Running in the heat

Last updated: 23-Aug-18

By James Eacott

Whether you lace up your kicks in Japan or California, France or the UK, chances are you’re preparing to hit the trails under a fairly intense sun. The world over trails are parched and runners are struggling to cope with training and racing in the heat.

In this article we explain what happens to your body when you run in the heat and, importantly, what you can do to manage the impact this has.

So, what happens when temperatures crank up? Well, without wanting to state the obvious, your core temperature increases (just like when you have a fever) so when you run you’ll notice your rate of perceived exertion (RPE) increase.

As your core temperature rises, your body begins to divert blood from the muscles to the skin in order to cool itself through sweating and evaporation. The decreased blood supply to key running muscle groups means less oxygen for your muscles to utilise as fuel. Thus, your RPE increases.

To make it all worse, higher temperatures increase your risk of dehydration making it harder still for your body to cool.

It’s worth noting there are two basic forms of heat: wet and dry. Dry heat isn’t so bad, because our bodies are capable of sweating in excess of 2L per hour in the right conditions, meaning we’re pretty adept at cooling ourselves in these environments.

However, as humidity increases the already saturated air around your body will absorb less sweat from your skin, thus slowing the cooling process. As a result, you’ll find running at a 30⁰C and 90% humidity significantly harder than in 40⁰C ‘dry’ heat.

But fear not, this doesn’t mean running has to be side-lined. You just need to adapt your approach to training and racing to accommodate for these unusually high temperatures. Here’s some tips to get you started.

Nail your fluids during training…

This has got to be top of any list for coping with higher than normal techniques. As mentioned above, hydration is imperative to allow the body to cool itself.

We can’t give you an absolute figure of how much you should drink (you need to find what works for you) but this will give you a good start point: calculate your sweat rate by weighing yourself before and after a run. Record how much fluid you drink, too.

If you run for an hour and lose 0.75kg (750ml of fluid) but drink 500ml, then your sweat rate is about 1,250ml (1.25L) per hour. This is a good starting point to aim for.

…but don’t forget about when you’re not training

It’s all good and well making sure you’re hydrated when you’re running, but don’t forget about the rest of the day. Being constantly hydrated will ensure your body is prepared to train and also it has what it needs to recover.

I find keeping a water bottle on me on the commute, in the car and at work does the trick of reminding me to swig frequently.

Give your body a chance to adapt

Take it slow to begin. We’re in pretty good shape at this time of year, but don’t expect the same paces to come as easy in the heat, particularly if you’ve spent most of the last eight months in chilly climes. Your body will acclimatise quickest if you keep runs shorter and less intense to begin.

Make use of mornings and evenings

Many of you already rise with the sparrows to squeeze your training in, but if you’re not someone who trains early (or late) in the day, then now’s the time to start. Avoid the sun and get your training done before breakfast!

Keep the electrolytes coming

Most of us save electrolyte supplementation for particularly tough sessions or races only but running in the heat depletes you of key minerals which need to be replaced.

Sodium is the key protagonist here, but potassium and magnesium are also important. Your body needs them to ensure your cells retain the right amount of water. Balance is crucial – too much water and not enough electrolytes will tip the balance and could get you in trouble.

Most sports nutrition companies have ‘electrolyte only’ drinks or dissolvable tabs which you can put in water, so you can avoid sugar-dense energy drinks if you want.

Pre-cooling techniques

Literally cooling yourself before a session can help your body cope with the added pressure of training in the heat. Wrap a bag of frozen peas in a towel and hold this against your head, neck, wrists and arms.

If you’re heading out into really cooking temperatures, you can wet a towel and wrap that around your torso for a while!

Adjust your pace strategy

Running in the heat is all about managing expectations. When you set off, expect to run slower for the same effort or heart rate. Mindlessly pushing to stick to your normal mile splits will just mean you’re working too hard and not getting the benefits of training within that zone.

Be wise and know when to call it a day. If you’ve a 4 x 10’ tempo session planned, understand that you might need to pull the plug after three. In high temperatures, that’s ok – don’t stress it. Bag what you can, move on and chill out.

Get your kit right

Utilise lightweight, easy-wicking fabrics that help your body cool. We have stacks of reviews on RunUltra to get you kitted out.

Head indoors

I know for many it’s sacrilege even in mid-winter let alone mid-summer to train indoors but hitting the treadmill could be your answer to maintaining training miles and getting the intense sessions completed. Stick the headphones in, find a motivational video and crank up the air conditioning!

Know when to call it a day

We’re not robots. If you find yourself in a really dark place, then it’s likely your body telling you it’s not happy. Do not underestimate the damage you can do by pushing yourself too hard in the heat.


If you take it easy and gradually build duration and intensity training in the heat, and ensure you’re giving your body all it needs, then you’re onto a winner.

Not only will running (and racing) in the heat become easier, but it’ll bring physiological adaptations which will also make running in cooler climes feel easier too.



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