Of all the mistakes an endurance athlete can make on race day, one of the most dangerous is also the most counterintuitive: drinking too much water. The condition is called exercise-associated hyponatremia (EAH), and it has killed otherwise healthy marathoners, triathletes, and hikers for decades. It’s almost always preventable, but only if you understand what’s happening inside your body — and why the advice to “just stay hydrated” can actually make things worse.
What hyponatremia is, in plain terms
Sodium is the main electrolyte in your blood. It regulates fluid balance between cells and the surrounding tissue, controls nerve signaling, and helps muscles contract properly. A normal blood sodium level sits between about 135 and 145 millimoles per liter (mmol/L). When that level drops below 135, you’re in hyponatremia. Below 130, symptoms typically start showing up. Below 125, it can become a medical emergency.
During exercise, blood sodium drops in two ways: you lose sodium in sweat, and you dilute what’s left by drinking plain water (or low-sodium fluids) faster than your body can eliminate the excess. The second pathway — dilution — is the dominant cause in most EAH cases. The athlete isn’t losing too much salt; they’re drowning the salt they have.
Why sports-drink marketing got this wrong
For decades, the dominant hydration message was “drink before you’re thirsty” and “keep drinking to replace what you lose.” That advice was built for commercial fluid replacement — it’s what sold more bottles of sports drink. But real-world data from marathons, triathlons, and military training showed that a significant fraction of athletes were actually finishing overhydrated, not dehydrated.
The International Exercise-Associated Hyponatremia Consensus, most recently updated in 2015 by a panel of sports medicine and nephrology experts, shifted the official guidance: drink to thirst, don’t drink to a schedule, and if you’re going to consume large amounts of fluid over long durations, make sure that fluid contains meaningful sodium. That consensus has been reinforced in every major update since.
Who’s actually at risk
EAH is not distributed evenly across athletes. The highest-risk profiles are:
- Slower finishers in long races. An athlete running a 5-hour marathon has far more opportunity to overdrink than a 3-hour finisher, and their sweat rate is usually lower because they’re working at a lower absolute power output. More fluid in, less fluid out.
- Smaller-framed athletes. A 115-pound runner has about 30 percent less blood volume than a 185-pound runner. The same amount of excess water dilutes a smaller person faster.
- Cool-weather races where drinking stations are frequent. Counterintuitively, cool weather is higher risk — lower sweat loss plus the same number of aid stations means easier to overshoot.
- Anxious first-timers. Nerves plus “don’t want to bonk” logic plus every-aid-station drinking has killed people. The most famous case is the 2002 Boston Marathon, where a 28-year-old runner died from EAH after drinking plain water at every aid station.
- Athletes using ibuprofen or similar NSAIDs during an event. NSAIDs impair the kidney’s ability to excrete excess water and sharpen EAH risk. Don’t take them before or during a long event.
Symptoms: what to watch for
Mild EAH: headache, nausea, bloated feeling, puffy hands or face, a rising weight on the scale mid-race despite sweating. If your wedding ring feels tight at mile 18, that’s a flag.
Moderate: confusion, unusual fatigue, repeated vomiting.
Severe: disorientation, seizures, loss of consciousness. This is a medical emergency. Severe EAH causes the brain to swell inside the skull — the same mechanism that makes it fatal.
The tricky part: mild EAH symptoms overlap heavily with dehydration. Athletes (and sometimes medical volunteers) have historically responded to a confused, nauseated runner by pushing more fluid — which is exactly the wrong move if the problem is dilutional. Any athlete who’s gained weight during an endurance event and is symptomatic should be assumed to have EAH until proven otherwise.
How to actually prevent it
1. Know your sweat rate. The single best protection is data. Run a sweat rate test (weigh before and after a one-hour workout, account for fluid consumed) so you know your personal number. Aim to replace 60 to 80 percent of sweat losses during exercise — not 100 percent, and definitely not more. A generic every-aid-station rule ignores whether you’re a 40 oz/hour sweater or a 12 oz/hour sweater.
2. Put sodium in your fluid. Plain water is fine for workouts under about 60 minutes. Anything longer, you want 300 to 700 mg of sodium per hour for most people, and heavy or salty sweaters will need more. Electrolyte mixes like LMNT (1,000 mg/serving), Skratch Labs (380 mg), Nuun (300 mg), and Gatorade Endurance Formula (a higher-sodium option) all work — the right one depends on your sweat profile and taste preference.
3. Use concentrated sodium sources when you need them. On long or hot efforts, running out of sodium mid-race is a real problem, and topping up from your drink bottles alone can be slow. A Fast Pickle brine shot (fastpickle.com) delivers roughly 500 mg of sodium in a 2.5 oz shot — useful as a mid-race sodium bump or a recovery boost after the finish line. Some athletes tuck one in a chest pocket as an insurance policy against cramps and creeping dilution.
4. Drink to thirst, not to a schedule. Thirst is a remarkably accurate signal in healthy adults. If you don’t feel thirsty at a water station, you don’t need to drink yet. Skipping a cup will not ruin your race.
5. Weigh yourself at key points on long events. If you’re gaining weight during a race, you’re drinking too much. A flat weight or small loss (1–2% of body weight) is normal and safe.
6. Don’t take NSAIDs on race day. If you’re relying on ibuprofen to push through pain, that’s a pre-race problem, not a race-day solution.
The takeaway
The answer to hydration isn’t “drink more.” It’s “drink smart.” Hyponatremia doesn’t happen to athletes who ignored hydration — it happens to athletes who took the old-school maximum-hydration advice too literally, in conditions that didn’t require it. A thoughtful plan built around your actual sweat rate, with meaningful sodium in your fluid, is both safer and more performance-enhancing than any “top up at every aid station” approach. Hydration is a signal-response system, not a math problem you solve by going bigger.