Are you a triathlete, runner or cyclist who has chosen a race this year where you are likely to be competing in temperatures significantly higher than those you are used to and for at least three hours or considerably longer? Examples might include the Ironman™ World Championships in Kona, the Marathon Des Sables, or any half or full iron distance triathlon, marathon, ultramarathon or endurance cycle event that usually takes place in temperatures over 30⁰C. If so, this blog is for you.

The effects of racing in the heat

An athlete suffering from exertion-induced heat stroke is in a dangerous place. Signs to look out for include fever, dizziness, fatigue, loss of co-ordination, nausea, vomiting and diarrhoea. Immediate medical attention and rehydration are essential. Think Jonny Brownlee at the ITU Grand Final in Cozumel, Mexico in 2016, being helped over the line by his brother Alastair. So, if you are racing in the heat, particularly if you are unaccustomed to doing so, you need to take all the steps you can to prevent this happening to you. In the last few years, particularly 2016-18, there have been several useful research papers published on this topic, and the aim of this blog is to help you understand why problems may occur when racing in the heat and what you might be able to do to help minimise them.

So, what happens in your body when you push yourself as hard as you can in hot temperatures? Initially, this is no different to exerting yourself in cooler conditions. As your muscles work to produce energy and your heart pumps oxygen around your body, your body temperature rises and heat must dissipate from your body as sweat to maintain that temperature within optimal limits. Meanwhile, blood flow is redistributed from internal organs such as the gastrointestinal (GI) system to your muscles to optimise energy production. The difference in hot temperatures is that your body temperature rises faster, and you need to sweat more to dissipate that heat and keep your internal temperature down.

Meanwhile, the lack of blood flow to the GI system may result in damage to the lining of the gut, making it more permeable. Gram negative bacteria residing in the gut may  disintegrate, releasing endotoxins called lipopolysaccharides (LPS) through the damaged wall of the gut into the blood stream in such quantities that your liver may not be able to clear them. This increase in LPS exacerbates the strain on the body already caused by exerting yourself in the heat and initiates an inflammatory process, resulting in the release of cytokines such as Interleukin 6 (IL-6).  In turn, the inflammatory response may be exacerbated in the heat due to the impact of the increased permeability of the gut lining and, in the worst case scenario, may eventually result in severe endotoxemia and potential organ dysfunction, as seen in cases of exertion-induced heat stroke. 1 The risk of developing gastrointestinal distress is also more likely, resulting in a reduced ability or failure to take on fluids and nutrition which is likely in turn to impact adversely on performance or result in failure to finish the event. GI problems are not necessarily related to developing intestinal permeability or endotoxemia, however. Dehydration, lack of blood flow to the stomach and consequent slower emptying time may also be factors.

racing in the heat - nutritional and hydration strategies for endurance athletes

A good illustration of the effects of exercising in a hot temperature of 35⁰C compared with a mild 22⁰C is a recent study in which 10 endurance runners completed a two hour session at 60% VO2max at both temperatures. Circulating endotoxins increased post-exercise in both trials. At the lower temperature, anti-endotoxin antibodies increased by 28%, implying an ability to deal effectively with the endotoxins. But at the higher temperature, the same anti-bodies decreased by 21%. Inflammatory cytokine levels were also greater after the higher temperature trial and increases in body temperature were positively associated with damage to the gut lining, nausea and the urge to vomit. 2

Individual factors that increase the risk of racing in the heat

These issues do not happen to everyone exercising in hotter conditions than they are used to. A range of factors increase the risk:

  • How hot is it? The hotter the conditions, the greater the physiological strain on your body. But this is also individual. For some people the risk of developing heat stroke might occur in temperatures of 30⁰C, for others it might not be a potential problem until exercising in 40⁰C heat, if it occurs at all. The ability to stay hydrated is also a factor here.
  • Are you acclimatised? Training for several days in the race environment enables adaptations that improve tolerance to exercising in the heat and reduces the physiological strain. You can also acclimate to the heat in a controlled environment like a heat chamber if you are unable to travel early to your event. 1
  • Do you have a previous history of gastro-intestinal problems in races? Such as nausea, vomiting, diarrhoea and abdominal discomfort? Or have been diagnosed by a doctor with Irritable Bowel Syndrome (IBS)? If so, it would be advisable to pay close attention to your pre-race nutrition and consider some of the pre-race supplementation strategies which will be covered below.
  • Are you susceptible to illness and infection during periods of increased training load? Higher IL-6 concentrations could make you more susceptible to heat strain due to impaired anti-inflammatory responses. 3 A nutrition and supplementation strategy aimed at limiting the inflammatory response to exercise may be important for you. This will be covered further below.

How likely is the heat to be a problem in your race?

While you may or may not be pre-disposed to problems when racing in the heat, as outlined above, there are several other factors which may contribute to the effect on the heat on your race:

  • How long is the race? The longer you are exerting yourself in the heat, the greater the risk of developing problems. The release of LPS into the blood stream has a cumulative effect, so day-long or multi-day events pose a greater risk than those lasting only a few hours.
  • What type of event is it? The effects of the heat are more problematic in running than cycling due to the cooling effects of airflow while riding. 4 This is particularly significant for triathletes where the impact of the heat will be more noticeable during the run leg of the event. This effect is also exacerbated by the run being the final stage, so the cumulative effect of racing in the heat is likely to be felt at this point.
  • What type of heat is it? You lose more body heat via sweating when conditions are hot, dry and windy than you do when racing in a hot, humid and still environment. This is because your sweat more readily evaporates and does not remain on your skin. This ‘sweating efficiency’ is important because it is the evaporation of sweat from the body, and not the production of it, that liberates heat from the body and brings down your core temperature. The accumulation of heat within the body greatly increases the risk of heat-related illness and injury. 4
  • How well are you able to stay hydrated? Dehydration increases the physiological strain on your cardiovascular system and reduces your sweat rate, increasing the amount of heat stored in your body. 5 It also contributes to reduced intestinal blood flow, potentially increasing intestinal permeability and the subsequent risk of endotoxemia and GI issues. 1 Losing more than 2% of your body weight is generally considered to be the point at which dehydration starts to impact on performance, although again this is individual and may be affected by pre-race carbohydrate loading. Part of the subsequent weight gain is water, as every gram of stored glycogen will be accompanied by 1-3g of water. This is released for use by the body when you burn through the glycogen as energy. As a result, you may lose more than 2% of your weight without being dehydrated. Fluid losses are greater in faster, more competitive runners.

So, given the potential risks to health and performance from racing in the heat, what might you do to help reduce those risks? My recommendations fall into two categories: nutritional strategies to support gut health in the weeks prior to your event and hydration/nutritional strategies to follow immediately before and during your event.

Preparing to race in the heat: supporting gut health

  • Optimise your gut bacteria, known as the microbiome, which plays a significant role in both digestive health and immunity. It is thought that the more diverse species of beneficial bacteria in your gut – ie probiotics – the more robust the microbiome, which might allow your gut to cope better with the challenges presented by exerting yourself in the heat, as discussed above. 1 Here are some ways to improve the diversity of your gut bacteria:
    • Increase the amount and variety of fibre in your diet. Fibre isn’t just found in wholegrains but also in vegetables, fruit, legumes (beans and lentils), nuts and seeds. Ideally, we need to consume 30g of fibre each day, but it’s thought that the average person in the UK only gets 18g. The fibre passes undigested through your small intestine and is then used as fuel by the probiotic bacteria in your colon. Please note: do still avoid a high fibre diet in the 48 hours prior to your race to prevent  too many toilet stops!
    • Consume fermented foods like sourdough bread, bio-live yogurt, sauerkraut, kefir and kombucha. These are natural sources of probiotic bacteria.
    • Take a probiotic supplement for around 12 weeks before your event. Choose one with a range of bacterial strains, not just one. A study of 30 recreational long distance triathletes, who supplemented probiotics or a placebo for 12 weeks prior to an iron distance race, showed lower endotoxemia levels 6 days post-race in the probiotic group. 6 The length of time taking probiotics may be significant too. Another study, where 8 runners exercised for 2 hrs at 60% VO2max in 34⁰C heat, found no meaningful change in endotoxin or cytokine concentrations after 7 days of taking a single strain probiotic (Lactobacillus casei) compared with 7 days of taking a placebo. 7
    • Supplement the anti-oxidants alpha lipoic acid and N-acetyl cysteine in addition to a probiotic. The study on 30 triathletes mentioned above also showed that this combination reduced endotoxemia pre-race as well as post-race, 6 which would reduce the ‘load’ effect of low level endotoxin release during training. This may be due to increases in glutathione levels stimulated by taking ALA and NAC.
  • Protect the integrity of the gut lining, to help prevent the development of intestinal permeability linked to both endotoxemia and general gastrointestinal issues. This may be a particularly relevant if you have a history of GI issues, either generally or in races. Ways to do this include:
    • Take probiotics for 12 weeks prior to the event. The study on 30 triathletes 6 showed an increase in intestinal permeability after the race in those taking placebo, but not in those taking the probiotic.
    • Supplement 500mg of curcumin for three days prior to the event. A small study with 8 participants who ran at 65% VO2max for 60 minutes in 37⁰C heat found that GI barrier function and associated inflammatory response improved when they had taken the curcumin vs a placebo. 8 This supplement should be tried in training before using prior to an event.
    • Please note that it is not recommended to supplement bovine colostrum to help prevent intestinal permeability. Not only are the results of various trials contradictory and of uncertain value in long distance events 3 but colostrum should not be taken by athletes who are drug-tested under the WADA code due to the growth factors and other peptides that it contains, which may result in adverse test findings.
    • It is also worth being aware that GI barrier integrity may be compromised by over-the-counter anti-inflammatory medications like NSAIDS, which are commonly taken by athletes to reduce pain or prevent anticipated musculoskeletal pain during an event. Doing so when exercising in the heat may increase the incidence of heat related injury or GI distress. 3

Hydration and Nutrition while racing in the heat

    • Plan your hydration strategy in advance, together with your fuelling strategy, considering the expected environmental conditions, event duration and expected intensity level. 5 Your hydration needs will be very individual and should cover sodium requirements as well as water. The amount of sodium lost in sweat can vary hugely between people, and if you sweat heavily and lose a lot of sodium, this can have a significant impact on your race performance if you are not keeping your fluid and sodium levels up. I recommend taking the free online sweat test found at as the first step in planning your race hydration. Do this several weeks before your event.
    • Pre-hydrate for 24 hours prior to the event using an electrolyte drink. Sodium, chloride, magnesium and potassium are usually included, with the greatest amount of electrolyte coming from sodium which helps the body to retain fluid.
    • Take a glutamine powder supplement in water two hours prior to your event. The dose should be at least 0.25g/kg, although better results may be seen with higher doses up to 0.9 g/kg. This was shown to reduce intestinal permeability in a trial with 10 recreationally active men running on a treadmill in 30⁰C heat, although any links with reduced GI symptoms were unclear. 9 It may also be worth supplementing glutamine in the same way prior to your longest training sessions in the 12 weeks prior to the event, particularly if training in the heat, and gives you the opportunity to test your tolerance before race day.
    • Implement your hydration plan during the race. Underconsumption is common. Don’t wait to drink until you are thirsty. Be aware that your sensation to thirst is less sensitive during exercise 5 and that it is easy to become dehydrated in hot conditions, particularly when it is still and humid which reduces sweat evaporation and therefore increases risk of harmful rises in body temperature, as discussed above.
    • Consume carbohydrate regularly. Compared to water alone, this has been shown to help prevent intestinal injury and permeability, 10 which may reduce the risk of GI distress and endotoxemia when racing in the heat. Avoid consuming too much protein as this may increase the incidence of GI symptoms.
    • Use ice slurry drinks or icy water during races to reduce body temperature, particularly in hot, humid and still conditions where sweat evaporation is reduced. Be aware that using these to pre-cool before the event will delay the onset of sweating and may result in a greater rate of heat storage and core temperature rise during the initial stages 4


Racing in hot temperatures without experiencing heat-related issues that may adversely affect performance requires careful planning. It is recommended to learn as much as possible about the expected environmental conditions, plan your hydration and fuelling well in advance, and take steps to improve gut microbiome diversity and gastrointestinal barrier stability prior to the event. Then follow your plan on the day from the start of the race, adjusting it to suit the conditions you encounter if necessary.


Jo Scott-Dalgleish BSc (Hons) is a BANT Registered Nutritionist, writing and giving talks about nutrition for endurance sport. Based in London, she also works as a Registered Nutritional Therapist, conducting one–to–one consultations with triathletes, distance runners and cyclists to help them eat well, be healthy and perform better through the creation of an individual nutritional plan. To learn more about these consultations, please visit


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