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 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%.

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 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.