The New Rules of Hydration

Remember when the only guideline for staying hydrated during exercise was to drink—and drink often? And plain water took the podium as the perfect sports drink? Thanks to new insights on how our bodies process fluids and other nutrients while we're working up a sweat, the conventional wisdom on when and what to drink is evolving. And although the rules may have changed, the objective remains the same: improved performance and optimal health.

Here's a look at the old and new views on hydration.

More: Measure Your Sweat Loss for Optimal Hydration

Old: Drink ahead of your thirst.
New: Drink according to your thirst.

For years, sports nutrition experts advised athletes to drink "ahead of thirst," that is, to drink before getting thirsty and more frequently than what thirst dictated during exercise. Experts warned that by the time you feel thirsty, you've already become dehydrated. However, recent studies show that being in this state of slight dehydration has no negative impact on performance or health.

For example, in a study from the Sports Science Institute of South Africa, runners did three two-hour workouts while drinking a sports drink at three different rates: by thirst (roughly 13 oz. per hour), at a moderate rate (about four oz. every 15 to 20 minutes), and at a high rate (about 10 oz. every 15 to 20 minutes).

More: 15 Hydration Facts for Athletes

The study found no significant differences in core body temperature (rising body temperature hastens dehydration) or finishing times among the three trials. However, during the high-rate trial two of the eight runners suffered severe stomach distress and couldn't finish the workout, suggesting that drinking too much too often can cause problems.

"The idea that thirst comes too late is a marketing ploy of the sports-drink industry," says Tim Noakes, M.D., a professor of sport and exercise science at the University of Cape Town, South Africa. While thirst is not a perfect indicator of hydration status, it does appear to be a good indicator of the optimal drinking rate during exercise, according to Noakes. "The answer is just drink as your thirst dictates."

More: Hydration and Electrolyte Tips for Race Day

Old: Aim to completely prevent dehydration.
New: Aim to slow dehydration.

You've probably been told to drink enough fluid during exercise to completely make up for what you lose through sweat. In other words, the goal is to weigh the same before and after your workout. But the latest research has revealed three problems with this advice.

First, when athletes drink according to thirst, they usually replace only 60 to 70 percent of the fluid they lose, but studies have shown that this state of slight dehydration does not harm performance or health.

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