Exercise-Associated Hyponatremia: Risks and Prevention for Endurance Athletes

Introduction Exercise-associated hyponatremia (EAH) is a dilutional drop in blood sodium that can occur during or after prolonged activity. While most athletes worry about dehydration, research suggests overconsumption of low-sodium fluids (especially water) during long events may be the more common driver of EAH (evidence: strong). Understanding when and why it happens—and how sodium, sweat, and fluid intake interact—can help endurance athletes and active people make smarter hydration choices beyond the marketing of generic sports drinks.

What Causes EAH? The Science EAH reflects an imbalance between water and sodium, typically from excess fluid intake relative to losses and impaired water excretion during exercise.

• Excess hypotonic fluid intake: Drinking more fluid than the body can excrete dilutes blood sodium, especially when fluids are low in electrolytes (evidence: strong; observational data and consensus statements) [Almond 2005; Hew-Butler 2015, 2020].
• Non-osmotic antidiuretic hormone (ADH) release: Exercise, pain, nausea, heat stress, and hypoglycemia can increase ADH, reducing urine output and promoting water retention even when sodium is already low (evidence: strong; physiological and clinical consensus) [Hew-Butler 2015, 2020].
• Sodium losses in sweat: Sweat sodium loss varies widely among individuals; while sweat sodium loss alone rarely causes EAH, it can interact with excess fluid intake to lower blood sodium further (evidence: moderate-strong; mechanistic and review data) [Baker 2017].

Sweat Composition Is Personal Sweat rate and sweat sodium concentration vary dramatically by person and conditions. Reviews report several-fold differences among athletes in sweat sodium concentration and meaningful day-to-day variability for the same individual (evidence: strong) [Baker 2017]. This variability helps explain why a one-size-fits-all hydration or electrolyte strategy may not match individual needs.

Who Is Most at Risk? Epidemiology from large endurance events and clinical reports highlights several patterns:

• Prolonged events (often >4 hours) with frequent fluid availability (evidence: strong) [Almond 2005].
• Weight gain during the event, a proxy for overdrinking (evidence: strong) [Almond 2005; Hew-Butler 2015].
• Slower pace or longer exposure time, cooler race conditions, and smaller body size (less total body water) may increase risk when coupled with high fluid intake (evidence: moderate) [Almond 2005; Hew-Butler 2015].
• Use of nonsteroidal anti-inflammatory drugs (NSAIDs) has been associated with increased risk in some cohorts, likely via effects on kidney water handling and ADH pathways (evidence: moderate) [Hew-Butler 2015].

What It Looks Like—and How It Differs from Dehydration EAH exists on a spectrum. Early features can resemble heat stress or dehydration, which complicates on-course recognition.

• Mild to moderate EAH: Headache, nausea, bloating, dizziness, swollen hands, and unexplained fatigue (evidence: strong; clinical consensus) [Hew-Butler 2015].
• Severe EAH (acute hyponatremic encephalopathy): Confusion, vomiting, seizures, or collapse; this is a medical emergency (evidence: strong) [Hew-Butler 2015].
• Distinguishing clues: Body-mass gain from start to finish suggests overdrinking; profound thirst is less typical in EAH than in dehydration, though overlap exists (evidence: moderate; field data and consensus) [Almond 2005; Hew-Butler 2015].

What Research Suggests May Reduce Risk There is no single protocol that fits everyone. However, several evidence-based themes emerge.

1. Let thirst guide fluid intake during prolonged exercise

• Consensus statements recommend a “drink to thirst” or “avoid overdrinking” approach rather than fixed, aggressive fluid schedules for most endurance scenarios. This appears to lower EAH incidence without increasing heat illness risk in typical conditions (evidence: strong) [Hew-Butler 2015, 2020].

2. Individualize based on sweat losses and conditions

• Because sweat rate and sweat sodium vary, athletes may benefit from observing their body-mass changes across training sessions and noting environmental heat/humidity to tune intake patterns over time (evidence: moderate; field studies and reviews) [Baker 2017; Hew-Butler 2015].

3. Sodium in fluids or foods may help maintain fluid balance, but is not a cure-all

• Sodium in drinks or foods may enhance fluid retention and support plasma volume during and after exercise, especially when sweat losses are high (evidence: moderate-strong; RCTs on post-exercise rehydration and beverage retention) [Shirreffs 1998; Maughan 2016]. However, sodium alone does not prevent EAH if total fluid intake is excessive relative to losses (evidence: strong; consensus) [Hew-Butler 2015].

4. Oral rehydration science informs smarter choices

• Oral rehydration solutions (ORS) leverage sodium-glucose co-transport in the small intestine to enhance water and sodium absorption. This mechanism is well-established in clinical dehydration and cholera care and can improve fluid retention compared with plain water in non-clinical settings (evidence: strong clinically; moderate in sports contexts) [Maughan 2016]. In athletic use, ORS-style formulations may help when rapid rehydration matters or when GI tolerance limits volume, but they still require attention to overall intake relative to sweat losses (evidence: moderate) [Shirreffs 1998; Maughan 2016].

5. Be cautious with unnecessary medication use around events

• Some reports associate NSAIDs with higher EAH risk via effects on renal prostaglandins and free-water clearance; minimizing non-essential use around endurance events may reduce risk (evidence: moderate) [Hew-Butler 2015].

Traditional Hydration Wisdom—In Context Across cultures, traditional beverages aim to replace both water and salts lost through sweat:

• Coconut water is naturally rich in potassium and contains some sodium; small trials suggest it rehydrates similarly to many sports drinks, though it may cause fullness in some people (evidence: emerging; small RCTs) [Kalman 2012].
• Salted broths and soups have long been used in endurance communities and by traditional cuisines to restore salt and fluid after exertion. Their higher sodium content may aid fluid retention and palatability post-exercise, though controlled trials are limited (evidence: traditional/emerging).
• “Drink to thirst” mirrors long-standing ancestral practices of listening to bodily cues. Modern consensus recommendations align with this principle to reduce overdrinking risk (evidence: strong) [Hew-Butler 2015, 2020].

Marketing vs. Meaningful Supplementation Not every session requires a sports drink or electrolyte product. For many short, lower-sweat activities, water and regular meals may sufficiently replace electrolytes over the day (evidence: moderate; position statements and reviews) [Hew-Butler 2015]. For longer, hotter, or saltier sweaters, research suggests that beverages or foods containing sodium may help maintain fluid balance and reduce urine output post-exercise (evidence: moderate-strong) [Shirreffs 1998; Maughan 2016]. The key is matching intake to personal losses and conditions, rather than assuming more fluid or more electrolytes is always better.

Bottom Line

• EAH is primarily a dilutional problem driven by overconsumption of low-electrolyte fluids relative to sweat losses and reduced water excretion during exercise (evidence: strong).
• Risk rises with longer events, weight gain during competition, and possibly NSAID use (evidence: moderate-strong).
• Research supports drinking to thirst and individualizing strategies based on sweat rate and conditions to lower EAH risk (evidence: strong).
• Sodium-containing foods or beverages may aid fluid balance, especially after heavy sweating, but cannot counteract excessive fluid intake (evidence: moderate-strong).
• ORS principles—sodium-glucose co-transport—are strongly validated in clinical settings and may help with efficient rehydration in sport when applied thoughtfully (evidence: strong clinically; moderate in athletes).
• Traditional options like coconut water or salty broths can be part of a culturally familiar approach; their effectiveness depends on sodium content, total fluid volume, and individual tolerance (evidence: emerging/traditional).

References

• Almond CS et al. Hyponatremia among runners in the Boston Marathon. N Engl J Med. 2005;352:1550-1556.
• Hew-Butler T et al. Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference. Clin J Sport Med. 2015;25(4):303-320.
• Hew-Butler T et al. Exercise-associated hyponatremia: 2017–2020 update. Wilderness Environ Med. 2020;31(1):50-61.
• Baker LB. Sweating rate and sweat sodium concentration in athletes: a review. Sports Med. 2017;47(S1):111-128.
• Shirreffs SM, Maughan RJ. Whole-body water loss and post-exercise rehydration in man: effects of volume and sodium content of ingested fluids. Med Sci Sports Exerc. 1998;30(4):607-614.
• Maughan RJ, Watson P, Cordery PA, et al. A randomised trial to assess the potential of different beverages to affect hydration status: development of a beverage hydration index. Am J Clin Nutr. 2016;103(3):717-723.
• Kalman DS et al. Comparison of coconut water and a carbohydrate-electrolyte sport drink on measures of hydration and physical performance. J Int Soc Sports Nutr. 2012;9:1.