The key to retaining water during exercise is to take in salt to replace the salt you lose in your sweat. Until recently, many scientists did not appreciated how important salt is to retaining fluid in your body. Thirst is a late sign of dehydration. You lose water during exercise primarily through sweating, and sweat contains a far lower concentration of salt than blood. So exercisers lose far more water than salt, causing the concentration of salt in the blood to rise. A person will not feel thirsty until the concentration of salt in the blood rises high enough to trip off thirst osmoreceptors in the brain and it takes a loss of between 2 and 4 pints of fluid to do that.
You need to take salt to retain the fluid you drink while exercising. If you lose two pints of fluid, you can replace it with two pints of water if you also take salt, but if you don't take salt, it can take four pints of fluid to replace two pints of sweat because the water you drink will pass out through your kidneys. In one study, female competitive distance runners took in drinks with different concentration of salt during a four-hour run (British Journal of Sports Medicine, Volume 37, Issue 4, 2003). Ninety-two percent of those who took in plain water with no additional salt developed low blood levels of salt. 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. You can keep yourself fresh during extended exercise by eating foods with salt and drinking frequently, before you feel hungry or thirsty. Once you feel hungry or thirsty, you will find it very difficult to regain your strength.
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