Yes — and on a long day it's more dangerous than dehydration.
Drink more than you lose during hours of exercise and you can dilute the sodium in your blood below a safe level. It's called exercise-associated hyponatremia, and the counterintuitive part is that more electrolytes isn't automatically the fix. Here's who's at risk and what the consensus actually says — honestly, even though we sell electrolytes.
See what you actually lose →Confusion, a worsening headache, vomiting, or unusual behavior during or soon after a long effort — especially in someone who drank heavily — can be severe hyponatremia and needs immediate medical care. Do not treat suspected severe cases by drinking more fluids. This page is education, not medical advice.
What it actually is.
Hyponatremia means the sodium concentration in your blood has fallen too low. The exercise version, EAH, is defined as a serum sodium under 135 mmol/L during or within 24 hours of exercise, and it's graded by how far it falls:
| 130–134 mmol/L | Mild — often no symptoms, or vague nausea and bloating |
| 125–129 mmol/L | Moderate — headache, nausea, feeling "off" |
| below 125 mmol/L | Severe — a medical emergency; the brain swells |
It is the mirror image of dehydration, and it's caused not by losing salt so much as by adding too much water to the blood you have.
Why more electrolytes isn't automatically the answer.
This is the honest part, and it matters more than anything we'd sell you: the sodium in a typical sports drink is well below the concentration of your own blood and sweat. So if you over-drink — even an electrolyte drink — you can still dilute your blood sodium. The primary driver of EAH is fluid volume, not salt content. Pouring in more low-sodium fluid on a schedule is exactly how people get into trouble.
Who's most at risk.
The international consensus on EAH names a consistent set of risk factors. You don't have to be elite — the classic case is a slower participant who drank at every aid station:
- Over-drinking water or sports drinks beyond thirst — the main one.
- Longer events — marathons, ultras, long hot rides, all-day efforts.
- Smaller body mass, and slower finishing times (more hours, more chances to over-drink).
- NSAIDs (ibuprofen, naproxen) taken around the event.
Cramping, by contrast, sits at the other end — usually too little sodium replaced, not too much water. If that's you, this is the page →
The consensus advice, plainly.
Drink to thirst, not to a schedule. The single clearest prevention message from the EAH consensus is to let thirst guide fluid intake rather than forcing a fixed amount per hour.
Match sodium to real losses on long or salty days — not by chugging more, but by making the fluid you do drink carry appropriate sodium. That's what the calculator sizes.
Be extra careful if you're small, slow, on a long course, or taking NSAIDs. If in doubt, less forced fluid is safer than more.
Sources: Third International Exercise-Associated Hyponatremia Consensus Development Conference (Hew-Butler et al., Clinical Journal of Sport Medicine, 2015), summarized and updated in Frontiers in Medicine, 2017; overview in StatPearls (NCBI Bookshelf). EAH defined as serum sodium <135 mmol/L during or within 24 h of activity.
Keep reading
What you actually lose
Your sport and how salty you sweat → the sodium to replace, and when not to.
Why you cramp
Usually too little sodium replaced — the opposite failure from over-drinking.
Why salt, not just water
The OVLT salt sensor and why plain water can leave you thirstier.