Gabe Mirkin, M.D.
A study from Switzerland shows that when you exercise for more than three hours, you should take in salt as well as fluids.
During a four hour run, female competitive distance runners took in drinks with different concentration of salt. Ninety-two percent of those who took in plain water with no additional salt developed low blood levels of salt, which could kill them. Taking in fluid without also taking in adequate amounts of salt dilutes the bloodstream, so that the concentration of salt in the blood is lower than that in brain cells. This causes fluid to move from the low-salt blood into the high-salt brain causing the brain to swell which can cause seizures and death. Taking in extra salt during prolonged exercise increases thirst so you drink more fluids, and prevents blood salt levels from dropping so low that you become tired, develop muscle cramps, and can even die.