Myth: Pure water is best for hydration.
Truth: Although water is a great way to hydrate, it may not be the best choice in all situations. For an easy, hour-long run on a coolish day, sipping water is fine. But if you’re running 10 miles on an August morning and are a salty sweater (you have white salt streaks on your face or clothes postrun), you need to ingest some sodium as well. “Salt helps you retain water,” says Yeargin. “You’re less likely to pee it out.” A sports drink, such as Gatorade, and water enhanced with electrolytes, like Nuun, are good options; taking high-dose salt tabs before a run is less so. “There’s no way to ‘preload’ with sodium to negate sodium loss,” says Yeargin. “You just pee out anything you don’t use.”
Source from Runner’s World