How hot weather affects the body
Credit: Getty / Rivers Dale

How hot weather affects your body and your ability to train and race

Hot weather can cause physiological challenges for athletes when training and racing. Sports scientist and Precision Hydration founder Andy Blow explains its impact

Although most of us would probably rather be training and racing in warm and sunny conditions, the physiological challenges are significantly greater than when it’s cold outside. Sports scientist and Precision Hydration Founder Andy Blow gave us the lowdown on the impact the heat has on your body when you’re exercising and shared his tips for coping with hot conditions.

How heat affects your body when you’re training and racing

Exercising in the heat is tough for the body to cope with because of the challenges heat poses to controlling your core body temperature via a process known as thermoregulation.

The thermoregulation process has one key objective, to maintain your core body temperature within a very narrow range. The core body temperature of a healthy human is usually close to 37°C (98.8°F) at rest and, if it increases to above ~40°C (104.5°F), or decreases by ~2°C for prolonged periods, you can be in significant trouble. There’s very little flexibility either way.

At rest, your basal metabolic processes produce something in the region of 100w of heat energy, about the same as a bright light bulb! This excess heat can usually be radiated and convected away to the external environment pretty easily. It's primarily done via your blood being diverted to flow close to your skin, taking heat from your core and ‘dropping it off’ into your external environment.

This is precisely why you start to go red when you get hot and more blood rushes to your skin. But, if you start riding a bike or running hard, your muscles can produce way more heat because humans are only about 25% energy efficient (similar to a car engine) at converting food energy into mechanical work. For example, cycling at 250w at the pedals can produce ~1,000w of excess heat and running 6 minute miles can kick out ~1,500w!

In a cold environment this excess heat production from your muscles is not a huge issue. In fact it’s potentially very useful as it helps keep your core body temperature from dropping too low - so, if you work hard enough, you can run in surprisingly light clothing even on a very cold day.

5 key ways cold weather affects your body and your training

However, when you workout in hot conditions, you still have to try to dump the excess heat off to your external environment and this cannot be achieved by radiation/convection alone (or at all if the air temperature is higher than your skin temperature).

As a result, you have to sweat to prevent overheating. Sweating works because the evaporation of water into vapour consumes lots of energy and results in a dramatic cooling effect. This is why you get cold quickly when you get out of the sea and there’s a breeze in the air.

Why do I sweat a lot?

How does sweating help the body?

It’s also why exercising in a hot and humid environment is so much harder than in a hot but dry climate - if the air is too wet for your sweat to evaporate easily, then the sweat just drips off you, taking very little heat with it.

It’s also the reason why the ‘Heat Index’ gives us a better idea of how hot a given temperature will really ‘feel’, it takes air temperature and humidity into account.

What happens if you do overheat?

The consequences of dramatically overheating can be scary and even fatal. “Heat illness” is the catch-all term used to describe a collection of conditions that occur when the human body overheats and heatstroke is at the really scary (and possibly terminal) end of that spectrum.

Heatstroke occurs when the thermoregulation process is overwhelmed by heat production and there's insufficient ability/opportunity for your body to cool itself down. High humidity, lack of air flow, excess clothing, high air temperatures, a lack of acclimatisation to the heat and high levels of metabolic work all contribute to the risk of heat stroke in athletes.

Ultimately, if a person experiencing heat stroke is not cooled down rapidly enough it can be fatal, causing extensive brain and organ damage.

3 things have been suggested to contribute most to heat stroke risk in athletes…

1) Heavy clothing (e.g. American Football pads and kit), which inhibits cooling

2) Pre-existing illnesses, because these can drive up your resting core body temperature even before exercise is involved.

3) Stimulants or similar medications (e.g. A.D.D meds like Adderall).

This last one is an interesting one. In his excellent book ‘Endure’ sports science writer Alex Hutchinson sorts through some evidence that the influence of stimulants on the brain might be enough to mess with it’s ability to recognise and regulate your core temperature as closely as it should, leaving the door open for overheating to occur if the athlete is sufficiently motivated to push themselves until they drop.

It's thought that this may have been a major factor in the death of Tommy Simpson - a professional British cyclist - on the slopes of Mt Ventoux in the 1960s - he was taking amphetamines at the time of his collapse on a very hot day.

If you ever experience heat stroke or it’s suspected in an athlete near you, the key thing to do is to try to cool them down as fast as possible. Getting them in the shade, placing ice packs on major blood vessels (e.g. the groin, neck and armpits), using cold water sprays, fans and wet cold towels are all good if accessible.

Luckily, heat stroke in athletes is extremely rare, in large part because your body and mind do a great job at subconsciously slowing you down when you’re racing and training in the heat in an attempt to stop you from doing lasting damage to yourself.

Studies have shown that athletes even instinctively set off at a slower pace or lower intensity of exercise in the heat (when blinded to their actual pace). And, when you do start to overheat your brain does all it can to reduce your metabolic output by inhibiting the amount of muscle fibres it lets you recruit and by making you feel fatigued so that you're forced to slow down.

How to train/race better in the heat

1) Adapt through heat training and acclimatisation

The good news is that your body can adapt, becoming significantly better at cooling itself with practice. This is what we call heat ‘acclimation’ (adapting to a simulated hot environment) or acclimatisation (adapting to a real hot environment) and it happens relatively rapidly if done right.

Repeated exposure to the heat has been shown to kick off a handful of key physiological adaptations that combine to help you thermoregulate much more effectively. Interestingly these changes essentially all focus on helping your body cool itself more effectively, rather than working to significantly increase your tolerable core temperature range.

It seems that the upper limit for core body temperature doesn’t ever really budge upwards from about ~40.5°C, no matter what. It’s as though there’s a kind of ‘thermal cut off’ at that point that’s non-negotiable.

Some of the principle adaptations that come with acclimatisation include…

1) An increased sweat rate

2) A lower core temperature stimulating the onset of sweating

3) An increase in total blood volume

4) Improved blood flow to your skin

These changes start to kick in a matter of hours after exposure to the heat. Within about 5 days of exposure most people are about 70% as adapted to the heat as they’ll ever be. ‘Full’ acclimatisation takes about 14 days and the whole process is more effective if you undertake some light training in the heat (60-90 min a day) rather than just passively experiencing the temperature change.

Because some of the adaptations (particularly increased blood volume) are also advantageous for exercise in cool conditions, there has been a lot of interest quite recently in undertaking heat training to boost performance in temperate climates and this is something I’ve written about before when talking about heat training.

Heat training could offer same benefits as at altitude

Importantly, although these adaptations kick in pretty quickly, they also reverse pretty quickly too.

For most people the benefits last around a week before degrading rapidly from there if they’re no longer exposed to the heat. So acclimatising for a race in a hot country should be done in the immediate build up.

If travel to a hot environment isn’t possible, then training in cooler climates with extra clothing on can also be effective as it pushes your core body temperature up and stimulates the body to adapt in similar ways.

It’s well worth doing this if you have an event coming up in the heat but aren’t arriving early enough acclimatise fully. You can also take regular hot baths, use saunas and try to train indoors on a treadmill/bike with no air conditioning or fans to further help the process along.

If you do this though it’s important not to push it too far and really overheat, as heat stroke can be fatal. Caution is a must and you should never undertake heat training alone or unsupervised.

2) Pace yourself

Another critical factor in controlling core body temperature in the heat is pacing. Pacing matters because the majority of the excess heat you need to offload to the environment comes from your working muscles and the faster you go, the more heat you produce.

If you go off too hard in cool conditions and get a bit hot, when you slow down your temperature corrects itself pretty quickly and overheating is averted. But, when it's hot the penalties imposed for overheating early on are more severe. In some cases getting your temperature back down is impossible without a dramatic reduction in pace or even stopping altogether.

So, having a robust and conservative pacing plan than usual when you’re racing in hot conditions is critical if you want to avoid the pitfall of ‘cooking yourself’ by going out too hard early on.

3) Pre cool yourself

One proven tactic commonly employed by elite athletes is “pre-cooling” (i.e. reducing your core body temperature before the start of an event). It allow more ‘headroom’ before the thermal cut off point of about 40.5°C (105°F) is reached.

This can be done with special cooling jackets, ice baths or just by staying in cold rooms right before the start of an event. Drinking cold drinks can help too. Ice-slushies are best as ice is way better than just cold liquids as the phase change of ice to liquid in the stomach consumes a lot of energy, removing heat.

Science find slushies are best before start

As a minimum, avoiding long winded warm-ups that promote an increase in core body temperature and staying in the shade / under the air-con for as long as possible before the start is a sensible plan when it's really hot.

4) Pour water over your head

Another pretty effective tactic is to dump water over your head/body during an event as this will evaporate and act like sweat (assuming the humidity is right), whilst allowing you to sweat less and preserve body fluid and blood volume.

5) Preload on sodium / fluid to start well-hydrated.

Just drinking loads of water before exercising in the heat is definitely not a sensible strategy to adopt. You risk a condition called hyponatremia (lower blood sodium levels through dilution) which will really impact your performance and can even be dangerous.

Hyponatremia: What it is and how you can avoid it

Instead, sodium/fluid “preloading”, i.e. drinking a stronger electrolyte drink before races (or long/intense training sessions) in the heat can help to expand your blood plasma volume and aid your performance.


Precision Hydration's free online Sweat Test helps you get started with personalising your hydration strategy through some good old-fashioned trial and error in training.

Andy Blow has a few top 10 Ironman and 70.3 finishes and an Xterra World Age Group title to his name. He founded Precision Hydration to help athletes solve their hydration issues. He has a degree in Sport and Exercise Science and was once the Team Sports Scientist for Benetton and Renault F1 teams.


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