I’ve been thinking about the importance of hydration a lot while running in this heat wave.

If you’re anything like me, you sweat whenever you run and when the heat rises the sweat increases, tenfold! 

Last week I experienced what felt like the beginnings of serious dehydration, 22km into a 25km run and I’d been saving a bit of water for the final push but all of a sudden I felt physically sick, like I was going to throw up, my heart rate pushed way higher than it usually would for the pace I was doing and I felt like I might need to stop. 

I walked for a short while then managed to take down the last of my water and got through the final few km but when I’d recovered, I thought I should look into hydration in more detail, this is what I found.

Sweat is more than water leaving the body

When you run in the heat, your body diverts blood toward the skin to cool itself. Sweat evaporates, carrying heat away and helping maintain internal temperature.

It is a great system, but it comes at a cost.

Most runners lose between 400ml and 1.2 litres of sweat per hour, though in hotter conditions or at higher intensities this can exceed 2 litres per hour. Sweat rates vary, but the principle stays the same: the hotter the environment, the harder your body works to regulate itself.

And with that sweat goes sodium.

Typically, sweat contains 500–1,000mg of sodium per litre, though some runners lose significantly more.

This matters because hydration is not just about replacing water. It is about replacing the minerals that allow water to be absorbed, distributed, and used properly.

What the body can actually process

There is a limit to how much fluid the body can absorb during exercise.

For most runners, the gut can comfortably process around 400–800ml of fluid per hour, depending on heat, effort, and individual tolerance. Trying to force more than this can overwhelm the stomach, leading to sloshing, bloating, or nausea. Notice anything? Yes, it’s very possible that you lose more than you can physically take in.

A practical guideline in hot conditions is:

• 400–800ml fluid per hour

• 300–700mg sodium per hour

• 30–60g carbohydrate per hour for longer efforts

For ultra distance runners or those training beyond 2–3 hours, carbohydrate intake can increase to 90g per hour if using mixed glucose/fructose sources.

The goal is not full replacement as that is rarely possible, it’s more about minimising the deficit enough to preserve function.

What happens when you dehydrate

Dehydration begins without you noticing.

A 2% loss in body weight through fluid loss is enough to reduce endurance performance, increase heart rate, and impair temperature regulation. By 3–4%, concentration begins to decline more sharply. Decision-making slows. Perception of effort rises.

This matters more than many runners realise.

Running dehydrated is not simply harder, it changes the way the brain interprets effort. Studies show dehydration can reduce cognitive performance, impair short-term memory, and narrow attention. On technical terrain or in races where pacing decisions matter, that loss of clarity can carry consequences.

As dehydration progresses:

• Blood volume drops

• Heart rate rises

• Core temperature increases

• Sweat rate becomes less efficient

• Muscles receive less oxygen

• Energy delivery becomes compromised

The body begins prioritising survival over performance and we start to see the effects.

The signs are often subtle to begin with 

The earliest signs of dehydration are rarely dramatic.

A dry mouth. A slight headache. A pace that feels strangely challenging. A growing irritability or loss of focus.

Then the signs become more pronounced:

• Dizziness

• Chills despite heat

• Muscle cramping

• Goosebumps

• Dark urine after the run

• Nausea

• Reduced sweating

• Confusion

These are not just signs of “working hard.” They are signs the body is struggling to maintain equilibrium.

And in hot weather, that line can move quickly.

Hydration needs to start before you run

One of the common mistakes in summer running is starting already underhydrated.

Good hydration is cumulative. It begins in the hours before you lace up.

A simple marker: pale yellow urine, stable energy, no lingering thirst.

Then during the run, consistency matters more than volume spikes. Small, regular intake is easier to absorb than large infrequent drinks.

Because in heat, hydration is less about catching up and more about staying close so, have a look back up towards the top and revisit what we can process. My benchmark now is one 500ml bottle from my vest every 30 mins.

How to stay ahead

Like foot care, pacing, or recovery, hydration rarely feels important when it is going well.

It becomes visible only when it begins to fail.

But in hot weather, it is one of the few variables that affects everything at once - heart rate, temperature, concentration, muscular efficiency, and the ability to keep making good decisions.

The body can tolerate a lot but it performs best when it does not have to defend itself from preventable losses.

And sometimes, in the heat, the smartest runner is simply the one who drinks early enough not to notice the difference.

Bookmark this page for when it heats up again and work out your own plan and habits to make sure you stay hydrated.