Research
Exercise-Associated Hyponatremia: Sweat, Sodium, and Overhydration
Exercise-associated hyponatremia (EAH) is a low blood-sodium condition that develops when endurance athletes drink more fluid than they lose in sweat and urine, diluting the sodium in their blood. It is defined as a serum sodium concentration below 135 mmol/L occurring during or within 24 hours of prolonged activity. In one landmark study of the 2002 Boston Marathon, about 13% of runners tested were hyponatremic. Most cases are mild and symptom-free, but severe EAH can cause brain swelling, seizures, and death, which makes it one of the few sweating-related conditions where drinking too much, not too little, is the danger.
By the Sweat Explained Editorial Team · Published 2026-07-13 · Last reviewed 2026-07-13 · Educational information, not medical advice.
Key statistics at a glance
13%
of Boston Marathon runners tested had hyponatremia (serum sodium ≤135 mmol/L), out of 488 finishers sampled in 2002
Almond 2005, NEJM
0.6%
had critical hyponatremia (≤120 mmol/L), about three runners
Almond 2005, NEJM
<135 mmol/L
the consensus serum-sodium threshold that defines EAH
Hew-Butler 2015 consensus
5–51%
reported incidence of asymptomatic EAH after endurance events
StatPearls review
0.1–1.0%
of endurance athletes experience symptomatic EAH
StatPearls review
What exercise-associated hyponatremia is
Sweat contains water and sodium. During prolonged exercise the body loses both, but many athletes replace only water, and sometimes drink far more of it than they are actually losing. When fluid intake exceeds the combined losses from sweat and urine, the sodium already in the blood becomes diluted. The 2015 international consensus statement defines exercise-associated hyponatremia as a serum, plasma, or blood sodium concentration below 135 mmol/L occurring during or up to 24 hours after prolonged physical activity.
Overdrinking of hypotonic fluids (plain water or sports drinks) is the central cause. A second contributor is the non-osmotic release of arginine vasopressin (antidiuretic hormone, ADH) during hard exercise, which tells the kidneys to hold on to water rather than excrete the excess. Together these mechanisms mean that a well-meaning "stay ahead of your thirst" strategy can, in some people, tip blood sodium into a dangerous range.
What the Boston Marathon study found
The most-cited human data come from Almond and colleagues, who collected finish-line blood samples at the 2002 Boston Marathon and published the results in the New England Journal of Medicine in 2005. The figures below are from that single study of nonelite runners.
| Measure | Finding |
|---|---|
| Runners providing a usable finish-line blood sample | 488 (64% of the cohort) |
| Had hyponatremia (serum sodium ≤135 mmol/L) | 13% |
| Had critical hyponatremia (≤120 mmol/L) | 0.6% (about 3 runners) |
| Associated with weight gain during the race | odds ratio 4.2 (95% CI 2.2–8.2) |
| Associated with racing time ≥4 hours (vs <3:30) | odds ratio 7.4 (95% CI 2.9–23.1) |
Female sex, the type of fluid consumed, and NSAID use were not independently associated with hyponatremia in this study.
What raised the risk
In the same study, two factors stood out in the multivariate model. Both point to the same underlying picture: fluid taken on faster than it can be lost. Weight gain during a race, the clearest sign of net fluid overload, and a long time on course each sharply raised the odds of hyponatremia.
| Group | Value |
|---|---|
| Weight gain during race | 4.2× odds (vs runners who did not gain weight) |
| Racing time ≥4 hours | 7.4× odds (vs finishing in under 3:30) |
Source: Almond CSD, et al. N Engl J Med 2005;352:1550–1556. Chart is an original rendering of the cited data.
How common it is
Prevalence depends heavily on how EAH is defined and measured. Most cases are asymptomatic and detected only by blood testing: reviews report asymptomatic EAH in roughly 5–51% of participants across different endurance events, with the wide range reflecting differences in event length, conditions, drinking behavior, and testing methods. Symptomatic EAH (low sodium plus nausea, headache, confusion, or worse) is far less common, affecting an estimated 0.1–1.0% of endurance athletes.
The risk is not limited to marathons. EAH has been described in ultramarathons, triathlons, hiking, and military training; one review cites an incidence of about 6.9 cases per 100,000 person-years in a military population. The common thread across all of these settings is prolonged activity combined with high fluid intake.
Why severe cases are dangerous
When blood sodium falls low enough, water moves into brain cells and causes swelling (hyponatremic encephalopathy). Symptoms can progress from headache, nausea, and vomiting to confusion, seizures, coma, and death. Failure to recognize and correctly treat severe EAH is a documented cause of event-related fatalities in marathon runners and military recruits; the Boston Marathon study itself was undertaken in the wake of a runner's death.
A dangerous pitfall is that EAH can look like dehydration (both can cause collapse and fatigue) yet the treatment is opposite. Giving ordinary intravenous fluids to someone who is actually overhydrated can make hyponatremia worse. This is why medical guidance emphasizes measuring sodium before assuming a collapsed endurance athlete simply needs more fluid.
The prevention message: drink to thirst
The consensus prevention advice is straightforward: drink according to thirst rather than to a fixed schedule or to "stay ahead" of fluid loss. Using thirst as the guide tends to limit overdrinking while still preventing meaningful dehydration. Warning signs that fluid intake has outpaced losses include gaining weight over the course of a long event, along with nausea, headache, puffiness, or feeling increasingly unwell despite drinking.
This page describes what the research shows and is not medical advice. Anyone who becomes confused, severely unwell, or collapses during or after prolonged exercise, especially after drinking heavily, needs urgent medical assessment, and anyone planning long endurance events can discuss a personal hydration approach with a qualified clinician.
Methodology and limitations
This page draws on a primary cohort study (Almond 2005, a cross-sectional analysis of 488 Boston Marathon finishers), the 2015 Third International Exercise-Associated Hyponatremia Consensus Statement for definitions and prevention guidance, and a peer-reviewed clinical review (StatPearls) for incidence ranges and pathophysiology. Each figure was traced to its source and confirmed.
Limitations: the 13% and 0.6% figures come from a single race in one year and one location, so they should not be read as the rate for all runners or all events. Incidence estimates vary widely (5–51% asymptomatic) because studies differ in event type, climate, drinking behavior, and how and when sodium was measured. Odds ratios describe associations within the Almond model, not proven cause and effect for any individual. Nothing here is a diagnosis or medical advice.
Frequently asked questions
- What causes exercise-associated hyponatremia?
- Drinking more fluid than the body loses through sweat and urine, which dilutes blood sodium. Non-osmotic release of antidiuretic hormone during prolonged exercise adds to it by causing the kidneys to retain water. It is defined as serum sodium below 135 mmol/L during or within 24 hours of activity (Hew-Butler 2015 consensus).
- How common is EAH in marathon runners?
- In the 2002 Boston Marathon study, about 13% of the 488 runners tested were hyponatremic and 0.6% had critical hyponatremia (Almond 2005). Across endurance events generally, reviews report asymptomatic EAH in roughly 5–51% of participants and symptomatic EAH in about 0.1–1.0%.
- Isn't drinking a lot of water during exercise a good thing?
- Not without limit. Replacing more than you lose can dilute blood sodium and, in severe cases, cause brain swelling. Gaining weight during a long event is a red flag for fluid overload. Consensus guidance recommends drinking to thirst rather than to a fixed schedule.
- How is EAH different from dehydration?
- They can look similar (both may cause fatigue or collapse) but they are opposite problems. EAH is too much water relative to sodium, while dehydration is too little fluid. That is why sodium should be measured before assuming a collapsed athlete simply needs more fluid; the wrong treatment can worsen EAH.
- Can exercise-associated hyponatremia be fatal?
- Severe cases can be. Very low sodium can cause hyponatremic encephalopathy (brain swelling leading to seizures, coma, or death) and unrecognized severe EAH is a documented cause of event-related deaths in runners and military recruits. Most cases, however, are mild and symptom-free.
- Who is most at risk?
- In the Boston study, gaining weight during the race (a sign of net fluid overload) and spending 4 or more hours on course were the strongest associations. Slower runners who drink heavily throughout a long event are a recognized higher-risk group.
Sources
Primary peer-reviewed studies and official sources first, then reviews and institutional framing (secondary).
- Almond CSD, Shin AY, Fortescue EB, Mannix RC, Wypij D, Binstadt BA, et al. Hyponatremia among runners in the Boston Marathon. N Engl J Med. 2005;352(15):1550–1556. PMID 15829535. 13% hyponatremic; 0.6% critical. PubMed
- Hew-Butler T, Rosner MH, Fowkes-Godek S, Dugas JP, Hoffman MD, Lewis DP, et al. Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. Clin J Sport Med. 2015;25(4):303–320. PMID 26102445. Definition <135 mmol/L; drink-to-thirst. PubMed
- Nelson JL, et al. (StatPearls Publishing). Exercise-Associated Hyponatremia. StatPearls [Internet]. NCBI Bookshelf, NBK572128. Incidence 5–51% asymptomatic; 0.1–1.0% symptomatic; pathophysiology. (secondary) StatPearls
How to cite this page
Sweat Explained. Exercise-Associated Hyponatremia: Sweat, Sodium, and Overhydration. Published 2026-07-13; last reviewed 2026-07-13. Available at: https://sweatexplained.com/research/exercise-associated-hyponatremia-research
Please cite the original studies for the underlying figures. Journalists are welcome to link to this page; the charts are original renderings of the cited data.