Performing and running in heat


Throughout all our running careers I’m sure that we have all over-cooked things from time-to-time, especially during a hot summers day, I know I have! Feelings of fatigue, dizziness, nausea, decrease and/or an increase in appetite, dehydration, muscle cramps, uncontrollable shivering; these are all symptoms of heat exhaustion and/or heat stroke, and are a sign that you have overdone things by running in extreme heat.

Thermoregulation plays a vital role in our bodies homeostatic balance, so vital in fact that it can be fatal when things go slightly haywire physiologically when exercising during hot environments. A human can tolerate drops in core temperature of ~18°F (10°C), yet can only tolerate increases of ~5°C (9°F) (1). Understanding thermoregulation, especially during extreme exercise environments can dramatically reduce heat injuries/stress/stroke, and even death.

There are many ways of staying safe and cool when running in hot weather, and not have the summer heat cause detrimental effects to both health, and performance; here are our top tips from Mountain Fuel®:


 Control your core temperature:

The human core temperature at rest is suggested to be 98.6°F (37°C) and it is vital for performance (and health) that this does not fluctuate any more than 5°C (9°F), however, human performance will be negatively affected before this peak is reached. Therefore, it is absolutely essential that core body temperature is regulated. Depending on the event distance, duration, intensity, and the temperature & humidity of the environment, various cooling strategies have been adopted to help regulate core temperature ie: drinking cold water, pouring water over your head & body, in-race fine-spray showers, water-bottle sprays, eating ice-slurry, wearing ice-vests, ice-hats and/or ice-bandanas will all aid in thermoregulation (1).


Physical performance can be significantly impaired when ~2-4% of total body mass is lost through sweat, and >4% can be detrimental to health (2). Building-up to your event (2-4 hours pre-race) consume in the range of ~350-490mL/hr (3) of a carbohydrate and electrolyte drink, Mountain Fuel Xtreme Energy fuel is a good example of this mixed with plain water to help continued hydration, and also to top up glycogen stores.

Athletes are advised not to depend on thirst mechanisms during exercise as the thirst signalling mechanism sent by your brain can be blunted by exercise or overridden by the mind (3). During your event, you should be well practiced in hydration strategies due to knowing your own personal sweat-loss rate. However, it is good practice to aim for the consumption of ~0.4-0.8L/hr of fluids, (4)  ideally a carb / electrolyte drink like Mountain Fuel or equivilant, although this range will need to be customized to the individual athletes needs and tolerances and timing in the race (plain water, carbohydrate drinks, electrolyte drinks, exercise duration & intensity, environment, weather, etc).

Electrolyte intake:

In addition to water, sweat contains electrolytes (sodium being the main one) and these are essential to maintain homeostasis, optimal body function, physical performance, and the perception of well-being, which is why athletes should strive to replenish electrolytes during exercise as part of their hydration strategy. Consequently, to exercising in hot environments, individual sweat rates will increase significantly, typically with a rise in temperature and therefore electrolyte loss is increased, and should be carefully replaced through such sources as Mountain Fuel Xtreme Energy Fuel drink which contains all the essential electrolytes lost through sweating.

In races or training in extreme heat we often suggest diluting down Mountain Fuel as you will naturally drink more ensuring your glycogen replenishment but to aid the additional sweat loss we suggest also taking a regular salt capsule. Salty snacks at feed stations are also a possible option at feed stations to replace sodium although you need to consider the impact of the actual food on your stomach and digestive system which will already be under stress due to the heat.

Be aware of physiological stress signals:

There are many ‘normal’ physiological demands which are similarly incurred by our body’s which are induced by exercising, and exercising in a hot environment, such as an increase in sweat rate, heart rate, thirst, and time to exhaustion. Also, decreases in stroke volume of the heart and maximal performance are seen more regular when we exercise in the heat. These are not unusual when physically active in a hot environment, however, there are a number of physiological stress signals which we should be more aware of, and be able to recognise. These stress signals are ‘alarm bells’ to warn you that you may be beginning to ‘over-do’ such activities in the heat and you should take action immediately, yet they may also be induced through the nature of the event ie: (Ironman, ultra-marathon running, cycling sportive events, and other prolonged endurance events etc.), and this is where ‘knowing your own body’ is beneficial. ‘Alarm bells’ to be aware of include: Heavy/Severe sweating, feelings of discomfort, muscle cramps, dark-very dark urine colour, flushed skin, rapid heart rate, breathlessness, dehydration, weakness, feelings of dizziness & nausea, headaches, vomiting, fainting, and in more dangerous situations, severe vomiting, severe headaches, dizziness, confusion, hallucinations & delirium, drowsiness, breathlessness, and possible heart palpitations.  

Slow down/Reduce intensity:

In some situations, ‘beginning slower’ or simply ‘slowing down’ and/or ‘reducing the intensity’ which you may be currently performing at could be enough to help cool you down, and see you through a potentially more difficult phase in your event. Slowing down and limiting and/or removing the exercise intensity will also bring about a reduction in core temperature, heart rate, sweat rate, and energy requirements; all which will consequently aid in balancing thermoregulation. If the intensity is slightly more difficult to control, and/or is a combination of several intense factors ie: (mountainous terrain, high altitude, dry & arid environment and/or elevated humidity) then you may need to stop, reassess your situation and take appropriate action for you.

The tortoise and the hair scenario is often borne out in hot racing conditions, managing the stress on your body, muscles and stomach in the early stages by starting slowly can result in a steady pace throughout the race and see you appearing to finish strongly while others are crashing and burning.


Ideally, acclimatization should be carried out in the environment ie: (country, weather, temperatures, humidity, terrain etc.) in which you will be competing in. However, unfortunately we are not all as privileged as some athletes who are able to do this in preparation for an event. Yet, not all is lost, as there are alternative interventions which may be useful when preparing to compete in a hot environment, such as repetitive bouts of training (>60-min) in an environmental chamber (+30°C), regular hot baths and saunas, and/or even training in enclosed heated rooms such as steam rooms and conservatories are all suggested to aid in heat acclimatization.

Heat acclimation will bring about several positive physiological adaptations such as increased blood plasma volume, increased sweat rate, and begin to sweat sooner. Decreases in core temperature, heart rate, and sodium loss will also be seen after periods of heat acclimation (5).


Competing in hot environments can induce all kinds of nutritional issues, therefore it is imperative for individuals to be organised, and to be aware of their own nutritional strategies before, during, and even post-event in such extreme environments. Training and/or competing in hot conditions consequently will bring about an increased muscle glycogen need due to increased physical and physiological demands (5). Therefore, in prolonged bouts of endurance exercise conducted in hot environments it is vital you continue to replenish glycogen stores throughout the duration to reduce the risk of premature fatigue and/or exhaustion. This can be done in various ways such as solid foods, energy gels if used liberally and/or bars, or through carbohydrate-loaded drinks such as our Xtreme Energy Fuel, which, when chilled is much more refreshing on a hot day than attempting to chew through some sticky energy bar.

Remember on top of having a Recovery Fuel to begin the replenishment and re-synthesis process after a hot training or race to also rehydrate post-event too with Mountain Fuel Xtreme Energy Fuel, as this is very important for replacing those vital electrolytes. Plain water is also essential after such demanding events in such extreme environments.

Climate controlled clothing:

The debate on whether climate controlled clothing actually works is still rolling on, and the ‘research jury’ are still out on this one I’m afraid. However, like so many things in endurance sports, it seems to come down to personal preference, and personal experience. Sweat wicking garments with the promise of helping us stay dry and cool are all the range now for endurance athletes…but do they really do what they say on the tin?…or would you be better off not wearing a shirt (guys)? Some individuals cannot wear polyester clothing otherwise they have problems with sore skin, and does the colour of your clothing have any effect on thermoregulation, again there are conflicting arguments on this issue to, so I suggest you be sensible with your attire, dress for the occasion and go with experience and preference on this one, and use a combination of the other suggested cooling strategies when training and/or competing in hot environments. Two pieces of clothing advice I would suggest for running in hot sunny environments: Peaked cap and sunglasses are essential!

In summary;
Keep hydrated by sipping a combination of liquid carbohydrates and electrolytes, ideally in the same drink to avoid taking on too much fluid

Too much fluid, particularly water alone can be harmful

Consider taking salt caps to supplement sweat loss on a very hot day

Pace yourself, on a hot day the majority of people simply can’t run at their usual pace

Make the most of available water sources to soak hats, bandanas etc

Be aware and know your own body and be prepared to act upon the onset of physiological stress  


1Katch, V.L, McArdle, W.D; Katch, F.I. (2011). Essentials of Exercise Physiology (Fourth Edition). Philadelphia, USA. Lippincott Williams and Wilkins

2– Seebohar, B. (2011). Nutrition Periodization for Athletes. (Second Ed.) CO, USA, Bull Publishing Company

3– Maughan, R.J and Noakes, T.D. Fluid Replacement and Exercise Stress. Sports Med. 12:1, 16-31, (1991)

4Sawka, M.N and Burke, L et al. ACSM Position Stand: Exercise & Fluid Replacement. Medicine and Science in Sports and Exercise. 39:377-390, (2007)

5– Ryan. M. (2012) Sports Nutrition for Endurance Athletes (3rd Ed.). Colorado, USA, Velo Press