In order to avoid dehydration, an athlete needs to drink sufficient fluid to match their sweat loss. A generic fluid replacement plan can not be advised as the amount of sweat lost is highly individual, being governed by various factors such as environment, length and intensity of exercise, metabolic rate and even genetics. As such, the amount of water needed to remain in fluid balance is different for every athlete and situation. The American College of Sports Medicine (2007) recommends that hydration needs vary between individuals and under different environments.
Their main message is that the goal of drinking during exercise is to prevent excessive (>2% body weight loss from water deficit) dehydration and excessive changes in electrolyte balance to avert compromised performance. By estimating sweat rate during exercise, an athlete can gauge the amount of fluid lost and therefore have the ability to be guided on how to match losses during exercise. Sweat rate can vary from day-to-day and if you are serious about determining your fluid requirements, you can estimate your sweat rate under different conditions and exercise intensities. Methods include tracking pre- and post-body mass or simply checking urine colour.