Overhydration: The Underappreciated Risk of Drinking Too Much Water
Most hydration advice focuses on drinking more. The risks of drinking too much are poorly understood by the general public but real - particularly for endurance athletes.
Hyponatraemia: When Too Much Water Becomes Dangerous
Hyponatraemia - abnormally low blood sodium concentration (below 135 mEq/L) - can result from drinking excess water without sufficient sodium replacement. As plasma sodium falls, cells osmotically draw in water and swell. Brain cell swelling (cerebral oedema) is the mechanism behind hyponatraemia's most serious symptoms: headache, confusion, seizures, and in severe cases, coma and death.
Exercise-Associated Hyponatraemia
Exercise-associated hyponatraemia (EAH) is the most clinically significant form and primarily affects endurance athletes. A landmark study of the 2002 Boston Marathon found that 13% of finishers had blood sodium below 135 mEq/L, and 0.6% had critically low levels. The affected runners had all consumed significantly more fluid than they lost in sweat - following the old advice to "drink as much as possible."
"The shift from 'drink to prevent dehydration' to 'drink to thirst' was one of the most evidence-based corrections in sports medicine. Forcing fluid intake caused deaths in marathons that the old guideline was designed to prevent." - Dr. Tim Noakes, University of Cape Town
Who Is at Risk
- Slower marathon and endurance runners (more time exercising, more opportunity to over-drink)
- Women - lower body water volume means the same excess fluid produces greater dilution
- Events in cool weather (sweat rate lower, fluid loss lower, but drinking rate unchanged)
- Anyone following rigid "drink X litres per day" rules regardless of thirst or conditions
The Current Guidance: Drink to Thirst
Current sports medicine guidelines (American College of Sports Medicine, 2007 onwards) recommend drinking to thirst during exercise rather than pre-emptive drinking. Thirst is a well-calibrated signal for most scenarios and naturally prevents both dehydration and overhydration in the vast majority of people.
Overhydration in Practice
For everyday hydration: drink when thirsty, use urine colour as a guide, and aim for pale straw rather than colourless. Colourless urine consistently may indicate you are drinking more than necessary. For endurance events over 2 hours: drink to thirst and choose drinks containing sodium (sports drinks or electrolyte tablets in water) rather than plain water, which provides fluid without the sodium needed to maintain plasma osmolarity.
