system
Given the following post, respond as if you're commenting on Reddit:
human
Why does salt water dehydrate you when salt helps you retain water?
gpt
Part of the problem is, seawater is really salty, about 3.5% salt by weight. An 8 oz (227 g) serving of seawater contains about 8 grams of salt. That's a lot of salt! The recommended maximum per day for sodium is 2.3 grams, which corresponds to 5.8 grams of NaCl table salt, about the same as in 1 teaspoon.

By comparison, bodily fluids like blood serum, cytoplasm, or interstitial fluid, are only about 0.9% salt by weight. When seawater comes into contact with cells in the intestines, where water is usually absorbed, osmosis pulls water out of those cells until the concentration is equal on both sides of the cell walls. So for every 8 oz serving you consume, your intestines want to supply 31 oz of water to equalize the concentration of salt.

So right there you're going to have problems, but in addition to your body dumping a bunch of water into your intestines, it's also going to absorb some of that salt and put it into the bloodstream. Your kidneys will be like "Whoa! Too much salt!" and try to get rid of it. But the only way they have to get rid of it is to concentrate salt into your urine, and they can only make that a little less salty than seawater. That means they're going to have to get rid of water to get rid of salt. They're expecting you to resupply your body with freshwater, and they don't have a backup plan when you don't do that. So now you've got diarrhea and your peeing too much, and you're probably vomiting too - you're probably going to die of dehydration.

When eating salty foods, it's a different story, usually. The mix of liquids and food in your intestines isn't going to be nearly as salty as seawater, so your body will be fine with just absorbing the salt and the water. But if you eat a lot of salt, it's still going to want to pull water out of your cells, so your cells end up a little dehydrated unless you drink a lot of liquids. So then you get a lot of liquids in your blood, and you get high blood pressure. Then your doc prescribes a diuretic so you'll pee some of that extra water out, and the salt you ate along with it.