For years, the mantra was drummed into endurance-sports competitors: hydrate, hydrate, hydrate. Swig water at most marathon or triathlon fluid stations and drain your bottles during a cycling race—whether you’re thirsty or not.

Mounting research supports far different advice. Athletes are more likely to suffer severe harm by drinking too much during competition than by drinking too little, the evidence shows. In extreme cases, people have died after drinking too much liquid during a race.

Further, new studies suggest that 3% dehydration levels during competition, which experts once warned against, don’t hurt performance and might even help it. Increasingly, experts advocate a simple rule: During competition, drink when you’re thirsty.

Tim Noakes, a longtime sports medicine physician and emeritus professor at South Africa’s University of Cape Town, believes the body’s instincts are an athlete’s best friend. “If you drink to thirst, you maximize your performance,” he says. His 2012 book “Waterlogged” blames the sports-drink industry for encouraging athletes to drink more than needed.

Ultramarathoner Dean Karnazes says he drinks gallons of water while competing in events like the Badwater Ultramarathon, a 135-mile run across California’s scorching Death Valley. At November’s New York City Marathon, however, where temperatures were in the mid-40s, he drank only once: a half-cup of water near the 15-mile mark.

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