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.

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