Ultramarathon runners who took more sodium during a super long, super hot race tended to lose less body weight — the more sodium they took, the less weight they lost.

Ultramarathon runners who take sodium supplements during long races in extreme heat keep more of their body weight, but those who skip the supplements might actually stay better hydrated, according to the researchers.

Endurance runners in a super long, hot race who took sodium supplements every part of the way lost less body weight during the race than those who didn’t take any.

Ultramarathon runners who drank only when thirsty and skipped salt pills during a long, hot race lost about 3.4% of their body weight — and experts say that’s still a safe level of hydration.

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.

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.