the involuntary stimulus to drink. The optimum carbohydrate-electrolyte beverage is lightly sweetened and does not require further dilution. Sodium should be included at a level that approaches the rate of sweat loss (roughly 1.2g/l), but that doesn’t interfere with palatability.
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