Hyponatremia in Ultramarathons: The Danger of Overhydration and the Importance of Electrolyte Balance

Introduction

“Drink plenty of water” is a well-known exercise recommendation, but in the extreme environment of ultramarathons, overhydration can be more deadly than dehydration. Exercise-Associated Hyponatremia (EAH) is one of the most common serious medical emergencies in ultramarathon events. In mild cases, it causes headaches and confusion; in severe cases, it can lead to cerebral edema and respiratory arrest. Even more dangerous is that its symptoms are often mistaken for dehydration—if the athlete is given more water, the condition can deteriorate rapidly.

1. What is Hyponatremia?

Normal blood sodium concentration ranges from 135–145 mEq/L. A blood sodium level below 135 mEq/L is defined as hyponatremia. The severity of symptoms is highly correlated with the rate of sodium decline:

2. Causes of EAH

Primary Mechanism: Overhydration Dilutes Blood Sodium

The most common cause is consuming excessive low-sodium or sodium-free fluids (plain water, electrolyte-free drinks), which dilutes the sodium in the blood.

Secondary Mechanism: Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH)

Prolonged exercise, pain, nausea, and certain medications (NSAIDs) can stimulate excessive secretion of antidiuretic hormone (ADH), causing the kidneys to retain too much water and lowering blood sodium.

High-Risk Groups

• Smaller runners (smaller fluid space)
• Athletes exercising for more than 10 hours
• Slower finishers (slower pace = less sweating = but possibly the same drinking habits)
• Runners who follow a fixed “must drink X ml every hour” plan without considering thirst
• Those who hydrate primarily with plain water rather than electrolyte drinks

3. Distinguishing EAH from Dehydration Symptoms (Critical!)

This is the most important and most error-prone assessment:

Golden Rule for Assessment: If an athlete has nausea and headache but no significant weight loss, or has swelling in the extremities, suspect EAH rather than dehydration. Absolutely do not give water!

4. Prevention Strategies for EAH

Drink According to Thirst

The most important preventive measure: Drink only when you feel thirsty; do not force fluids on a fixed schedule. This recommendation is supported by multiple ultramarathon medical studies.

Replace Plain Water with Electrolyte Drinks

• For prolonged exercise (>2 hours), electrolyte drinks should be the primary fluid source
• Sodium intake target: 400–600 mg per hour (including food sources)
• Avoid drinking large amounts of plain water over extended periods

Monitor Urine Status

• Urine colorless and clear → reduce fluid intake, increase electrolyte intake
• Urine dark yellow → increase fluid intake

5. Emergency Management Principles (For Medical Stations)

If EAH is suspected:

1. Immediately stop all fluid intake
2. Administer concentrated saline solution (if the athlete is conscious)
3. Transport to a medical station for blood sodium measurement
4. Severe cases require intravenous hypertonic saline (administered by medical personnel)

Remember: Giving water to a hyponatremic athlete is the most dangerous mistake, even if they appear to have “dehydration-like” symptoms.

Conclusion

Knowledge of hyponatremia is essential safety education for every ultramarathon runner. Understanding its causes, recognizing its symptoms, and establishing a thirst-based hydration habit not only protects yourself but also enables you to identify dangerous situations in fellow runners on the course and seek help in time.

Related Reading

• Exercise-Associated Hyponatremia: The Fatal Mistake of Drinking Yourself into Water Intoxication
• Exercise-Associated Hyponatremia (EAH): Hydration Mistakes and Prevention in Endurance Events Over 4 Hours
• Exercise and Hyponatremia: Drinking Too Much Water Is Actually More Deadly Than Dehydration
• Water Intoxication and Overhydration in Athletes: The Silent Killer of Hyponatremia—How to Hydrate Correctly