In the 1990s, endurance athletes were advised to stay ahead of their thirst and drink as much as they could stand during training and races. A decade later, almost everyone had been schooled with the knowledge that hydrating to excess can cause hyponatremia—essentially, intoxication caused by consuming too much water, a potentially fatal condition in which cells swell with the excess fluid.
However, whether dehydration is equally troublesome and a hindrance to peak performance remained up in the air. But according to a 2011 review of time-trial studies of dehydration, losing up to 4 percent of body weight during exercise does not alter performance. Results from endurance events seem to bear that out: during the 2009 Mont-Saint-Michel Marathon in France, researchers measured the weight loss of 643 competitors and compared it with their finish times. The runners who lost the most water weight were also the fastest. Most of those who finished in less than three hours lost at least 3 percent of their body weight to sweat.
Get over it: “Drink when you feel thirsty,” says James Winger, M.D., assistant professor at Loyola University’s Stritch School of Medicine in Chicago, who conducted a survey of distance runners last year and found that misconceptions about hydration were rampant, even among endurance athletes. “Thirst is an exquisitely finely tuned indicator of your body’s actual hydration status,” Dr. Winger says. “Listen to it.”