Drinking too much pure water during exercise can be lethal
We are often told to hydrate continuously during workouts, but drinking excessive amounts of plain water without electrolytes can lead to exercise-associated hyponatremia. When you drink far more water than your kidneys can excrete, your blood sodium concentrations plummet. Water rushes into body tissues to balance sodium levels, causing dangerous swelling in cells—including brain cells—which can lead to confusion, seizures, or death.
The Delicate Balance of Blood Sodium
Sodium is the primary electrolyte in the extracellular fluid that surrounds the cells in the human body. Under normal physiological conditions, serum sodium concentration is maintained within a narrow window, typically between 135 and 145 millimoles per liter. This balance is critical because sodium dictates osmotic pressure, acting as a chemical anchor that keeps the volume of water inside cells and outside cells in equilibrium.
When the concentration of sodium in the bloodstream drops below 135 millimoles per liter, the condition is classified as hyponatremia. When this drop happens during or shortly after continuous physical exertion, it is specifically designated as exercise-associated hyponatremia. Rather than being caused by a severe deficiency of dietary salt, exercise-associated hyponatremia is overwhelmingly a disorder of fluid balance, driven by the consumption of hypotonic fluids—such as plain water or standard commercial sports beverages—at a rate far higher than the body can eliminate them.
The Cellular Physics of Swelling
The fundamental danger of hyponatremia lies in basic osmosis. Water moves naturally across semipermeable cell membranes from regions of lower solute concentration toward regions of higher solute concentration to equalize osmotic pressure. When an athlete drinks excessive amounts of water, that excess fluid dilutes the blood, making the extracellular space abnormally dilute relative to the interior of the cells.
To balance this gradient, water rushes out of the bloodstream and into the cells, causing them to swell. While most soft tissues can accommodate a modest amount of swelling, the brain cannot. Because the brain is enclosed inside the rigid, unyielding bone of the cranium, intracellular swelling quickly leads to cerebral edema. As brain cells expand, intracranial pressure rises sharply.