Most of us were taught one rule about hydration, and taught it well: don't get dehydrated. Drink lots, drink early, never let yourself get thirsty. So that's what we aim at, and on a hot long run we aim hard, setting off weighed down with bottles and a plan to empty every one.

The trouble is there are two ways to get hydration wrong, not one. Too little water is the failure everybody knows about. Too much plain water is the one nobody warns you about, and it can leave you feeling worse than the problem you were trying to avoid.

Here's what hydration actually is for a runner, why more isn't automatically better, and how to get it right without doing any maths.

The two ways to get it wrong

When you sweat, you don't just lose water. You lose salt with it, sodium in particular. Hydration isn't really about the volume of fluid sloshing around in you, it's about keeping water and sodium in the right balance. Both errors are a break in that balance, just in opposite directions.

Drink too little and you dehydrate: the classic one, where your blood volume drops, your heart works harder, and everything feels heavier than it should. But pour plain water back in without any salt, faster than you're losing it, and you dilute the sodium in your blood. That's a condition called hyponatraemia, and at the mild end it feels like a sloshing stomach, a headache, and nausea.

The catch, and the reason this matters, is that watered-down sodium looks almost identical to dehydration from the inside. Same headache, same queasiness. So the instinct is to drink more, which is exactly the wrong move, and the hole gets deeper.

Why more water isn't automatically safer

This is the bit that runs against everything we've been told, so it's worth saying plainly. On a normal run, in normal conditions, you are far more likely to be gently over-thinking your fluid intake than to be dangerously behind on it. Forcing litres in "to be safe" is not a free action. Past a certain point it stops helping and starts causing its own problem.

None of this means dehydration isn't real or doesn't matter. It does, especially in heat and on long efforts. It means the goal isn't "as much water as possible." The goal is balance, and your body has a surprisingly good gauge for it built in.

Getting it right without the maths

The good news is you don't need a spreadsheet or a sweat-rate lab. A few simple habits cover almost every run.

Drink to thirst

Thirst is a reliable signal for almost everyone on almost every run. Drink when you're thirsty, in the amount that satisfies it, and stop there. You don't need to force fluid in before you're thirsty, and you don't need to finish the bottle on principle.

It's salt, not just water

Anything over about an hour, or any run where you're sweating heavily, wants electrolytes rather than plain water. A proper hydration tab, or a pinch of salt in the bottle, does more for you than another mouthful of plain water ever will. For shorter, cooler runs, plain water is completely fine.

Aim for pale straw, not clear

Urine the colour of weak apple juice is the target. Completely clear isn't a gold star, it can be a sign you've overdone the water and thinned things out. Pale straw is the sweet spot.

Start topped up, don't cram

Hydration is a slow habit, not a last-minute fix. Sip normally through the day before a hot long run, rather than downing a pint on the start line and hoping it does the job. What you drink in the hours before matters more than a panicked gulp right before you set off.

The stop signs

Most of the time, getting the balance slightly off just means a slightly worse run. But it's worth knowing when a wonky feeling is asking you to stop rather than push on.

  • A sloshing stomach that won't settle.
  • A headache that builds rather than eases.
  • Nausea, especially if it comes on suddenly.
  • Puffy fingers or a ring that's gone tight.
  • Feeling foggy, muddled, or oddly off.

If any of these land, don't just reflexively drink more, because both too little and too much can look like this. Slow down, get to shade, and take stock. If you've been drinking plenty of plain water already, reach for salt or an electrolyte rather than another bottle. If things get worse instead of better, treat it seriously and get help.

Balance, not volume

I used to treat hydration as a number of litres to hit, and read a full bottle as a job well done. What I learnt, usually the hard way on a hot long run, is that the number was never the point. Come home sloshing, headachy and slightly smug about how much you drank, and you've still got it wrong.

Hydration is a balance to keep, not a target to beat, and your body already runs a decent version of it for free. Drink to thirst, salt the long ones, aim for pale straw rather than clear, and stop counting litres. Carry a couple of bottles on a warm run by all means. Just carry them knowing the goal was never to empty them.

This article is general information, not medical advice. If you have a medical condition affected by fluid or sodium intake, or you become unwell during or after a run, please speak to a GP or seek medical help.