The Real Relationship Between Water and Electrolytes

Hydration sounds simple: drink water, stay hydrated. But biologically, it’s much more interesting than that. Our bodies are constantly balancing water and electrolytes, especially sodium, to keep blood pressure stable and circulation, muscles, nerves, kidneys, and virtually every cell functioning normally.

One of the most important concepts to understand is this: Water follows sodium

Sodium is the major electrolyte in the fluid outside our cells, including our bloodstream. Wherever sodium goes, water tends to follow because the body is always trying to maintain the right concentration of particles on either side of a cell membrane. This movement of water is governed largely by osmosis. (Mini med school recap: If sodium concentration is higher inside the cell, water moves INTO cells from the bloodstream. If sodium concentration is lower inside cells, water flows OUT of cells, into the bloodstream and ultimately OUT of the body in our urine.) It’s one reason sodium plays such a powerful role in maintaining blood volume and blood pressure.

Our kidneys are the master regulators of this system. Throughout the day, they continually adjust how much sodium and water we retain or excrete. Hormones including antidiuretic hormone, or ADH, and aldosterone help fine-tune the process. When the body senses that blood volume is falling or that the blood is becoming too concentrated, it holds onto water. When there’s excess water, healthy kidneys generally allow more of it to leave through the urine.

Then we exercise, and things get even more interesting

When we sweat, we aren't simply losing water. Sweat contains both water and electrolytes, primarily sodium and chloride. Anyone who’s finished a hard workout with white, salty marks on their clothing or their baseball hats has actually seen evidence of this.

Mini med school: Electrolytes include anything with an ion charge, such as sodium, potassium, magnesium, and calcium.

How much we lose varies dramatically from person to person. Temperature, humidity, exercise intensity, duration, body size, acclimatization, and even genetics affect both how much we sweat and how salty that sweat is. One person can finish a workout having lost a relatively modest amount of fluid, while another can lose liters.

For most of us doing an ordinary workout for an hour or less, drinking water and then eating a normal meal is generally enough to replace what we’ve lost. Electrolyte drinks are not automatically necessary every time we break a sweat, though they do increase hydration efficiency somewhat—the ease with which water gets into the cell is increased when water is mixed with electrolytes.

But during prolonged, intense exercise, particularly in hot and humid conditions, relying on water alone may not adequately replace what’s being lost. In extreme circumstances, drinking very large amounts of plain water while simultaneously losing significant sodium through sweat can dilute the sodium concentration in the blood. This condition, called exercise-associated hyponatremia, can be dangerous.

The opposite problem matters too

If we fail to replace enough fluid during significant sweating, blood volume can fall. Heart rate may rise, performance can decline, and symptoms such as thirst, fatigue, headache, dizziness, and weakness can appear. Severe dehydration can become medically serious.

This is why I think about hydration less as hitting a magical number of ounces every day and more as maintaining balance.

Your needs on a cool day spent working at a desk are not necessarily your needs after a 90-minute workout in August. Thirst, urine output, exercise duration, weather, and sweat losses all provide useful information.

Water matters enormously. But hydration isn't just about water. It’s an elegant physiological relationship between water, sodium and other electrolytes, our kidneys, our hormones, and what we ask our bodies to do.

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