Do people who exercise need to eat more salt?

Short answer

There was no need to eat extra. Sweat loses about 1,000 mg of sodium an hour, while people in Korea usually eat 3,255 mg a day. Only when salt was cut sharply before long exercise in the heat did body temperature rise faster. Only a 160 km run with heavy drinking needed sodium during exercise.

Do people who exercise need to eat more salt?
'Sweat → salt loss → eat more': is it true?
Sweat contains sodium, the main part of salt, so some say people who exercise do well to eat extra salt. We checked how much is actually lost in sweat, how much people usually eat, and what happened when salt was cut or eaten in large amounts, in that order.
Sodium lost in sweat: about 1,000 mg an hour
About 830 mg per 1 L of sweat on average
In sweat data from 506 people, 1 L of sweat held about 830 mg of sodium on average, ranging from about 420 mg to 1,630 mg, nearly a 4-fold difference between people. People sweated 1.21 L an hour on average. That works out to losing about 1,000 mg of sodium (about 2.5 g of salt) an hour.
A two-fold difference by sport
American football and endurance sports lost the most
In data pooled from 1,303 people, sodium lost in sweat per hour was about 1,290 mg in American football, about 1,190 mg in endurance sports such as running and cycling, about 800 mg in soccer, and about 630 mg in baseball. This analysis had no data for weight training alone.
So how much do people usually eat?
3,255 mg a day, 1,255 mg over the upper limit
In a national survey of 116,284 people, people in Korea ate 3,255 mg of sodium a day in 2018. That is 1,255 mg over the recommended upper limit of 2,000 mg, and still over the limit after subtracting one hour of sweat. In a study comparing 40 people, sodium intake did not differ between those who exercised and those who did not.
The other way: what if salt is cut sharply?
The body ran hotter during long exercise in the heat
When 9 people ate little salt for 9 days and then cycled for 3 hours in the heat, their blood sodium was lower and their heart rate and body temperature were higher than after eating a lot. When 15 people ate only about 1,050 mg a day (for a 70 kg body weight) for 3 days, body temperature rose faster, and there was no difference in body temperature or heart rate at the end.
When replacing it during exercise was needed
Running 160 km while drinking a lot
In a model that calculated sweat and fluid intake, a soccer match and an elite marathon needed no extra sodium during exercise. Only in a 160 km ultramarathon, when people drank more than 80% of the water they lost and sweat held about 920 mg of sodium or more per 1 L, did they need to replace 47% or more of the sodium lost.
The burden of eating a lot remained even with exercise
More activity meant a smaller blood pressure response, but it did not disappear
1,906 people ate about 1,180 mg of sodium a day for 7 days and about 7,080 mg a day for another 7 days. On the high amount, the top blood pressure number rose by 3.96 in the most active group and 5.21 in the least active group. A study pooling many analyses found that each extra 1 g of sodium a day came with a 4% higher risk of heart and blood vessel disease.
Summary: there was no need to eat extra
A normal diet gives more than 3 times one hour of sweat loss
Sweat loses about 1,000 mg of sodium an hour, and people in Korea eat 3,255 mg a day. Even with exercise, the blood pressure burden of eating a lot remained. Replacing sodium during exercise was needed only when running 160 km while drinking a lot. Keeping your usual diet is enough; the one thing to avoid is cutting salt sharply and then exercising for a long time in the heat.