Hydration for runners: how much you actually need
Most hydration advice is written by people selling electrolytes. Here is the short version: drink to thirst, and understand why drinking too much is the more dangerous error.
Table of contents
Hydration is the area of running nutrition with the widest gap between what is sold and what is supported.
The short version
Drink to thirst. For the overwhelming majority of runs, by the overwhelming majority of runners, that is the entire protocol.
Thirst is a well-calibrated mechanism. The idea that it lags dangerously behind real need — and that you must therefore drink on a schedule — came largely from marketing rather than evidence.
Under an hour
You do not need to carry anything, in temperate conditions. Drink normally before you go and normally afterwards.
Starting a short run slightly under-hydrated has no meaningful performance cost. Starting it with a stomach full of fluid has an immediate and obvious one.
Over an hour, or in heat
Carry water and drink when you want it. A rough working figure is 400–800ml per hour, but the range between individuals is enormous — sweat rates vary by a factor of several — so the number is a starting point, not a target.
If you want your own figure: weigh yourself before and after a long run, without clothes, and note how much you drank. Each kilogram lost is roughly a litre of sweat. Do it a couple of times in different weather and you will know more about your own hydration than any general guideline can tell you.
Why overdrinking is the bigger risk
This is the part that gets underplayed.
Exercise-associated hyponatraemia — dangerously diluted blood sodium from drinking more fluid than you lose — is a genuine medical emergency, and it has killed runners at mass-participation events. Mild dehydration, by contrast, makes you slower and uncomfortable.
The risk profile is not symmetric, and the people most at risk are slower participants at long events who have been told to drink at every station regardless of thirst.
If you finish a long run heavier than you started, you drank too much. That is the single clearest signal.
Electrolytes
Genuinely useful when:
- You are running longer than about two hours
- It is hot and you are sweating heavily
- You are a salty sweater — white residue on skin and dark marks on kit afterwards
- You have had cramping that correlates with hot, long efforts
Not needed for a 40-minute run in mild weather, whatever the packaging says. For ordinary training, food covers it.
Caffeine
Mildly diuretic, but the effect is small and offset by the fluid the coffee comes in. Habitual coffee drinkers do not need to count it against their intake. Pre-run coffee is a gut-motility question rather than a hydration one — that is covered here.
Checking without weighing yourself
Urine colour is a reasonable rough guide — pale straw is about right, consistently clear suggests you are drinking more than you need. Bear in mind that some supplements, notably B vitamins, turn it bright yellow regardless.
Where this fits
Fluid is the easy half of fuelling. The half that matters more for how you feel over a training block is eating enough.
