The science of saline – why can we drink pure water, but when we put water into our bodies in other ways, like an IV, must we add minerals to it?
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I am not so sure any of the top comments actually answered your question, honestly.
So most of them mention tonicity or osmosis and how pure water can make your cells burst and make you die. All that is true but doesn’t really answer why you can drink pure water (and for all intents and purposes tap water is pure because it’s far from isotonic).
The reason is that when you drink it, your body can regulate how much it takes up and it will also take up salts from the foods you eat and your kidneys will then measure the concentration of salts in your blood and make sure the concentrations stay about the same, because otherwise you will die (keeping everything about the same is called homeostasis). That’s why when you eat lots of salty things you will feel thirsty, and when you drink lots of water without any salty things you will pee a lot (and usually your pee will then be less salty).
You CAN also die from drinking too much water, but you’d have to drink many liters in a short amount of time without having any food in your stomach. This actually happens. Sometimes because of stupid bets but often it happens to athletes like long distance runners if they keep sweating out salts and water and drink lots of water but no salts (hyponatremia). That’s why when you do sports in particular, isotonic drinks are important.
When you get an IV with salt-less water it will just make your blood too dilute too quickly so your body doesn’t really have time to regulate itself.
EDIT: spelling
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