During exercise, offloading metabolic heat to the environment is one of the biggest challenges the body has to overcome. Despite being well adapted, cooling in extreme environmental conditions can put an unbelievable and dangerous strain on thermoregulation.

So, understanding and defining your own limits is incredibly important for any athlete wanting to perform well, and safely, in the heat.

What is extreme heat? 

There are two definitions of extreme heat: :

  1. A number of unusually hot days occurring in succession
  2. Weather that’s much hotter (or humid) than the average for a particular time and place

When it comes to determining whether heat is 'extreme', much will also depend on the individual. Some people have a lower ‘threshold’ than others and may interpret conditions as extreme on occasions when others deem it OK. A person's degree of acclimation and their training level play a big role in this thermal perception.

You might have heard the old saying, “it’s not the heat, it’s the humidity”, and this does hold some truth. Humidity is the percentage of moisture saturation in the air and it can greatly influence our perception of the ‘feels-like’ temperature by hindering our body's ability to cool.

This ‘feels-like’ temperature is measured by the Heat Index.

The Heat Index takes into account air temperature and humidity, and is widely accepted as a more accurate measure of an environment’s overall heat stress.

Image Credit: Wikimedia

Below is a table indicating possible complications at varying levels of heat index values.

Image Credit: Calculator.net

How does the body deal with extreme heat?

The brain’s hypothalamus, sometimes referred to as the body’s thermostat, continually balances heat gain with heat loss. Exercise or exertion means there’s a greater need for heat loss. Almost 80% of the energy produced by working muscles is in the form of heat which must be dissipated to maintain equilibrium. Problems can arise when the body generates more heat than it can offload to the environment and as a result our core body temperature rises.

An increase in internal temperature of a single degree triggers the thermoreceptors (which detect changes in temperature) in the hypothalamus and initiates the body’s cooling responses.

So, we start sweating and redirect the body’s blood flow away from the core towards the skin. This redistribution of blood to the peripheries allows the blood to cool as it passes closer to the skin.

This process is helped by the dilation of peripheral blood vessels. Whilst this shunting of blood is necessary for optimal cooling, it creates a degree of competition between the muscles and skin, both of whom are signalling for increased blood flow during exercise.

In order to meet this competitive demand under hot conditions, the cardiovascular system must work harder (i.e. increase heart rate) which in turn can raise the perceived effort of the activity.

Over time an upward trend in heart rate despite no change in pace or intensity is known as  ‘cardiovascular drift’. As stroke volume decreases, heart rate increases to maintain cardiac output.

Sweating is the body’s main cooling process. As sweat evaporates from the skin, it removes heat and cools us. High humidity can significantly limit performance because it reduces the effectiveness of sweating by hindering evaporation. If sweat is pooling on the skin because the air is already highly saturated with water, then it’s not doing much cooling.

The normal limits of resting core body temperature are generally accepted as being 36.5-37.5°C (97.7-99.5°F). The human body is far less tolerant to rises in core temperature than it is to reductions. A rise of just a couple of degrees (>40.0°C / 104°F) can spark a cascade of negative and life-threatening responses in worst case scenarios.

Around 1,000 deaths were associated with heat-related illnesses last year in the UK. In hotter climates like Australia, numbers can be much higher and are set to soar as climate change increases global temperatures.

Most heat-related fatalities are people aged 65+, often with predisposing medical conditions, but deaths amongst athletes are also common. It’s thought the young and highly motivated are susceptible to heat-related illnesses because they can ignore or suppress the body’s early warning signs and bypass the ‘threshold’.

Exertional heat illness

Exertional heat illness is the broad term for a spectrum of heat-related illnesses which occur as a result of the body overheating during exercise or exertion and can be categorised more specifically depending on the symptoms and severity.

Symptoms which fall under the same bracket of heat illness include heat rash, heat syncope (dizziness and fainting from the dilation of blood vessels) and heat cramps (muscular cramps due to depleted sodium and fluid levels). At the more severe end of the scale, there’s heat exhaustion and heat stroke.

Because of the variation between individuals, there’s not a precise body temperature which marks the onset of these various heat illnesses. But there’s a number of risk factors which may increase a person’s susceptibility, including a preceding viral infection, fatigue, obesity, lack of sleep, poor physical fitness or conditioning, and/or lack of acclimatisation.

Progression of heat illness.
Image Credit: Abby Coleman

Heat exhaustion

An athlete showing signs of heat exhaustion is the first indicator of trouble. It usually develops in unacclimated persons during the first summer heatwave or the first hard training session on a hot day.

It’s identified by a headache, muscular weakness or cramps, and possible vomiting or nausea. Generally, an individual is still sweating profusely; a major differentiating factor between heat exhaustion and heat stroke.

Exercise-related heat exhaustion primarily occurs because the thermoregulatory system is working ineffectively. This is most frequently a result of a limited fluid intake with excessive sweating causing a reduction in plasma volume.

Examples of this can be found in professional sport. There’s the iconic moment when a heat-exhausted Jonathan Brownlee was helped over the finish line by brother Alistair at the Grand Final of the 2016 ITU World Triathlon Series in Cozumel. It was hot and humid and Jonny hadn’t drunk enough.

An example which will always stand out for me is Scottish marathon runner Callum Hawkins. He was competing in the 2018 Commonwealth Games on the very hot Gold Coast and was determined to get gold.

With a two minute lead on the rest of the field and just over a mile to go, he looked set to do it until gut-wrenchingly he started weaving and stumbling in a similar fashion to Jonny Brownlee. With no Alistair to get him to the finish line, Hawkins soon collapsed.

Whilst being taken away in an ambulance, he reportedly asked the ambulance staff, ‘Did I win?’, unable to recall what had just happened.

Like Brownlee, he also attributed the incident to being severely dehydrated. Though it’s likely that his actual collapse had more to do with heat illness than simple fluid loss.

For a person who’s experiencing symptoms of heat exhaustion, the advice is stop exercising, move to a cooler environment, rest, and strive to replenish fluids and electrolytes effectively. Failing to cool down could mean heat exhaustion continues to develop into dangerously high core body temperatures and eventually heat stroke.

Heat stroke

Heat stroke is a very serious form of heat illness and can occur when a person’s core temperature rises above 40°C (104°F). It’s a life-threatening condition which must be recognised early and treated urgently with whole-body cooling.

Within high school athletes in the US, exertional heat stroke ranks as one of the top three leading causes of death. American Football players are most at risk because their protective equipment exacerbates sweating and limits evaporative capacity.

As mentioned above, one of the main distinguishing features between heat exhaustion and heat stroke is sweating. Whereas those suffering from heat exhaustion continue to sweat, individuals who have developed heat stroke stop sweating.

This reflects the failing of the body’s thermoregulatory system and the skin becomes hot, dry and red. As a result, body temperature rises even higher (and quickly) to 41°C (105°F) and above, marking the lower boundary of heatstroke territory.

Each heatstroke victim responds differently to these extreme core temperatures but early symptoms include extreme irritability and convulsions through the core and limbs. Additionally, the person may  deteriorate into a state of confusion and delirium.

If rapid cooling isn’t undertaken and internal temperatures continue to creep up, vomiting may ensue. Eventually, body cells begin to break down as the body’s proteins are destroyed and rendered useless. The sheaths of the blood vessels begin to leak, causing haemorrhaging throughout the body. Thinking the blood vessels have been severed, the body then responds by initiating a ‘clotting cascade’.

In the most severe cases, if left untreated, heatstroke can cause multiple organ failure, circulatory collapse and damage to the central nervous system.

One of the biggest challenges when dealing with heat-related illnesses is the speed of development. Once overheated, it’s easy to continue on that trajectory of getting hotter, but difficult to cool once the body is past a certain point, especially if the individual can’t escape the hot environment.

Prompt recognition of symptoms is critical as rapid cooling is the cornerstone of treatment. The odds of survival depend on the duration of overheating and how quickly the individual can be cooled. The ‘gold standard’ treatment for exertional heat stroke is whole-body immersion in cold water or ice. But, ice packs to the neck and fluid replacement are valuable alternatives.

Sadly, survival doesn’t guarantee a full recovery. One in three individuals who survive a near-fatal case of heat stroke suffers permanently with multisystem organ dysfunction.

Once an athlete suffers a bout of exertional heat illness, they’re then more susceptible to experiencing symptoms again. Furthermore, their subsequent performance may be impaired, even in less hot conditions.

On top of treating symptoms with cooling when they arise, the best method of prevention is  limiting heavy training on hot days, unless suitably acclimated..

For a real account of what it’s like to experience heat stroke, read this article written by Peter Stark who had a close shave when hiking to find the perfect surf in South America.

Heat Exhaustion Heat Stroke
Excessive sweating No sweating
Cold, pale, clammy skin Body, dry skin
Fast, weak pulse Rapid, strong pulse
Dizziness/fainting Headache/unconsciousness
Nausea/vomiting Nausea/vomiting
Muscle cramps High core body temperature (>40°C or 104°F)

A comparison of heat exhaustion and heat stroke symptoms

It would be easy to assume that the longer you’re exercising in a hot environment, the more at risk of heat stroke you are. But, counterintuitively, the data shows heat stroke is more common in races of 30-90 minutes duration (10km events are common culprits) due to the high intensity pace on shorter courses, compared to longer events where the relative intensity is lower.

So, it seems intensity matters more than duration.

Why does extreme heat cause GI issues?

There’s one other side effect of extreme heat worth discussing and that’s disturbed gastrointestinal (GI) function. GI problems and poor gut health is one of the leading reasons for DNFs in endurance events. If you can’t maintain a healthy GI system you won’t finish – or at least, you won’t finish well.

At the 2016 Rio Games, the world witnessed a stark example of this. French racewalker Yohann Diniz, who holds the world record for the 50-km event set back in 2014, was leading the race comfortably when he began to experience severe gut issues. He collapsed several times and appeared to be losing blood from his bowels. Remarkably, he refused to give up and was hailed 'a hero’ for finishing in eighth place.

The temperature in Rio that day was ~28°C (82°F) and 75% relative humidity in the latter stages of the event (equating to a ‘feels like’ temperature close to 31°C / 88°F and a ‘caution’ warning on the Heat Index).

So why is the GI system so affected?

Intense heat exacerbates overall exercise stress and can have negative implications on GI function due to reduced blood flow to the stomach. It comes back once again to the competing demand of blood flow in the body. The skin and muscles’ need for blood flow trumps that of the gut.

As a result, the body dramatically shunts blood away from the GI system which compromises the absorption rate of nutrition and results in issues like bloating and diarrhea.

It’s recently been suggested that intense thermoregulatory strain may cause injury to the GI system. Blood levels of intestinal fatty acid binding proteins (I-FABP), a marker of GI integrity, were seen to markedly increase when the ambient temperature was increased from 22°C (71°F) to 35°C (95°F) during exercise. This certainly seems to fit with Diniz’s experience in Rio.

Factors which compromise GI function and integrity during exercise. Image Credit: Abby Coleman

How to maintain your performance in extreme heat

We can mitigate the risks and reductions in performance by acclimatising and undertaking quality training, along with some other simple preventative measures.

Heat Adaptation  

When discussing heat adaptations, we’re talking distinct changes to physiology which may take days or weeks, not immediate changes in response to acute heat exposure. Adaptations allow the body to respond better to the heat and heat training could also help us perform better in cooler environments – something Andy’s weighed in on before.

There are two major ways heat adaptations can be attained;

  1. Heat Acclimation
  2. Heat Acclimatisation

Heat acclimation is the process of exposing an individual to repeated heat stress in a controlled or artificial environment (e.g. saunas) with the aim of increasing whole-body temperature and inducing profuse sweating.

Heat acclimatisation describes the same process but when it happens in naturally hot environments.

Adaptations include:

  • Increased blood volume
  • Increased stroke volume and cardiac output
  • A reduction in heart rate and core body temperature
  • Improved thermal comfort and reduced rating of perceived exertion
  • An earlier onset of sweating (the body learns to activate its cooling response at lower temperatures)
  • Increased sweat rate at any given body temperature

In turn, these heat adaptations allow you to hold a higher work rate at a given temperature. It can also improve cognitive-perceptual measures like attention, reaction time and decision-making.

The majority of adaptations occur over the course of 7 to 14 days of heat acclimation.

Image Credit: Science Direct


Athletes in the heat will commonly sweat between 1-2 litres per hour (maybe up to 4L in some super sweaty individuals). In those with high sweat rates, they’ll be drinking less than they sweat and the result is dehydration.

Sweating with inadequate fluid replacement decreases plasma volume, which compromises cardiovascular function, impedes heat dissipation, and causes a steep rise in core temperature. This will eventually diminish an athlete’s exercise capacity and send them off towards heat illness.

It’s no coincidence that both Jonny Brownlee and Callum Hawkins attributed their undoing to severe dehydration.

Adequate fluid and electrolyte replacement helps maintain plasma volume so that circulation and sweating can progress optimally. Check out these articles if you’re interested in reading further:

Cooling strategies

Athletes may benefit from pre-cooling before workouts and races. Evidence suggests it can reduce the stress on the thermoregulatory system by lowering the core body temperature ahead of exercise, essentially creating a bit more of a ‘buffer’ by lowering the starting point.

This could be done by taking a cold shower, using cooling jackets or drinking an ice slurry drink (though this one may influence our perception more than have an actual cooling effect!).

Avoiding overheating before exercise is important so keep cool by sticking to the shade (or hogging the fan) prior to being exposed to heat stress.

During exercise in hot conditions, using cold towels or pouring cold water over your face, head and neck at least provides the sensation of cooling and will be a psychological boost.

Whether there’s a physiological benefit to this is still debated. Certainly, in drier conditions it’s feasible that pouring water over your skin could act like “free sweat”.

Body composition

There’s a reason that the athletes who perform best in the heat are often those who are lean. Generally, decreasing body fat increases heat tolerance - not because fat acts as an insulator but because fat heats up more easily in the body.

Extra fat isn’t the only bulk problem which can impede heat tolerance. The same can be said about excess muscle.

Take American Football, if a 270-pound player adds 30 pounds of muscle, he can generate more heat, but he doesn’t add enough extra surface area to shed that extra heat. As such, huge lineman are at a greater risk of overheating.

Typically, individuals who are shorter than average, slender and long-limbed cope better in the heat. This offers the maximum ratio of cooling surface area to heat-generating body mass.


It’s crucial in times of high heat stress that you learn to pace yourself. It’s not only important to preserve performance but also to protect from heat illness.

We offload heat mainly through the evaporation of sweat but when conditions are hot or humid, offloading that heat is more difficult. Therefore, inappropriate pacing (e.g. setting a brutal pace early on) may come with a heavy price. It can cause overheating that’s tricky to reverse when you’re stuck in that environment.

In many ways, you also have to accept that you’re not going to be able to perform at the same intensity in extreme heat. Being comfortable with that fact will go a long way to aid success in these conditions.

Other measures

  • Avoid the midday sun - exercise in the early morning or late evening if possible. If not, try to stick to a shaded location.
  • Wear appropriate clothing - lightweight, loose-fitting, breathable. Avoid dark colours which absorb heat. Wear a hat!
  • And sunscreen

It’s getting hotter…

Athletes should be prepared to face more extreme conditions in their training, racing and everyday lives as temperatures creep up each year.

Those who have a good understanding of how their body reacts to the heat and how best to mitigate the most extreme conditions should have an edge on days when the heat is on!

Further reading