What do electrolytes and sodium have to do with hydration? Is water alone not enough?

Question

Ever since I started running, all I know is that I need to drink a sh*t ton of electrolytes. But I would like to understand why on a physiological level.

In: 576

Answer 1

There is no ELI5 version, but I can give you an “ELI15 and understand osmosis” version. As you remember from biology class, water will diffuse towards an electrolyte across a semipermeable membrane. This is called “osmosis.”

[There are three basic compartments separated by semipermeable membranes in the body:](https://images.app.goo.gl/FxrkatQ18YyArFK27)

1. The intracellular space, which is the fluid *inside* of cells
2. The interstitial fluid, which is the fluid *around* cells
3. The intravascular space aka plasma fluid volume, which is the fluid *inside your blood vessels*

Dehydration is when you deplete the volume in the intravascular space (“plasma volume” or the volume in your blood vessels). When this tank is low, your blood pressure drops, and you feel dehydrated. Your organs aren’t seeing the perfusion they’re used to. We need to preferentially fill this tank to “rehydrate” you.

When you drink pure water, it distributes to each of your spaces equally, so only **one twelfth** of that glass of water will redistribute into your blood vessels. You need to drink twelve glasses of water to get one glass of water into your arteries and veins!

Now here’s what’s special about sodium.

Sodium is actively pumped out of every cell of your body (and water follows); it is preferentially relocated out of the intracellular space. When you increase your total supply of sodium, you **increase the volume of the extracellular space,** including the volume in your blood vessels.

So when you intake sodium with water, that water is preferentially relocated into your blood vessels rather than everywhere all at once, rehydrating you faster.

Adding sodium will thus **triple** water’s ability to rehydrate you.

​

**EDIT:**

The gatorade commercials lied to you. Under normal circumstances, sweat does **not** deplete your total store of sodium in a physiologically meaningful way.

**EDIT 2:**

While the kidneys (via the hypothalamic-pituitary-adrenal axis and the renin-angiotensin-aldosterone system) **are** do play a role in “blood saltiness,” they **are not** the primary reason why eating salt/electrolytes restores intravascular volume and are not necessary to answer OP’s question.

Source: I’m a doctor. I give salty IV fluid boluses every single day.