Eli5: Why do electrolytes hydrate you but saltwater dehydrates you?
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It’s because saltwater from the ocean is saltier than urine. The saltiest that urine gets is about 10 grams of salt in each liter (and normally your urine is a lot less salty than that). Ocean water has about 35 grams of salt in each liter.
So if you drink a liter of seawater (yuck), your body needs more than two additional liters of fresh water, just to get rid of the salt that came with that one liter of seawater.
If your kidneys were more efficient at concentrating the salt in your blood, then you could drink seawater. For example, ocean fish do that and they’re just fine.
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