Nutrition and heat stress - how do they interact?

What happens to the body in hot conditions?

When exercising in the heat, the body's primary focus is thermoregulation (i.e. keeping itself cool!).

Whilst this cooling response is essential, it comes at a large physiological cost - especially in unacclimatised athletes:

• Increased cardiovascular strain (Higher heart rate at the same pace, when racing in the heat) 
• Impaired aerobic performance
• Increased fluid and sodium losses via sweat, increasing the rate of dehydration 
• Elevated core body temperature

Perhaps the most frustrating aspect for athletes, is the disparity between pace and effort. In extremely hot conditions,  perceived effort drastically increases, whilst pace stays the same and often drops.

Underestimating the heat can not only cost you your race but, in extreme cases, your health. 

Why are some athletes unprepared for racing in the heat? 

Despite spending months building robust physiology to withstand the demands of race day, some athletes still arrive under-prepared for the environmental conditions. 

One of the main reasons for this is: training blocks/camps are often completed over winter and/or spring months where conditions are significantly cooler. Whilst athletes have developed the capacity to perform, they lack exposure to the thermal stress needed to be prepared to race when unpredictable conditions lead to a hot race day.  

For recreational athletes this problem is often compounded further. Accessibility to environmental chambers, warm weather training camps and controlled heat acclimation strategies is often limited. 

Nutrition strategies also become vulnerable in the heat. Fuelling plans practiced in cooler temperatures react poorly to heat, altering fluid requirements, gut tolerance, and overall demand for carbohydrates. 

As a result, many athletes may exhibit excellent fitness, appearing primed and ready to race, but poor tolerance to thermal stress ultimately unravels race execution. 

Appetite suppression, gut stress and the underfuelling problem 

One of the largest problems athletes experience in the heat is simply consuming enough fuel for their engine. 

As temperatures rise, the body prioritises thermoregulation over digestion.Two things happen: initially the body suppresses hunger through hormonal changes to devote more energy to staying cool. Parallel to this, redistribution of blood to promote skin blood flow for cooling reduces blood flow to the gut.

As a result, the gut’s ability to absorb carbohydrates is reduced. Often leading to:

• Nausea
• Bloating 
• Severe stomach cramps 
• Missed fuelling opportunities/deviation from fuelling strategy

This creates a dangerous physiological mismatch. Whilst carbohydrate demand increases significantly in the heat, appetite and gut tolerance are simultaneously reduced.  

As a result, athletes may begin underfuelling early into endurance events, not fully understanding the consequences. Whilst short term underfuelling may help relieve gut symptoms, the long term performance consequences are often catastrophic. 

The combination between reduced carbohydrate intake and depleted glycogen stores leads to the rapid onset of fatigue later in the race - often where energy demands to make a race-defining move are highest. This is typically the moment where a race plan collapses and an athlete may experience "hitting the wall”. 

Carbohydrate demands in hot conditions 

Paradoxically, while exercise in the heat reduces carbohydrate absorption in the gut, hot conditions also substantially increase the energy that the body burns during exercise , whilst shifting to favour carbohydrates for energy production (Burke, 2004). Practically, this means that muscle glycogen stores deplete more rapidly in hot conditions. 

In turn, athletes' reliance on exogenous carbohydrate provision (i.e. the carbohydrate they are consuming during the race in the form of gels, bars, drink mixes etc.) is increased to sustain race pace. 

Simple, right? In hot conditions, take on more fuel? 

Unfortunately not. Whilst the body cries out for more carbohydrates, emerging evidence indicates that hot conditions can reduce your ability to use what you consume by up to 20% (Moucin et al., 2025).

The practical implications of this are important - athletes' engines typically require more fuel in the heat, however, their engine is less efficient in using this fuel, tolerating much less. 

The challenge is, athletes are stuck between fuelling for increased carbohydrate demands, whilst simultaneously managing a gut and digestive system under heightened physiological stress. 

For this reason, it is essential that athletes not only design an appropriate fuelling strategy for hot conditions, but also rigorously test this in conditions which replicate the intensity and environmental demands of race day.  

The role of dehydration 

Initial increases in sweat rate allow the body to effectively cool itself, offloading heat and preventing dangerous rises in core temperature. However as exercise progresses, significant fluid loss decreases sweat rate and the body’s ability to stay cool becomes less effective.

As a result of fluid loss, blood plasma volume declines, substantially increasing cardiovascular strain, meaning the heart must work harder to maintain oxygen delivery to the working muscles (Thomas, Erdman, Burke, 2016). Consequently, even relatively small levels of dehydration (>2%) lead to rapid elevations in heart rate and markedly compromised aerobic capacity, increasing the relative effort of race pace (Cheuvront and Knefick, 2014). 

Alongside fluid loss, substantial variability in sodium loss adds to the complex problem of hydration. Sweat sodium concentrations can range from 200-2000mg/L, with the ‘saltiest’ sweaters losing an incredible amount of sodium over the course of a multi-hour endurance event (Baker, 2017).

Interestingly, sodium loss doesn't always align with fluid loss. Two athletes with a very similar sodium loss may exhibit vastly different sweat rates (between 0.3-2.4 L/h), as a result of training status, relative work rate and environmental conditions (temperature and humidity). 

For this reason the most effective hydration strategies are individualised and informed through lab testing. 

Overhydration

Whilst hydrating poorly can pose severe dehydration risks, overhydration is a real and dangerous possibility too. When fluid intake exceeds fluid loss, combined with poor sodium replacement, blood sodium concentration can become dangerously diluted (<135mmol/L). Exercise-induced hyponatremia is the result, leading to a breakdown in race performance and often serious health consequences. 

As such, it is important to replace a sensible proportion of total fluid loss and, if unsure, seek the advice of a performance nutritionist to help you tailor an individualised hydration strategy. 

Practical nutrition guidance for racing in the heat 

Successfully racing in the heat requires careful management of hydration, carbohydrate availability and thermoregulation to prevent physiological strain seriously compromising performance. 

• Start euhydrated - In the 24-48 hours leading up to race day, make a conscious effort to stay hydrated, especially if the ambient conditions of the race location are hotter or more humid than you’re accustomed to. However, there is no need to overdrink. In the hours leading up to the race, consume 5-10 ml per kilogram of bodyweight to step on the start line appropriately hydrated (e.g. for a 70kg athlete this would be between 350ml to 700ml).
  
• Fuel early into the race - Prioritise carbohydrate intake early into the race, sticking to your fuelling strategy before possible gut discomfort and appetite suppression kick in. 
• Prioritise your cooling strategy - Minimise rises in core body temperature throughout the race by drinking cold fluids, or even iced slushie drinks, alongside external cooling methods such as ice bandanas, sun shades and regular rinsing of water. These simple actions can improve fuel tolerance and help maintain race pace during hot conditions.
  
• Liquid fuelling - Sports drinks which include carbohydrates and electrolytes, may be easier tolerated as opposed to solid chews and gels during prolonged races in the heat.
• Carbohydrate mouth rinsing - If you are struggling to digest your fuelling, mouth rinsing with your sports drink or carb mix can offer a quick pickup via CNS stimulation, preventing adding additional stress to the gut. 
• Avoid overconsumption of fluid - Excessive water intake can cause bloating, stomach discomfort and offset electrolyte balance increasing the risk of exercise- induced hyponatremia. 400-800ml/h (Sawka et al., 2007),  is a good starting point but sweat-testing will always tell the true story. 
• Practice nutrition at race intensity in the heat - Gut tolerance can change significantly as a result of thermal and exercise stress. Easy long runs in the heat are insufficient for practicing your fuelling strategy, instead aim to match race intensity as well as the conditions. For example, if you aim to hit 90g/h, you should practice this during a multi-hour run in the heat where you spend at least an hour at marathon pace. 

Ultimately, race day nutrition and hydration strategies should be highly individualised in hot conditions. Sweat rate, sweat sodium concentration and fuelling tolerance can all vary drastically between athletes. 

Heat training recommendations 

In addition to a well-designed nutrition, pacing and cooling strategy, regular heat exposure over a prolonged period of time is typically more effective than one extreme ‘hero’ session in producing meaningful adaptations. 

• Train with additional thermal load (active heat strain) - When outside heat exposure is limited, adding additional layers during inside training with reduced airflow can simulate thermal stress promoting adaptation.
• Sauna (passive heat strain) - 20 to 30 minutes of sauna exposure 3-5 times weekly, as a rise in core temperature induces thermal stress.  Immediately following endurance training can also be a great time to get in a sauna session, maintaining raised core temperature and prolonging thermal stress and adaptation.
• Hot water immersion (passive heat strain) - Bathing in hot water (20 to 40 minutes, 2-4 times weekly) promotes thermoregulation and is a viable option for athletes who do not have regular access to a sauna. 

*If choosing to undertake any of the above strategies, do be mindful of your hydration, practicing the hydration advice mentioned above. 

Summary 

The athletes who perform best in the heat are not always the fittest, but the most prepared. Racing in hot conditions requires more than fitness, generic recommendations are insufficient and can cost you your race. When environmental stress is ignored, physiology eventually wins.

Therefore, evidence informed strategies built from lab based testing, with 1-to-1 practitioner support are key to building a personalised fuelling, hydration and cooling strategy - ensuring the heat doesn’t cost you that PB or your race.