Gabe Mirkin, M.D.
A study from new Zealand shows that taking extra salt during athletic competition helps to prevent loss of fluid, and therefore prolongs endurance. When you compete in sports for more than an hour, you lose a lot of fluid, which slows you down. Sweat contains far less salt than blood does, so sweating causes you to lose more water than salt and blood salt levels rise during exercise. You feel thirsty only when blood salt levels rise high enough to trigger osmoreceptors in your brain to tell you that you are thirsty. You don't feel thirsty until you have lost at least 2 to 4 pints of fluid. Drinking water and not salt during excise prevents blood salt levels from rising high enough to make you feel thirsty. So, taking salt with fluid causes blood salt levels to rise high enough to make you thirsty, so you take in more fluid and have greater endurance.