Ten miles into a training run last July, with the temperature nudging 29°C, I felt my legs turn to sand and my vision blur at the edges. I’d necked nearly a litre of plain water over the previous hour, doing Everything “right” by the old advice to just keep drinking. What I hadn’t accounted for was the sodium, potassium and other electrolytes streaming out through my skin with every drop of sweat, leaving my blood chemistry dangerously out of balance.
That wobble on the towpath taught me something most casual runners don’t learn until it’s too late: drinking water alone during long, hot runs can create a problem rather than solve one. It’s called exercise-associated hyponatraemia, and it happens when sodium levels in the blood drop too low, often because someone has drunk more water than their body can process while also losing salt through heavy sweating.
Key takeaways
- A runner nearly collapsed mid-run after drinking only plain water in a heatwave—and almost made it fatal by drinking more
- Sweat contains vital electrolytes like sodium and potassium; replace only water and your blood chemistry becomes dangerously unbalanced
- The early warning signs of electrolyte loss mimic ordinary dehydration almost perfectly, making it easy to take the wrong action
What sweat actually takes from you
Sweat isn’t just water. It’s a diluted saline solution containing sodium, chloride, potassium, magnesium and calcium, the very minerals your nerves and muscles need to fire correctly. Sodium losses vary hugely between individuals; some runners are “salty sweaters” who lose visible white streaks on dark kit, while others lose comparatively little. Research published by the American College of Sports Medicine has shown that sweat sodium concentrations can range from around 20mg to over 90mg per 100ml, meaning two people running side by side could have wildly different needs.
When you replace that fluid with plain water but no sodium, you dilute the sodium already circulating in your bloodstream. Push this far enough, particularly over several hours in heat, and cells throughout the body, including in the brain, start to swell as water moves in to balance the dilution. The symptoms creep up quietly: nausea, confusion, that heavy-legged disorientation I felt at mile ten, and in severe cases, seizures or worse. The Boston Marathon has been a site of important research into this exact condition, with a study published in the New England Journal of Medicine finding that a meaningful proportion of runners finishing that race had some degree of hyponatraemia, with the heaviest drinkers of plain water most affected.
What makes this tricky is that the early warning signs mimic dehydration almost exactly. Fatigue, headache, nausea, feeling faint, your instinct screams “drink more water”, which is precisely the wrong move if sodium is already low. I know because that’s exactly what I did for the first six miles that day, topping up my bottle at every water fountain, feeling worse each time.
Getting the balance right without overcomplicating it
You don’t need a sports science degree to run safely through summer, but a few practical habits make a genuine difference. For runs under an hour in moderate conditions, plain water is usually fine; your body’s existing sodium reserves can cover that window without trouble. Once you’re heading past 60-90 minutes, especially in heat, an electrolyte drink or tablets dissolved in your bottle become worth considering, giving your body sodium alongside the fluid rather than just diluting what’s already there.
Weighing yourself before and after a long run, unglamorous as it sounds, gives you real data rather than guesswork. Losing more than 2% of your body weight suggests you’re under-drinking; finishing heavier than you started, or barely lighter despite heavy sweating, can be a sign you’ve overdone the plain water relative to salt intake. Some runners also add a pinch of table salt to food the evening before a long, hot session, or choose an electrolyte drink with at least 300-500mg of sodium per litre as a rough guide, though individual needs vary enough that this isn’t a strict rule to follow blindly.
Thirst itself is a reasonably reliable guide for most healthy adults, contrary to how it’s sometimes dismissed. The mistake many of us make, myself included that July afternoon, is drinking on a fixed schedule regardless of thirst, “just in case,” rather than listening to what the Body Is Actually signalling.
When it becomes more than tiredness
The line between ordinary heat fatigue and something requiring urgent attention isn’t always obvious mid-run, which is exactly why it’s worth knowing the difference in advance. Ordinary dehydration tends to bring thirst, dry mouth, dark urine and tiredness that improves with fluid. Hyponatraemia brings confusion, puffiness in the hands or face, nausea that doesn’t ease with more water, and in my case, that disorientating sandy-legged feeling that didn’t match how hard I was actually working.
If you or a running companion start showing confusion, vomiting, or a seizure during or after exercise in the heat, that’s a situation for emergency medical help, not a lie-down and a bottle of water. I stopped at mile ten, sat in the shade, ate a banana and a handful of pretzels from my bag, and waited twenty minutes before walking the rest home. In hindsight, given how disoriented I felt, seeking medical attention would have been the more sensible call rather than assuming I’d sort myself out.
Runners training through this summer’s heat would do well to rethink hydration as a two-part job: replacing fluid, yes, but replacing what’s dissolved in that fluid too. A running vest with a soft flask of electrolyte solution alongside plain water, rotating between the two, works well for many people I’ve spoken to since that run. Others prefer electrolyte tablets they can drop into a bottle at the halfway point of a route, avoiding the need to carry pre-mixed drinks that turn warm and unpleasant after an hour in July sun.
One detail that surprised me researching this afterwards: sodium losses can persist for a while post-run too, which is why some sports scientists suggest salting your recovery meal a little more generously than usual after a long, sweaty session, rather than assuming the job ends the moment you stop moving. Do check with your GP before making significant changes to your hydration or diet, particularly if you have any existing heart, kidney or blood pressure conditions, since sodium recommendations aren’t universal and what suits one runner’s blood chemistry may not suit another’s.