Exercise-induced hyponatremia: A silent risk in ultra-endurance sports

Exercise-associated hyponatremia (EAH) is a potentially serious medical complication that can occur in any endurance athlete. 

This complication primarily results from fluid overload and/or inappropriate regulation of sodium metabolism during exercise. Unlike dehydration, hyponatremia often stems from excessive hydration leading to significant plasma dilution of sodium. 

In this article, we offer an analysis of the pathophysiological mechanisms of HAE as well as an explanation of prevention strategies adapted to ultra-endurance competitions. 

1. Mechanisms of Exercise-Induced Hyponatremia

Hypertension occurs when the plasma sodium concentration is below 135 mmol/L during a prolonged endurance event, or within 24 hours following exertion. Three main reasons can explain this phenomenon:

• Excessive consumption of (hypotonic) fluids

Excessive ingestion of water or low-sodium beverages leads to plasma sodium dilution. Studies on ultramarathon runners have shown that athletes with HAE consumed up to twice as much fluid as those without hyponatremia.

• Inappropriate secretion of antidiuretic hormone (ADH)

In some athletes, the antidiuretic hormone (ADH or vasopressin), which is responsible for maintaining a normal level of sodium in the blood, remains elevated despite fluid overload, reducing urinary excretion and exacerbating the dilution of plasma sodium.

• Storage of sodium in an osmotically inactive form

Recent hypotheses suggest that sodium may be temporarily retained at the extracellular level, contributing to functional hyponatremia without net sodium loss.

2. Epidemiological Data

The number of athletes affected by HAE varies depending on the sport and environmental conditions:

• Ultra-marathons (>100 km, hot conditions): 30 to 51% of runners

• Long-distance triathlons (Ironman, Triple Iron): 18 to 28% of athletes

• Ultra-endurance cycling: around 0.5%, due to more gradual sweat loss and easier access to sodium intake.

• Ultra-swimming: increased risk in women (36%) compared to men (8%), potentially explained by distinct hormonal regulation and a lower muscle mass/body water ratio

3. Physiological and Clinical Effects

HAE can be asymptomatic or cause manifestations ranging from impaired performance to fatal cerebral edema. Symptoms vary depending on the severity of the sodium dilution:

• Mild hyponatremia (130-135 mmol/L)

  • Nausea, headaches, lethargy

  • Muscle weakness, cramps

• Moderate hyponatremia (125-130 mmol/L)

  • Confusion, dizziness, vomiting

  • Edema of the hands and feet (observed in 20 to 30% of ultra-endurance athletes in cases of fluid overload)

• Severe hyponatremia (<125 mmol/L)

  • Convulsions, coma, respiratory distress (cerebral edema)

Rare cases, but recorded during ultra-triathlons and races in extreme conditions.

4. Prevention: Hydration and Sodium Intake Strategies

The risk of developing acute esophageal hemorrhage (AEH) can be reduced by adopting a regular but not excessive hydration routine. The following recommendations are based on field and physiological studies:

• For long-distance races, maintain hydration of around 400 to 800 ml/h of effort (depending on conditions and the athlete's needs/body weight).

  • This water intake helps prevent both dehydration and water overload.

  • While drinking to quench one's thirst would be sufficient for most athletes, during ultra-endurance races, perceptions are sometimes altered and sensations of thirst can be underestimated or overestimated; therefore, it is advisable to follow a pre-determined hydration protocol.

  • A study on a 161km ultramarathon showed that athletes drinking > 800ml/h had a 3 times higher risk of developing HAE.

• Adapting to environmental conditions

  • Hot climate: slightly increase sodium intake (+ 0.5 g/L).

  • Cold climate: increased risk of HAE due to prolonged ADH secretion and decreased sweat loss. Strict monitoring is necessary.

• Avoid pre-competition overhydration

  • Drinking excessively before the race increases water retention and decreases renal sodium regulation.

  • A study on ultra-cyclists showed that those consuming more than 2 L of water in the 2 hours before the start had a higher risk of sodium dilution in the middle of the race.

• Optimize sodium intake

  • For long and/or high-intensity efforts, ensure you consume between 1 and 1.5 g of salt per hour of effort (via electrolyte drinks, isotonic drinks or salty foods): Discover Hydration, the Electrolyte drink specially formulated for ultra-endurance athletes.

  • Studies on Ironman athletes have shown that athletes consuming sodium capsules (600-800 mg/hour) maintained stable serum sodium levels despite significant sweat losses.

• Monitor for early signs of HAE

  • Abnormal swelling of the extremities, nausea or confusion should raise concern.

  • In case of suspected HAE, immediately restrict fluid intake and promote moderate sodium intake.

Conclusion

Excessive fluid intake (EFI) is an underestimated threat in ultra-endurance sports, primarily due to excessive hydration not compensated by sufficient sodium intake. Optimal fluid management involves proper fluid management around and during exercise, appropriate sodium supplementation, and monitoring for early warning signs. For athletes and their coaches, understanding these mechanisms is essential to avoid serious complications and safely optimize performance. Discover Hydration, the Electrolytes drink specially formulated for ultra-endurance athletes.

Key References:

Knechtle B., Nutrition in Ultra-Endurance Racing – Aspects of Energy Balance, Fluid Balance and Exercise-Associated Hyponatremia. Medicina Sportiva, 2013.

Noakes TD., Exercise-Associated Hyponatremia: A Review of the Literature. J Appl Physiol, 2005.

Speedy DB., Hyponatremia in Ironman Triathletes: Incidence and Contributing Factors. Med Sci Sports Exercise, 1999.