Most runners know they should drink enough. Fewer understand what "enough" actually means during training, why plain water sometimes isn't sufficient, and how both too little and too much can cause problems.
Hydration is simpler than it's often made out to be. But there are a few things worth understanding properly.
Why Hydration Matters for Running
When you run, your body generates heat. Sweating is how it manages that heat – water evaporates from the skin and cools you down. As you lose fluid, your blood volume drops, your heart works harder to deliver oxygen to your muscles, and your perceived effort increases even if your pace stays the same.
You track your miles.
We'll handle the meals.
Even mild dehydration can meaningfully impair performance. [1] In the heat, that threshold comes faster and the consequences are more significant.
Drink to Thirst
The simplest and most well-supported hydration guideline is also the most intuitive: drink when you're thirsty.
For years the advice was to drink ahead of thirst – to stay ahead of dehydration by drinking on a schedule even when you didn't feel like it. That guidance has largely been revised. Research now shows that thirst is a reasonably accurate signal for most runners in most conditions, and that drinking beyond thirst can actually be dangerous by diluting blood sodium – a condition called hyponatremia. [2]
The exception is very long efforts in extreme heat, where thirst sensation can lag behind actual fluid needs. In those conditions, drinking on a loose schedule rather than waiting for strong thirst is a sensible precaution.
When Water Isn't Enough
For runs under an hour in moderate conditions, water is all you need. Once you're running longer, especially in the heat, you're losing more than just water. Sweat contains sodium, potassium, and other electrolytes that plain water doesn't replace.
This matters for two reasons. First, electrolytes – particularly sodium – help your body retain and distribute the fluid you drink. Second, drinking large volumes of plain water without replacing sodium dilutes the sodium concentration in your blood, which is what causes hyponatremia. It's rare but serious, and it happens more often from overdrinking than from underdrinking.
For longer efforts, adding a small amount of sodium to your hydration makes your fluid intake more effective and safer. The Maple Lemon Electrolyte Drink covers this simply:
Maple Lemon Electrolyte Drink
Before, During, and After
Before a run: Start well hydrated. If your urine is pale yellow, you're in a good place. Dark yellow is a signal to drink more before you head out. Don't try to hyperhydrate by drinking large amounts right before – your kidneys will just excrete the excess.
During a run: Drink to thirst. For runs over an hour, aim for roughly 400 to 800 ml per hour depending on conditions and how much you sweat, taken in small sips rather than large amounts at once. If you're taking gels, washing them down with water helps absorption and reduces gut irritation.
After a run: Rehydrate gradually. A useful rule of thumb is to drink about 1.5 times the fluid you lost – so if you lost 500 ml, aim for around 750 ml in the hour or two after finishing. Including some sodium in your post-run fluid or food helps your body retain what you drink rather than excreting it.
Sweat Rate Varies Enormously
One thing worth knowing: sweat rate varies dramatically between individuals. Some runners lose 500 ml per hour. Others lose over 2 litres. Some are heavy salt losers – you can see the white residue on your kit after a long run.
This means generic hydration advice only goes so far. If you consistently struggle with cramping, early fatigue, or a drop in performance in the heat, paying closer attention to your personal sweat rate and sodium losses is worth the effort. In practice, this usually just means noticing how you feel under different conditions and adjusting accordingly.
A Few Common Mistakes
Drinking too much too fast. A sloshing stomach during a run is almost always caused by drinking too much at once. Small, frequent sips are far better tolerated than large gulps.
Relying on sports drinks for all runs. Calorie-containing sports drinks are designed for long efforts. For a 40-minute easy run, plain water is fine.
Ignoring hydration until race day. Your hydration habits across the week matter as much as what you drink the morning of a race. Chronically under-hydrated runners start every run at a disadvantage.
Forgetting about alcohol. Alcohol is a diuretic. A few drinks the night before a long run will have you starting dehydrated. Not a reason to avoid it entirely, just worth knowing.
The Simple Version
Drink consistently throughout the day. Head out well hydrated. Drink to thirst during runs. Add electrolytes for longer efforts, especially in heat. Rehydrate properly after. Don't overthink it.
Most hydration problems runners experience come from ignoring the basics across the week, not from a failure to optimize on race morning.
Fuel.fit tags during-run recipes separately from pre-run and recovery options – including real-food hydration options built around the right balance of fluid and electrolytes.
Sources
[1] Goulet EDB. Effect of exercise-induced dehydration on endurance performance: evaluating the impact of exercise protocols on outcomes using a meta-analytic procedure. Br J Sports Med. 2013;47(11):679-686.
[2] Hew-Butler T, Verbalis JG, Noakes TD. Updated fluid recommendation: position statement from the International Marathon Medical Directors Association. Clin J Sport Med. 2006;16(4):283-292.
