Symptomatic hyponatremia during prolonged exercise in heat

Area of Science:

• Exercise Physiology
• Sports Medicine
• Endocrinology

Background:

• Hyponatremia (HN) is a recognized risk for endurance athletes, but its precise causes during prolonged exercise in heat remain debated.
• This case report investigates the physiological underpinnings of symptomatic hyponatremia in an athlete undergoing controlled heat acclimation and sodium intake.

Observation:

• A 21-year-old male volunteer developed symptomatic hyponatremia (plasma Na+ < 130 mEq/L) within 4 hours of mild exercise in 41°C heat.
• Physiological markers including heart rate, temperature, blood pressure, and sodium losses in sweat and urine were comparable to unaffected control subjects.
• During the hyponatremic episode, an inappropriate surge in vasopressin release coincided with a complete cessation of urine output.

Findings:

• The primary drivers of hyponatremia were a high fluid intake (10.3 L/7h) and significant water retention (2.77 L/7h), coupled with a borderline low initial plasma sodium concentration (134 mEq/L).
• Sodium losses via sweat and urine were within normal physiological ranges and did not significantly contribute to the development of hyponatremia, instead acting to worsen the condition.

Implications:

• This case highlights the critical role of excessive water consumption and impaired water excretion in the development of exercise-associated hyponatremia.
• Clinicians and athletes should consider total fluid intake and individual sodium status, alongside sweat losses, when assessing the risk of hyponatremia during strenuous exercise in hot environments.