Water alone doesn't hydrate you properly. Your body loses sodium, potassium, and magnesium through sweat and urine, and drinking more water without replacing them makes you pee more and dehydrates you further. Hospitals use sodium-rich IV fluids to treat dehydration, not plain water.
I'm answering the question, why do you have to hydrate with electrolytes and not just water? Well, when you go to the bathroom, you're losing electrolytes. That's potassium, magnesium, and sodium. And also when you sweat from things like a workout or the sauna. And so if you just drink more water, then that's going to send you back to the bathroom, and then you're going to lose more electrolytes out of your body. And so if you're not replenishing those electrolytes, then you can drink more fluids and still become dehydrated. So make sure that if you are drinking a lot of water, that you are still replenishing your body of electrolytes. And make sure, especially if you're working out and you're losing even more from sweating and things like that, that you are replenishing yourself. Everybody knows when you go to the hospital and you are dehydrated, they're going to replenish you and hydrate you with sodium fluids. So make sure that you are hydrating properly with electrolytes.
