Drinking a salty drink before working out in the heat might help your body hold onto more water and stay hydrated, but it might taste bad and be hard to drink a lot of.

If you're doing long workouts—like marathons or ultra-endurance events—especially in the heat, sipping in 300 to 600 mg of sodium per hour can help your body stay hydrated, keep your blood volume stable, and avoid crashing during the event. But how much you need really depends on how much you sweat and how salty your sweat is.

If elite cyclists drink as much as they want during a 3-hour ride in cool weather, taking in 700 mg of sodium per hour helps keep their blood fluid levels stable—or even slightly increases them—while skipping it leads to a small drop.

If endurance cyclists drink fluids with sodium during long rides, they pee less and keep more fluid in their bloodstream, which helps prevent dehydration—even though the salt level in their blood stays the same.

If endurance athletes drink a lot during long workouts and also take in sodium, it might help prevent low blood salt levels — but it probably won’t make them perform better just by itself.

Taking sodium supplements during a long bike ride in cool weather doesn’t really change your body’s sodium levels if you're already drinking fluids whenever you feel like it.