Why slow, easy runs are your friend during a heatwave

·7-min read

We’re at risk of stating the obvious here, but here goes anyway: it’s ridiculously hot right now. In fact, the UK has just (provisionally) recorded its highest-ever temperature today, in Charlwood, Surrey, with a reading of 39.1ºC before midday.

But while most are trying to figure out a way to work and sleep in the heat, runners are working out how to train in the heat – and rightly questioning whether it’s even safe to do so at all.

The good news is: you can still run safely during a heatwave – if necessary precautions are taken. But, it’s important to understand the risks involved. ‘The greater the temperature, the greater the risk that heat will negatively impact the person,’ says Jim Pate, senior physiologist at CHHP London. ‘The two main illnesses related to overheating are heat exhaustion and heat stroke. These are potentially life-threatening conditions and, if left untreated, can progress to death.’

For many, then, the obvious thing to do would be to take a couple of well-earned rest days, until temperatures cool down later in the week. But if you’re training for an event and don’t want to miss a session, or are simply itching to lace up your trainers, Pate advises opting for some slow, easy runs. That’s because lower intensity efforts produce less heat in the body. ‘Higher intensity efforts are more thermogenic (produce more heat) than relatively less intense efforts, so moderating intensity or planning in lower intensity focus sessions during a heatwave is a good strategy to keep training on track while minimising risks,’ he explains.

What happens to your body when running in hot weather?

Still tempted to smash out some 1km repeats on the track later on? Understanding what happens to your body when running in hot weather may just deter you. ‘Running and other forms of physical activity involve muscular work,’ explains Pate. ‘Movement is the intended result, but heat is also generated in the process. To manage the thermogenesis (heat production) associated with exercise, the body will sweat in order to cool itself and maintain a suitable operating temperature. A hotter environment places more demand on the body's cooling system and makes it more likely to fail.’

And the body’s cooling system is comprised further if you are dehydrated. ‘If people become dehydrated, they are not able to sweat and cool themselves effectively,’ he says. ‘Overheating and dehydration impact exercise capacity negatively, make efforts relatively more difficult, and can result in fatigue sooner.’

What precautions should people take if planning to run in the hot weather?

Pate advises runners maintain hydration before, during and after a run to ensure the body is able to cool itself correctly. Recommendations on how much additional water we should consume per hour when running vary from 300-800ml, but the main thing to remember is to carry a hydration pack and to sip water regularly.

The best thing runners can do to protect themselves, though, is to minimise exposure to heat as much as possible, says Pate. ‘Exercise in an air-conditioned environment or exercise outside during the cooler hours of the day. Minimising exposure to direct sun is also a good idea as it will exacerbate ambient heat.’

How long does it take for your body to acclimatise to heat?

With more heatwaves predicted this year, you might be wondering if it’s possible for the body to become accustomed to running in hotter temperatures – and exactly how it does so. ‘Exposure to hotter than normal environments and higher intensity training triggers changes in the sweating response,’ explains Pate. ‘Once acclimated, people tend to sweat sooner and more profusely cooling themselves more effectively.’

Current research suggests physiological changes are observed between three and 14 days of heat exposure during heat acclimatisation, says Pate. ‘Research has also shown that higher intensity training sessions, which are more thermogenic, help promote the improvements in sweating that heat acclimation causes.’

However, heat acclimation is a gradual process and should be done strategically and under expert guidance. So, for now, as temperatures nudge 40ºC, your most sensible option is a slow pod around a tree-covered park – wearing sunscreen, loose-fitting clothing and a hydration pack – and at the coolest time of day. Or, perhaps even more sensibly: grab an ice cream, put your feet up and run tomorrow instead. And if it's still hot then, here's some further tips from running coach and founder of Track Mafia Cory Wharton-Malcolm on how to run safely during the warm summer months...

Tips for running in the heat

1. Drink to thirst

In a 2016 study, athletes completed a 20km trail run, either drinking a set amount to replace their expected sweat loss or simply drinking to thirst – their finish times were identical. However, as a ballpark, recommendations on how much water we should consume per hour when running vary from 300-800ml.

It is also important to drink before and after running. Research suggests about 530ml of water before and after a workout of any sort will support recovery and help prevent dehydration post-exercise.

2. Don't forget salt

Running on a hot day means more sweating, and this sodium needs to be replaced. Nutritionist Renee McGregor estimates that at a temperature of 20C, the average runner loses about 1,230mg of sodium per hour, which can lead to gastrointestinal distress, dizziness and heat stress. Top up your sodium stores with salt tabs, sports drinks and real food.

3. Adjust your goals

A study into the effects of heat on marathon running, published in the journal Medicine & Science in Sports & Exercise, found that sub-elites tended to run 20 minutes slower in temperatures between 21C and 26C.

Don't chase the same split times as you would normally hit on a day when the weather is cooler. Monitoring your average heart rate can be helpful in gauging your effort levels comparative to runs in cooler temperatures, so try training by heart rate instead of by pace. As you adjust to running in the heat, you'll be able to go faster at the same number of beats per minute.

4. Add sunscreen

Some runners claim that sunscreen clogs up sweat pores and makes you overheat, but a study commissioned by the US military found that using sunscreen did not adversely affect heat-regulation variables such as skin temperature. That said, not all sunscreen is created equal. Go for at least factor 30 and apply liberally (most of us use about half the required amount). Make sure it's broad spectrum, offering protection from UVA and UVB rays. We've summed up the best sunscreens for runners here.

5. Head to the trail

Roads retain heat and radiate it back onto your body, making your run harder, so head to the grass and shade of some nearby trails. Carry water and a mobile phone with you.

6. Mix things up

'Use the nice weather as an opportunity to try other sports along with running, maybe a duathlon or triathlon. Run to a pool or reservoir or run out and bike back,' Cory advises.

7. Dress for the heat

'Wear loose, breathable clothing, a cap to protect you from the sun or sweat bands to stop the sweat from dripping in your eyes,' Cory says. Opt for a lightweight vest to keep you cool – and a pair of sunglasses, to keep the glaring sun out of your eyes.

8. Instead of avoiding running in the heat, adapt to it

A study in the European Journal of Sport Science found that even two, two-hour runs in hot temperatures stimulated marked improvements, such as decreases in core temperature and an increase in blood plasma, which helps maintain optimum temperature.

Cory says, 'I like to wait until the hottest part of the day at 12 and run along the canal or through the woods away from built-up polluted areas. I try and be a smart runner and just slow down and enjoy the heat... I feel like that’s one thing that helped me when I ran the Speed Project with friends through death valley in California... the fact that I'd always tried to run when it was hot in London and not waited for it to be cool.’

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