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 cyclists drink the same amount of fluid during a 4-hour ride, it doesn’t matter how much salt is in the drink — their blood salt levels stay steady even if their body handles water and salt differently.

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 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.

Endurance cyclists who take in 700 mg of sodium per hour during a 3-hour ride in cool weather tend to drink about 160 mL more fluid each hour than those who don’t — probably because the salt affects how thirsty they feel.

If you're biking for a long time and drink fluids with a lot of salt, your body holds onto more fluid outside your cells. But if you drink fluids with almost no salt, you actually lose fluid from that same space — even if you're sweating the same amount.