Hyponatremia: The Hidden Danger of Too Much Plain Water

Exercise-associated hyponatremia happens when blood sodium falls below normal (under 135 mmol/L), most often because an athlete drinks far more fluid than they sweat out. It's rare, but it can be serious, and it's largely preventable.

How it happens

Your body works hard to keep blood sodium within a narrow range. During long events, two things can push it down:

• Overdrinking: taking in more fluid, especially plain water, than you lose through sweat and urine. This is the main cause.
• Large sodium losses: heavy, salty sweaters who replace big fluid losses with low-sodium drinks over many hours.

The risk rises in longer events like marathons, Ironman triathlons and ultras, and in slower participants who have more time and more aid stations to drink at.

Warning signs

Early symptoms can look a lot like dehydration, which is what makes hyponatremia tricky:

• Nausea and bloating.
• Headache.
• Swelling of the hands and feet (rings or watch feeling tight).
• Confusion, disorientation or unusual fatigue.
• In severe cases, vomiting, seizures or loss of consciousness.

Severe symptoms are a medical emergency. If you or a fellow athlete shows confusion or extreme symptoms, alert medical staff immediately. Don't keep drinking fluid in the hope it helps.

Who's most at risk?

• Athletes who gain weight during an event, a sign they've drunk more than they've lost.
• Slower finishers in long events.
• Those who drink on a fixed schedule regardless of thirst or conditions.
• People who rely solely on plain water for very long efforts.

How to protect yourself

1. Know your sweat rate so you drink to match, not exceed, your losses.
2. Don't overdo it. You should finish long events slightly lighter, not heavier.
3. Include sodium in your drinks for efforts over two hours, especially if you sweat heavily.
4. Listen to thirst in cooler conditions rather than forcing fluid down.

The takeaway

More fluid isn't always better. Drinking to a personal plan, with enough sodium, keeps you safely in the middle ground between dehydration and overhydration. Endura Fuel electrolytes help you replace what you actually lose. This article is for education, not medical advice; see a doctor with any concerns.