Here is another study which indicates that rather than giving blanket advice to people to drink x amount of fluid - e.g. 8 glasses a day - thirst is the best guideline to intake. Of course people trying to sell sports drinks would have you believe something different!
Drinking policies and exercise-associated hyponatraemia: Is anyone still promoting overdrinking?
OBJECTIVES: The purpose of this review is to describe the evolution of hydration research and advice on drinking during exercise from published scientific papers, books and non-scientific material (advertisements and magazine contents) and detail how erroneous advice is likely propagated throughout the global sports medicine community.
DESIGN: Hydration advice from sports-linked entities, the scientific community, exercise physiology textbooks and non-scientific sources was analyzed historically and compared with the most recent scientific evidence.
CONCLUSIONS: Drinking policies during exercise have changed substantially throughout history. Since the mid 1990's, however, there has been an increase in the promotion of over-drinking by athletes. While the scientific community is slowly moving away from "blanket" hydration advice in which one form of advice fits all and towards more modest, individualized, hydration guidelines in which thirst is recognized as the best physiological indicator of each subject's fluid needs during exercise, marketing departments of the global sports drink industry continue to promote over-drinking.