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 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 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.

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 endurance-trained guys eat less salt for 9 days, their blood sodium drops — even if they drink enough water — especially when they bike in the heat. This suggests long-term salt intake might affect how their body handles hydration in hot weather.

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.