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Comment by kube-system | Hacker News Reader

> Some even de-hydrate (coffee, seltzer water etc.)

This is mostly a myth. Water hydrates you regardless of whether it's mixed with CO2 or with coffee beans. The thing about caffeine is that it is also a diuretic, so some of that water will be quickly expelled.

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There is such a thing as osmolar pressure. Simply put, water wants to dissolve things and it goes wherever the solute is. High enough concentrations of solute in the intestines cause actually cause water to leave body instead of being absorbed, often causing diarrhea. Oral rehydration solutions consist of water with appropriate amounts of sugar and salts such as sodium and potassium.

There are very simple experiments you can perform to demonstrate this phenomena. Grab plants or something similar and put them inside three different solutions: hypertonic, isotonic and hypotonic. There will be interesting results.

https://en.wikipedia.org/wiki/Osmotic_concentration

https://en.wikipedia.org/wiki/Osmotic_pressure

https://en.wikipedia.org/wiki/Osmosis

Coffee will hydrate you. The amount that you expel from the caffeine is typically less than the amount you have taken in.

But then why do they use saline instead of (relatively) pure water in IV bags? I would think it's fair to say there's some sweet spot when the goal is hydrating the body, though I doubt it's what the slimy advertiser above was intending to convey.

They use saline instead of pure water because it's isotonic with respect to serum/plasma. i.e. you're not going to go into electrolyte imbalance if a lot of it is put into your blood.

Normal saline is about one-quarter the concentration of sea water, by the way.

here I thought I was saving cash by injecting sea water

They use saline because it goes directly into your blood stream. If they used pure water, the osmotic pressure would force water into your cells, causing them to burst (hypotonic).

You can drink pure water because it "becomes" saline as it mixes with the food in your stomach.

Even unfiltered tap water doesn't have very many ions compared to blood or saline solution. Most of your ions come from your food.

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