If you're a distance runner, you've probably been told for years that you should drink a lot of fluids before and during your runs. Moreover, you've probably also been urged to drink even if you weren't thirsty. If you let yourself reach the point of thirst, the thinking went, you were already dehydrated.

It seems the conventional wisdom is changing. Some experts now recommend trying to balance fluid intake more precisely with the amount of fluids lost through sweat. (USA Track & Field, for instance, has guidelines on how to estimate your personal fluid loss.) And thirst has gained new respect as a reliable gauge of when you need to drink.

The impetus behind this paradigm shift? Experts have learned that endurance athletes who over-hydrate risk bringing their blood sodium levels too low, which can have serious health consequences.