When fit guys bike for 3 hours in the heat and drink about a liter of fluid per hour, they lose a little body weight — around half a percent. They might lose slightly less if their drink has more salt, but the difference isn’t strong enough to be sure it’s not just chance.

If fit guys bike hard in the heat and drink a lot, a sports drink with more sodium keeps their blood sodium steady, but one with less sodium causes it to drop a bit on average.

If fit guys sweat hard in the heat for about 3 hours and drink a lot of fluid, a sports drink with more salt (60 mmol/L) keeps their blood volume stable, but a low-salt version (21 mmol/L) causes their blood volume to drop by about 2%.

If fit guys sweat a lot for 3 hours in the heat and drink lots of fluid, a sports drink with more salt (60 mmol/L) helps keep their blood sodium up, while a low-salt version (21 mmol/L) causes it to drop — and the difference is clear.

If fit guys eat a low-salt diet for 9 days, their bodies heat up more and their hearts beat faster when they bike in the heat — even if they drink the same amount of fluids — suggesting their bodies aren’t cooling as well.

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.