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.

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 fit guys cut down on salt for 9 days, they pee out less sodium—even when exercising in the heat—showing their bodies are adjusting and they’re sticking to the low-salt plan.

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.

Taking 700 mg of sodium every hour during a long bike ride in cool weather doesn’t make well-trained cyclists finish faster — they perform about the same as those taking a fake pill.

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.