Sweat isn’t just water – it contains salts as well (that is what electrolytes are). If you sweat too much, your salt levels will decrease and this can harm athletic performance (your body needs salts for all sorts of things).
So when you are trying to rehydrate after sweating, you need something that replenishes both. Athletic drinks, like Gatorade, will have both along with some sugar (as athletes need calories as well).
If you’re someone in normal health and do fairly normal stuff daily, plain water is fine for hydration. Electrolytes are not really necessary most of the time because your body is fully capable of taking in a reasonable amount of plain water.
Adding electrolytes is necessary if you lose water very rapidly perhaps due to illness or extreme perspiration due to prolonged exertion and need to rehydrate quickly. In this case, the electrolytes help to maintain electrolyte balance in your blood. Too much plain water in too short a time can lead to low electrolyte levels which, in the extreme, can be fatal.
Your body needs water and electrolytes to function.
When you sweat or pee or lose water in any way, you also lose electrolytes because sweat and pee aren’t pure water, so you’re losing electrolytes as well.
If you only drink water to replace what you lost, you have fewer electrolytes in your system, so your body can’t function properly.
It’s the same reason overhydration exists. Adding pure water to.your body means the electrolytes in your body want to balance out with the electrolyte free water you just added, reducing the amount of electrolytes in your cells.
Because without electrolytes, you are primarily relying on passive transport of water across the cell membrane. When electrolytes (and glucose) are present, transport proteins will actively transport these nutrients into the cell, which creates an osmotic gradient, which in turn increases the rate of absorption via osmosis.
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