Thirst is a poor indicator of the need for fluid replacement - athletes must train themselves to drink regularly to maintain hydration. By the time an athlete is 2% dehydrated, their performance can be markedly impaired (up to 15%).
Athletes should ensure that they are well - hydrated in the 3 hours leading up to an endurance event by initially drinking between 500 and 1000ml of water or dilute glucose - electrolyte solution, then 300 - 500 ml of water in the 30 minutes prior to competition. During the ensuing competition, fluids should be replaced at the rate of approximately 150 - 200 ml every 20 minutes. The rate of fluid lost through sweating can exceed 1 liter per hour - the goal is to replace this loss by regular fluid intake. This can be in the form of water, but there is increasing evidence that solutions of up to 8% glucose can be beneficial, especially in events lasting over 90 minutes. The exercising athlete can absorb glucose solution at this concentration as easily as water, although it may take some practice to get used to drinking this type of formulation.
