Beat the heat — how can you keep your cool on race day?
Thomas Solomon PhD.
16th Oct 2021.
We are certainly mammals with an impressive capacity for exercising in extreme heat but that doesn’t mean we can’t improve our savvy ways… In Part 1, you can learn all about what heat does to your physiology and performance and why it is important to “keep your cool”. But, if you don’t care about that, here in Part 2 you will learn how to “keep your cool” during your sessions and races so you can keep on rocking...
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It's gettin' hot in here. So take off all your clothes... Well, this is the strategy Nelly used amidst his prolific running career. But, he also sported big puffy jackets in the heat of summer, so you might like to consider some other ways to combat the heat when you are deep into a race...
As you learned in Part 1, getting “hot” during exercise leads to increased cardiovascular strain (heart rate & breathing rate), increased perception of effort (RPE), increased glycogen breakdown, and so on… All these “hot” things cause fatigue. But, heat and humidity also make strenuous exercise a little riskier and can increase your risk of an exertional heat illness. Therefore, the aim is to “Be cool”, as John Travolta once said. And smart athletes do just that by using strategies to mitigate the effects of heat.
So, what do you need to consider to “Be cool” in the heat?
Hydration in the heat.
First off, you cannot “adapt” to dehydration. Deliberately training in a dehydrated state will simply bury you in a hole. You are much better off learning to stay adequately hydrated day to day — drink to thirst — and, if necessary, to learn to use a strategy that helps you stay hydrated during your sessions and races, especially if they are long and absolutely if you are training in a hot environment — combined drink to thirst and planned drink to a schedule depending on the logistics of the event (see Part 4 of my series on Hydration for practical strategies).
As I described in Part 1, for every litre of water that evaporates from your skin, ~580 kcal of heat is removed from your body. Sweating is awesome because similar to pain, sweat is like a weakness leaving the body — sweat puts water on your skin to remove excess heat to keep you cool and to keep you producing high power output for longer. Therefore, sweat is your friend and being sweaty during exercise is helping you. But, every yin has yang. Sweat also removes water from your total body water “tank” (aka dehydration — see Part 1 of my hydration series). Also, because higher sweat rates are correlated with higher sweat sodium concentrations, sweating at a high rate or sweating for a long time can also remove sodium from your tank.
Your “cooling” response to exercise in the heat is simply to sweat more — more water leaves the “tank”. If dehydration persists, plasma volume will drop making it harder for your blood to perfuse the skin to allow cooling. Therefore, you can probably surmise why starting exercise in the heat in a “hypohydrated” (or “dehydrated”) state and/or not adequately maintaining hydration status during exercise in the heat, make it more difficult to cool down (see my series on Hydration for more info). Furthermore, if dehydration persists during exercise in the heat, you will likely lose more sodium, which will lower plasma osmolality, causing fatigue and/or hyponatremia — double trouble (see Part 3 of my hydration series).
Whatever hydration strategy you use, always practice it in training to ensure it is palatable, safe, practical and works for you — know what it does to your GI system, whether you can handle it, and whether it makes you feel full and unable to eat… This brings me to the second important consideration...
Nutrition in the heat.
When your hypothalamus deals with signals from central and peripheral thermoreceptors, which have detected increases in your core temperature and the air temperature, one of the reflexes is to send a signal to the adrenal medulla to secrete epinephrine (aka adrenaline) to increase heart rate and cardiac output. One of the “annoying” side-effects of epinephrine/adrenaline is that it stimulates glycogen breakdown (glycogenolysis) and increases carbohydrate oxidation. Therefore, your muscles munch through your stored fuel more quickly when exercising in the heat… So, in the heat, appropriately “Fueling for the work required”might necessitate a higher rate of carbohydrate intake during long-duration sessions/races.
But, there are caveats...
When it is very hot, you will be moving slower than on a cooler day so your energy expenditure — your rate of “burning” stored fuel — will be slightly lower. Also, when it is super hot and humid, it is not uncommon to lose your appetite and find that your food (real food and fake food, aka gels, blocks, potions, etc) can taste like it has lost a Michelin-star while any fluid you are carrying will get pretty warm and less palatable. Plus, if you use sports supplements, a 2021 meta-analysis found that the ergogenic performance benefit is dampened in the heat for most supplements (including caffeine and nitrate/beetroot).
All that said, there is another more serious consequence of training/racing in the heat: the potential loss of gastrointestinal (GI) integrity, which can lead to GI problems like bloating and sickness and food aversion (you can go deep on that here and here). Consequently, if you become a veritable fountain of puke, sticking to a well-planned and practised fuel intake strategy can be tough.
When training in the heat, experiment with foods/drinks to find items that remain palatable when they are warm, your mouth is dry, and your appetite is low. Then, on a hot race day, you will have found what works for you and you will be ready.
Now you know to experiment with your hydration and nutrition strategies in training to perfect them in time for a hot race day. You also know that your natural cooling mechanism — sweat — is awesome (Part 1). But, in hot and humid conditions you will notice that more sweat accumulates and drips off your skin. And this is where the problem begins. Because if sweat is dripping off, it is still liquid and is not evaporating as a gas carrying your body heat into the air. Consequently, when it is hot and humid and/or when you notice more sweat dripping off you than usual, you need other cooling strategies because your sweat is not cooling you. So...
How to stay cool before and during a session/race.
Just watch what athletes do during ultra races in the heat for some clues about how best to mitigate the rise in your core temperature during a race — ice vests, ice caps, ice arm sleeve... At the 2004 Athens Olympics, I also recall images of athletes with their hands and faces in “cooling basins” and I remember the Australian athletes at the 2008 Beijing Olympics drinking from their ice slushy machine before events. All athletes in all sports could learn a lot from simply opening their eyes and being open-minded to other athletes’ habits.
During sessions and races, your goal is to maintain your thermal comfort for as long as possible! Of course, on a hot day, you can simply choose to train in the cooler and more shaded times of the day. But, on race day, you cannot choose the start time. So, to be like John Travolta, I suggest the following tried and tested strategies:
The sun’s radiation can be very intense in direct sunlight or from reflective radiation coming from any surfaces (roads, walls, etc), thus contributing to your heat gain. So, stay in the shade before your race, find shade at aid stations/rest stops, and do your best to navigate via the shady side of the trails and roads.
Wear sunglasses (with a polarising filter), wear a sun hat/trucker cap, and use sunblock (factor 30 or higher).
These might not keep you cool but they will protect you from the sun’s radiation. But… Some sunglasses become uncomfortable when worn for long periods or they jump around on your face while running. Some hats are like a tea-cosy and heat your head — the total opposite of your goal. And, some sunblocks can be exceptionally greasy, making it impossible to open wrappers/bottles when you’re mid-race. Furthermore, some sunblocks can reduce your sweat rate during exercise, potentially blunting your ability to cool down. That said the evidence on the effects of sunblocks on sweat rates is sparse and equivocal: work from Aburto-Corona & Aragón-Vargas found that inorganic sunscreens blunted sweat rates during exercise whereas organic chemical sunscreens did not, while Ou-Yang et al. and Connolly & Wilcox found that sweat-resistant sunscreens did not blunt skin cooling during exercise. So, as always, play around with what works for you.
Throw cold water on your head, neck, and shoulders, etc.
Remember, ~580 kcal of heat is removed from your body per 1 litre of water that evaporates from your skin. So, throwing water onto your skin is a great way to remove heat from your body. But, remember that to remove heat from your body water must evaporate not drip off. So, if it is hot and the relative humidity isn't super high, you’ll be able to cool down if you have the opportunity to pour water all over your body. Also note that throwing water over your head can also move sweat into your eyes and soak everything you are wearing, which can increase the risk of abrasion from clothing — the reason I put plasters (Bandaids) on my nips and lube around my bits when it's a day for sweatiness and/or water throwing.
During exercise, as brain temperature approaches 40°C, cerebral blood flow decreases and brain wave activity is impaired with a concurrent increase in RPE and reduction in maximal voluntary muscular force output (reviewed here and here). Therefore, a “cool” brain might help prevent fatigue, and there is certainly some evidence that supports the use of head cooling both beforeand during exercise. So, if humidity is too high to allow for evaporation, throwing cold water on your head may still help reduce brain temperature. My first memory of the immediate pick-me-up (lowering of RPE) and a Mario-cart-like “power-up” of speed I received as a consequence of cold-water-on-my-head cooling was during the 2004 World Duathlon Champs in Belgium. On a hot and humid day, the local fire service had installed a bunch of fire hose showers throughout the racecourse — it was magical running through them!
There are freshwater “brunnens” (springs) all around the mountains where I live. When it is hot, I always dunk my head and shoulder and cool my arms and legs in them, and top up whatever drinking vessel I have with me. In other parts of the world, when it is hot, I always plan runs where I know there will be access to water — a grocery store, a lake/river, or even the occasional cliff jump. Most races provide water for drinking but you can also douse yourself in it. Most trail and mountain races also pass streams/rivers/natural springs, providing many opportunities for cooling on a hot day.
Drink a pre-race slushy or ice-cold beverage and drink cold fluid during exercise.
Ice slushies (crushed ice in water) give you the sensation of cold and can be ingested during the hour before exercise (e.g. 2-servings of 14 ml of slushy per kg bodyweight consumed 30–60 min pre-exercise). Now, since you know that water evaporating from your skin cools you down because evaporation requires heat energy from your body, the same principle occurs when you drink ice slush and ice-cold fluid — your body heat warms them up, thus removing heat from the body via conduction. Since we know the specific heat capacity of water (4.184 KJ/kg/°C), we can even get really nerdy and calculate its heat storage capacity. For example, if you drink 500 mL (0.5 kg) of cold (10°C) water, you will remove 4.184 x 0.5 x (37-10) = 56 KJ (or ~14 kcal) of heat from your body. But, as you will notice, 14 kcal is a far smaller heat loss potential than for water to evaporate off your skin (580 kcal per litre).
A 2012 systematic review by Burdon et al. also showed that athletes find cold (0-10°C) or cool (<22°C) drinks more palatable than warmer ones (≥22°C) and that drinking cold/cool beverages increases ad libitum fluid consumption (moderate
effect sizea quantitative measure of the magnitude of the experimental effect. Less than 0.2 is no effect, 0.2 to 0.5 is small, 0.5 to 0.8 is moderate, greater than 0.8 is large.
= 0.87,
95% confidence intervalthe range of values within which the true value would be found 95% of the time if the data was repeatedly collected in different samples of people.
0.44-1.3) and better maintains hydration status (large
effect sizea quantitative measure of the magnitude of the experimental effect. Less than 0.2 is no effect, 0.2 to 0.5 is small, 0.5 to 0.8 is moderate, greater than 0.8 is large.
= 1.26,
95%CIthe range of values within which the true value would be found 95% of the time if the data was repeatedly collected in different samples of people.
But, remember, during exercise you can frequently pour far more water over your head, shoulders, and body than you can fit cold water or ice slush in your belly. Plus, some folks find that drinking ice cold fluid or slushies during running triggers gut problems and nausea. So,
Discover whether your stomach can tolerate ice slush or ice-cold fluid during training before trying it on race day.And,
If there’s water to pour on your skin and you’re training/racing in the heat, always use it!
Use ice vests, hats, neckerchiefs, and arm sleeves.
In the summer, some ice-in-a-hat is my go-to pre- and during-session “cool tool” and I religiously keep soft flasks in the freezer so I can carry them in hand or in my pack on longer runs — I learned this trick on a “CocaCola soccer skills” course many moons ago in a hot London summer of 1987!
Consider using ice-clothing before gun time, during breaks during your sessions, and even at aid stations during long ultras. But… Whatever you choose, don’t freeze yourself — the aim is to be cool not cold.
Wear clothing that allows your sweat to evaporate and keeps you cool.
Moisture transport and water absorption properties of clothing is another important consideration. Some clothing (e.g. rain jackets) and backpacks can be a barrier to evaporative and convective cooling. Running on a warm day when it is raining is a pain — if you go short, you don’t need a jacket but if you go long, there is always the toss-up between maintaining warmth and staying cool. On such a hot, wet, long day, I usually opt to wear a light rain jacket from the house then take it off when I feel warm and/or alter my plans to loop past home mid-run just in case. But, no matter the temperature, following much trial and error, in the rain I will always opt for a waterproof, windproof, and breathable layer of GoreTex.
Running clothing comes in a smorgasbord of choices. Some are designed to be windproof and/or waterproof, others are designed to be “wicking”. It’s a matter of utility and comfort. Sticky/moist clothing or clothing that absorbs water and becomes heavy is just annoying and an unnecessary distraction to your focus on forward progress. Choose clothing that works for you and feels good. But avoid cotton, it’s rubbish — it absorbs water, gets heavy, prevents sweat evaporation, and chafs thy nips.
So...
Which is the best cooling strategy?
Well, let’s turn to
systematic reviewsA systematic collation of experimental evidence according to pre-specified eligibility criteria that answers a specific research question and to help to inform decision, guideline, and policy.
and
meta-analysesIf the qualitative synthesis of a systematic review is sound and the evidence collated has low bias, a quantified statistical analysis—a meta-analysis—can be completed to determine the overall effect size of an intervention.
of all known experimental evidence to examine the
effect sizesa quantitative measure of the magnitude of the experimental effect. Less than 0.2 is no effect, 0.2 to 0.5 is small, 0.5 to 0.8 is moderate, greater than 0.8 is large.
95% confidence intervalthe 95% CI is a range of values within which the true value would be found 95% of the time if the data was repeatedly collected in different samples of people.
d=−0.32a type of effect size that quantities the average change score relative to the standard deviation (i.e. the range) of the change scores.
;
95% confidence intervalthe 95% CI is a range of values within which the true value would be found 95% of the time if the data was repeatedly collected in different samples of people.
Systematic reviews of during-exercise cooling (Tyler et al. 2013, Bongers et al. 2014, Ruddock et al. 2016, & Douzi et al. 2019) find similar effects. Using a face spray, fan, or ice vest has a moderate beneficial effect on endurance performance (d=0.72; 95%CI 0.58 to 0.94) while using ice packs, ice vests, or cold water/slushy ingestion has a small beneficial effect on endurance performance (d=0.40; 95%CI 0.15 to 0.66) as does using neck cooling, cold fluid ingestion, or ice slurry ingestion/mouthrinse (d=0.48; 95%CI 0.78 to 0.18). Once again, these effects are unique to endurance exercise as the effect on anaerobic exercise (lasting less than 75-seconds) is negligible (d=0.27; 95%CI 0.04‐0.50).
effect sizea quantitative measure of the magnitude of the experimental effect. Less than 0.2 is no effect, 0.2 to 0.5 is small, 0.5 to 0.8 is moderate, greater than 0.8 is large.
for pre-exercise cooling is 0.72 standard deviations, which in the mens' race in Tokyo is equivalent to an endurance performance improvement of 263 seconds (4m23s). Therefore, any of the runners behind Kipchoge who finished in 2:13:01 or faster, which includes the 2nd to 12th place finishers, might have won gold if they used a pre-cooling strategy. Similarly, the most recent meta-analysis of during-exercise cooling found an effect size of 0.6 standard deviations, equivalent to an improvement of 220 seconds (3m40s), meaning that anyone finishing behind Kipchoge who was faster than 2:12:18 might have won if they used a during-cooling strategy. Of course, my inferences are mights and I make the (false) assumption that all athletes did not use cooling strategies, but they give you an idea of the potential impact of something so simple.
Most of the systematic reviews commented on the large range of cooling protocols used, poor study quality, the near complete lack of ecologically-valid outdoor race-like studies, and publication bias (Tyler et al. 2013, Douzi et al. 2019, and Bongers et al. 2014) in favour of only positive-findings being published. Therefore, the experimental evidence should only be used as a guide… Consequently,
The best cooling strategy is the one that keeps you cool and is practical to use while not freezing you, annoying you, or making you uncomfortable or nauseous.
Before wrapping up my rant on how you can stay cool in the heat, there is one very important and quite topical question to consider...
What about menthol — should you trick your brain into thinking you are cool?
When cool things are in your mouth, your brain senses the cool and goes “ahh”. This reduces thermal sensation and discomfort but does not lessen your body’s heat load. Ice slushies in the mouth “trick” your brain into thinking you are cool. But, the good thing with ice is that heat energy is lost from your body to melt the ice, contributing to cooling. Ice is good.
Tricking yourself to be like John Travolta is dangerous because when your brain is telling you to stop, it is doing so because its data analysis indicates that the rate of increase in your body temp is too rapid or that your body temp has reached a critical threshold. In other words, you are “not cool” and your brain is protecting you from exertional heat illness. For this reason,
Anything that makes your brain “think” you are cool while not actually removing heat from your body may increase your risk of an exertional heat illness.
So, in my opinion, stick to the safe ways to be cool… Hydrate, stay in the shade, and use cold water/ice/vests immediately before and during exercise.
You will reach fatigue during exercise in the heat at an elevated core temperature that is unique to you. Starting exercise with an elevated core temperature or having a high rate of increase of your core temperature during exercise, simply means you reach your thermal threshold quicker. Therefore, aim to start cool and, once the gun has fired, aim to be cool.
During very short-duration sessions and races (e.g. up to 5 to 10-mins), you simply do not have enough time to raise your core temperature UNLESS it is already elevated to begin. During longer-duration sessions and races (i.e. longer than 5-10-mins), the quicker you “burn” the faster your core temp will rise; and, the longer you “burn”, the hotter the furnace will become . These scenarios elevate your risk of “overheating”, causing fatigue sooner than you want while increasing your risk of developing an exertional heat illness.
To stay safe when training or racing in the heat,
Never underestimate the heat — when the air temperature reaches and exceeds body temperature (~37°C or ~ 98°F), it becomes difficult for heat to leave your body so you must use strategies to minimize the risk of heat stress and heat-related illness.
So…
Always stay aware of the signs of heat stress and exertional heat illness. This might include: disorientation, confusion, dizziness, irrational or unusual behaviour, inappropriate comments, irritability, headache, inability to walk, loss of balance and muscle function resulting in collapse, profound fatigue, hyperventilation, vomiting, diarrhoea, and delirium.
And
Always stop and seek help if you are developing an exertional heat illness.
On a hot and humid day, use the following free (or very cheap) and highly effective heat mitigation strategies to minimise the risk of an exertional heat illness and to resist fatigue and keep on rocking for as long as possible...
Stay aware of your feeling of thirst — if you are thirsty, drink some fluid; if you crave something salty, eat something salty. Carry fluid with you, ready to drink when you need it.
Drink ice-cold fluid and/or try drinking ice slush — your body heat will be used to melt the ice.
Train at cooler times of the day — this is usually in the morning.
Start your sessions cool — I cover my skin in cold water just before leaving the house, and permanently have a hat and a soft flask of water in the freezer ready to take out with me.
Slow down to reduce heat production — during your sessions, lower your intensity (speed/pace) compared to normal.
Take extra breaks and/or longer rest-intervals during your sessions — this reduces your heat production.
When you see shade, go in it and try to stay in it — minimize your time in direct sunlight to decrease heat gained by the body from solar radiation.
When you see water, cover your skin in it — regularly spraying or splashing water on your skin will remove heat from your body when the water evaporates from your skin.
And,
Place ice packs on your skin, wear an ice vest, and/or put ice in your hats, neckerchiefs, and/or arm sleeves.
But, The goal is to be cool, not cold.
And… Be sensible and don't try to be a hero — if you feel shit, stop immediately and find somewhere to cool down.
Thanks for keeping cool during today’s session. Until next time, “Be cool” like John Travolta.
×
Disclaimer: I occasionally mention brands and products but it is important to know that I am not affiliated with, sponsored by, an ambassador for, or receiving advertisement royalties from any brands. I have conducted biomedical research for which I have received research money from publicly-funded national research councils and medical charities, and also from private companies, including Novo Nordisk Foundation, AstraZeneca, Amylin, A.P. Møller Foundation, and Augustinus Foundation. I’ve also consulted for Boost Treadmills and Gu Energy on their research and innovation grant applications and I’ve provided research and science writing services for Examine — some of my articles contain links to information provided by Examine but I do not receive any royalties or bonuses from those links. These companies had no control over the research design, data analysis, or publication outcomes of my work. Any recommendations I make are, and always will be, based on my own views and opinions shaped by the evidence available. My recommendations have never and will never be influenced by affiliations, sponsorships, advertisement royalties, etc. The information I provide is not medical advice. Before making any changes to your habits of daily living based on any information I provide, always ensure it is safe for you to do so and consult your doctor if you are unsure.
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About the author:
I am Thomas Solomon and I'm passionate about relaying accurate and clear scientific information to the masses to help folks meet their fitness and performance goals. I hold a BSc in Biochemistry and a PhD in Exercise Science and am an ACSM-certified Exercise Physiologist and Personal Trainer, a VDOT-certified Distance running coach, and a Registered Nutritionist. Since 2002, I have conducted biomedical research in exercise and nutrition and have taught and led university courses in exercise physiology, nutrition, biochemistry, and molecular medicine. My work is published in over 80 peer-reviewed medical journal publications and I have delivered more than 50 conference presentations & invited talks at universities and medical societies. I have coached and provided training plans for truck-loads of athletes, have competed at a high level in running, cycling, and obstacle course racing, and continue to run, ride, ski, hike, lift, and climb as much as my ageing body will allow. To stay on top of scientific developments, I consult for scientists, participate in journal clubs, peer-review papers for medical journals, and I invest every Friday in reading what new delights have spawned onto PubMed. In my spare time, I hunt for phenomenal mountain views to capture through the lens, boulder problems to solve, and for new craft beers to drink with the goal of sending my gustatory system into a hullabaloo.
When training/racing in the heat, it is essential that you start exercise euhydrated and aim to maintain hydration status during exercise.
Anything that makes your brain “think” you are cool while not actually removing heat from your body may increase your risk of an exertional heat illness.
So, in my opinion, stick to the safe ways to be cool… Hydrate, stay in the shade, and use cold water/ice/vests immediately before and during exercise.
. These scenarios elevate your risk of “overheating”, causing fatigue sooner than you want while increasing your risk of developing an exertional heat illness.
Wind is the perfect cooling device. 
Stay in the shade at rest and during the run. 
Wear sunglasses (with a polarising filter), wear a sun hat/trucker cap, and use sunblock (factor 30 or higher). 
Throw cold water on your head, neck, and shoulders, etc. 
Drink a pre-race slushy or ice-cold beverage and drink cold fluid during exercise. 
Use ice vests, hats, neckerchiefs, and arm sleeves. 
Wear clothing that allows your sweat to evaporate and keeps you cool. 
Buy me a beer.
