Eli5: Why does sea water kill us but electrolyte solutions actually hydrate us? Aren’t they both water + salts?


Edit: Question answered. Thanks!

Don’t be too hard on me, I almost failed chemistry:'(

In: 2220

While sea water and electrolyte solutions both contain water and salts, they have different concentrations of salts and different types of salts that affect how they interact with the body. Sea water typically has a much higher concentration of salt than the human body. Drinking sea water can actually dehydrate you because your kidneys have to work extra hard to get rid of the excess salt. As a result, the body loses more water than it gains, leading to dehydration and potentially even death.

Electrolyte solutions, on the other hand, have a balanced concentration of salts that is similar to the body’s own electrolyte balance. These solutions are designed to replace fluids and electrolytes that are lost during sweating, vomiting, or diarrhea, which can lead to dehydration. When you drink an electrolyte solution, your body is able to absorb the water and salts more effectively, helping to rehydrate you.

Concentration is different.

A sports drink has *about* 500mg of salt per liter.. sea water has 3 *grams* per liter.
Corrected: 35 *grams*, didn’t move the decimal over enough. The point still stands.

Everything is toxic if you take too much or too little of it.

In the case of electrolytes, it moderates the electrical nervous signals in your body.

Too little, signals cannot reach the body parts which is dangerous.

Too much, irregular signals will be sent throughout the body causing arrhythmia or other dangerous conditions.

Seawater is much saltier than electrolyte solutions.

Seawater is about 3.5% salt. Your body tissues are about 0.9%. Your kidneys can concentrate your urine to more than 0.9%, but not to 3.5%, meaning that your urine is always removing less salt-per-water than seawater. If your kidneys work as hard as they can, they can manage about 2%. That means that, to clear the salt from 1 liter of seawater, you need ~1.75 L of water in your urine. +1 L of seawater, -1.75 L of urine equals -0.75 L of water overall.

A 20 oz bottle of Gatorade contains 270 mg of sodium. That’s an implied content of about 681 mg of salt. 20 oz of water is about 600 mL, or about 600 g of water, meaning that Gatorate’s salinity is about 0.1%. That’s less than your body tissues, and far less than seawater.

A saline solution pumped in by IV at the hospital is typically *isotonic* – equal to your body’s natural ~0.9% salinity – and would taste very salty if you tried to drink it.


EDIT: a point that has come up in some of the comments here is that, because your body absorbs water by osmosis, you can’t actually absorb water from a 3.5% saline solution at all. This is true: seawater in your gut will simply pull water *out* of your body osmotically, and in fact laxatives usually work on this exact principle.

That said, adding 1 L of seawater to 10 L of fresh water would result in a mixture your body can absorb, but which would result in less hydration than just drinking 10 L of fresh water would, for the reasons described above.

> Aren’t they both water + salts?

Yes, but VERY different amount of salts. The sports drink has a pretty low salt concentration. Just enough to replenish the salt lost by your body by sweating. Sea water contains many times more salt per volume, and is much saltier than your body itself. It’s so salty that when you drink it, your body has to use up its *existing* water trying to dilute the salt enough to pee it out before it kills you by frying your nerves (nerves use salt to send signals, so getting flooded with salt messes them up).

It comes back to the classic saying “**the dose makes the poison**”, which just means “anything is toxic if you consume enough of it”. Heck, even plain water by itself is toxic if you drink too much. Eating some salt on your pasta is fine. Eating a pound of salt out of the bag is not. Seawater is much like option 2 there. **Only about 13L (3.5 gal) of seawater contains a** ***pound*** **of salt.** That’s only a few days’ worth of water-drinking. Imagine eating a pound of salt in a couple days. That’s why seawater isn’t drinkable.