Hot Weather Racing: The Science Of Performing In The Heat

Your training says you’re ready for a PB. Then race morning hits 30°C and the wheels fall off by halfway. Sound familiar? You’re not alone. Even the world’s best marathoners lose time when it heats up, and the slower you are, the more the heat takes from you¹.

The good news? Most of that lost time is yours to win back, and none of it comes down to being tougher. Here’s the science-backed playbook for beating the heat: acclimation, cooling, fluid and sodium, and smart pacing.

What Heat Does To Your Body
Heat Acclimation: The Biggest Lever
Cooling: Before & During the Race
Sweat, Sodium & Fluid
Race-day Plan

What Heat Does To Your Body

Racing in heat forces your heart to do two jobs at once: feed your working muscles and pump blood to the skin to get rid of heat. As you sweat, blood volume drops, so your heart rate creeps up at the same pace and the speed you can hold falls.

Three things drive the slowdown:

Rising core temperature → Your body has a temperature ceiling. Starting cooler doesn't raise it, but it buys you more time before you hit it.
Everything feels harder → Hot skin and a hot core make the same pace feel tougher, so you back off, often without realising.
You burn through fuel faster → Heat increases how much carbohydrate you use, so it's a fuelling problem as well as a cooling one.
Humidity makes it worse → Sweat only cools you when it evaporates from your skin; sweat that drips off removes almost no heat. In humid air less sweat can evaporate, so you lose your main way of cooling down. On a humid race day, expect to slow more than the thermometer suggests and lean harder on ice, cold sponges and dousing.

Dehydration makes all of this worse, and the effects start to show once you've lost around 2% of your body weight.

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Cardiovascular strain → competing demands for blood flow to muscle and skin, plus falling plasma volume, drive heart rate drift and reduce sustainable output in the heat².
Core temperature ceiling → trained cyclists stopped exercising at almost the same core temperature (about 40°C) whether they started cool or warm; pre-cooling simply extended the time taken to reach it³.
Fuel use → a 2025 meta-analysis found heat stress increases carbohydrate use during endurance exercise, with glycogen use roughly 20-25% higher in hot conditions⁴.
Humidity → evaporation of sweat is the main route of heat loss during exercise, and it falls as the moisture gradient between skin and air shrinks². At 30°C, cyclists lasted 68 minutes at 24% relative humidity but only 46 minutes at 80%, despite sweating more in the humid trials ⁵.
Dehydration → performance impairment in warm-to-hot conditions typically appears once fluid deficits reach about 2% of body mass².

Heat Acclimation: The Biggest Lever

If you do one thing before a hot race, acclimate. Training in the heat teaches your body to carry more blood, run cooler, keep your heart rate lower at the same pace, and start sweating sooner. The longer you stick with it, the bigger the payoff.

How to do it:

Timeline: 1-2 weeks. You'll notice changes within a few days, most are in place after a week, and two weeks gets you the full benefit.
Dose: at least 60 minutes a day that gets you properly hot and sweating.
Match the race: train in similar conditions. If you can't travel early, use a heated room, extra layers, or a hot bath or sauna after training.
Timing: finish the block in the last 1-2 weeks before race day and keep a few top-up sessions going, because the benefits fade if you stop.

No heat where you live?

Bring it indoors. The CORE Heat Training Suit works like a personal sauna suit during indoor sessions, trapping heat so you reach your heat-training zone faster and with less effort. Pair it with a CORE body temperature sensor to see how hot you're actually getting and keep each session in the right zone. Drink before, during and after, and stop straight away if you feel dizzy, nauseous, faint or confused.

Sessions in the heat will feel harder, so drop the pace and train by heart rate and effort rather than splits.

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Adaptations: repeated exercise-heat exposure expands plasma volume, lowers resting and exercising core temperature, reduces heart rate at a given workload, and brings on earlier and greater sweating⁶.
Performance: a meta-analysis of 96 studies found these adaptations improve exercise performance in the heat, with longer protocols producing larger gains⁷.
Protocol: the international consensus recommends 1–2 weeks of daily sessions of at least 60 minutes that raise core and skin temperature and stimulate sweating. Most adaptations occur within about a week, with two weeks recommended to maximise them².

Cooling: Before & During The Race

Cooling works, whether you cool down before the start (pre-cooling) or keep cooling during the race (per-cooling). Combining a few methods gives the best results.

Strategy	When	Why use it
Ice slurry	30 min before; sips during	Drops your core temperature before the start, so you have more room before you overheat.
Cooling vest	Warm-up	Lets you warm up without heating up, so you start the race cooler.
Head, face and neck cooling	Warm-up and during	Makes you feel much more comfortable, though it barely changes core temperature.
Menthol (mouth rinse or on skin)	During	Makes you feel cooler without actually cooling you down.
Cold-water immersion	Before or between events	Cools the whole body quickly when you have time and access to water.

Gear that makes head and neck cooling easy:

East Peak Ice Bandana: load it with ice and wear it around your neck. The insulated shell slows melting, and the wide zip makes mid-race refills at aid stations quick.
Omius Aid Station Headband: graphite cooling pieces soak up your sweat and spread it out so more of it evaporates, so it keeps working as long as you do.
PURPOSE Aid Station Glacier Hat: splash it with water for evaporative cooling, or add ice to the hidden pocket on hotter, longer days. It also gives UPF 50 sun protection.

On humid days, evaporative options like the headband and hat work less well because sweat and water can't evaporate as easily, so ice-based cooling such as the bandana or ice in the hat pocket is the better bet.

Two cautions. Feeling cooler isn't the same as being cooler: head cooling and menthol can hide a rising core temperature, so keep watching your pace, fluids and any heat-illness symptoms. And trial every method in training before race day.

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Overall effect: across 28 studies, cooling improved performance in the heat by about 6.7%, with pre-cooling (+5.7%) and per-cooling (+9.9%) giving similar benefits. Combining methods was most effective before exercise, and ice vests were most effective during it⁸.
Ice slurry: before running to exhaustion at 34°C, ice slurry lowered core temperature by 0.66°C (vs 0.25°C for cold water) and extended run time by about 19%⁹.
Cooling vest: wearing a vest during an active warm-up gave runners a cooler start and a 5 km about 13 seconds faster¹⁰.
Head, face and neck cooling: reliably improves thermal comfort and lowers local skin temperature, but has little effect on core temperature or heart rate; performance results are mixed¹¹. Some 5 km studies show small gains¹²,¹³.
Menthol: improves thermal sensation and can improve performance without lowering core temperature¹⁴.
Cold-water immersion: whole-body at 22–30°C for about 30 minutes, or legs at 10-18°C².
Evidence gaps: most cooling studies use short, lab-based efforts (under 5 km) with mostly male participants, so effects in ultras and in women are less certain¹¹.

Sweat, Sodium & Fluid

Heat turns your sweat rate up, so fluid and sodium plans that work in winter can fall short in summer. Some people sweat a little, some pour it out, and how salty that sweat is varies hugely from person to person. Your teammate's plan may not suit you.

Are you a salty sweater?
Signs include white salt marks on clothing or caps, gritty skin after a session, sweat that stings your eyes, and kit that dries stiff. Your hourly sodium loss is sweat rate × sweat sodium concentration:

Sweat rate (L/h)	Sweat sodium (mg/L)	Sodium lost (mg/h)
0.6	500	~300
1.0	700	~700
1.5	700	~1,050
1.5	1,400	~2,100

How to plan it:

Fluid → weigh yourself before and after training in race-like heat to estimate your sweat rate. Aim to keep losses under about 2% of body weight, but never finish heavier than you started.
Don't overdrink → Drinking far more than you sweat is what causes dangerously low blood sodium (hyponatraemia), and taking extra salt won't protect you if you keep overdrinking.
Sodium → for long sessions, about 300-600mg per hour is a sensible starting point. Fine-tuning really matters only for events over about two hours where you're replacing most of your fluid losses. Spread it across drinks, gels, chews and food.
Don't overdo it → Light sweaters don't need extra salt "just in case"; too much can cause bloating and nausea.

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Individual variation → sweat rates range from about 0.5 to 3.5 L/h, and sweat sodium from about 17 to 92 mmol/L, a more than fivefold range¹⁵,¹⁶. Most of this variation is individual rather than explained by diet, exercise or environment¹⁷.
Fluid targets → limit body-mass losses to about 2% while avoiding weight gain during exercise².
Hyponatraemia → overconsumption of fluid relative to losses is the primary cause of exercise-associated hyponatraemia, and sodium supplementation cannot prevent it when excessive drinking continues¹⁸.
When to personalise sodium → modelling suggests precise matching matters mainly for events over about two hours with fluid replacement of roughly 70% or more of losses¹⁹.
Performance → no study has yet shown that matching sodium intake to sweat losses improves performance over simpler approaches²⁰.

Race-day Plan

The athletes who race well in the heat start cool, pace conservatively and keep fuelling. A simple checklist:

In the 2 weeks before

Complete 7-14 days of heat acclimation, finishing close to race day. No hot weather? Try the CORE Heat Training Suit for indoor sessions.
Rehearse your cooling, fluid and sodium plan in race-like heat.
Estimate your sweat rate with weigh-ins before and after training.

Race morning

Drink about 6mL per kg of body weight 2-3 hours before the start (about 420mL for a 70kg athlete).
Pre-cool: ice slurry in the 30 minutes before the start, and a cooling vest or East Peak Ice Bandana during your warm-up.
Keep the warm-up short and in the shade.

During the race

Start slower than your cool-weather target pace.
Pace by effort and heart rate, not splits.
Keep carbs coming: 30-60g per hour, up to 90g per hour for events over 2.5 hours.
Keep cooling going with an Omius Aid Station Headband, PURPOSE Glacier Hat or East Peak Ice Bandana, and top them up with ice and water at aid stations, along with cold sponges and cold drinks. Drink to thirst plus your rehearsed plan, never to excess.
Stop and get help if you feel confused, stumble, get chills or nausea, or stop sweating. These are warning signs of heat illness.

After

Replace about 100-120% of the weight you lost, with sodium, over the following hours.

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Pacing → across seven major marathons, elite men slowed from 1.7% off course-record pace at a wet-bulb globe temperature of 5-10°C to 4.5% at 20-25°C, and slower runners were affected more¹.
Pre-race fluid → about 6mL/kg body mass every 2-3 hours before exercise, with the last intake 2-3 hours out².
Pre-cooling → ice slurry and vest-assisted warm-ups improved endurance and 5 km performance in the heat⁹,¹⁰.
Carbohydrate → 30-60g/h for exercise over 1 hour, up to 90g/h for events over 2.5 hours², especially relevant given higher carbohydrate use in the heat⁴.
Drinking to thirst → recommended as the primary strategy to limit both excessive dehydration and overdrinking¹⁸.
Recovery → replace about 100-120% of body-mass losses after exercise².

Ash Miller
Dietitian and Nutritionist (Masters)
Bachelor of Physical and Health Education
Instagram: @ashthomo_nutrition

References

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González-Alonso J, Teller C, Andersen SL, et al. Influence of body temperature on the development of fatigue during prolonged exercise in the heat. J Appl Physiol. 1999;86(3):1032–1039.
Mougin L, Macrae HZ, Taylor L, et al. The effect of heat stress and dehydration on carbohydrate use during endurance exercise: a systematic review and meta-analysis. Sports Med. 2025;55:2825–2847.
Maughan RJ, Otani H, Watson P. Influence of relative humidity on prolonged exercise capacity in a warm environment. Eur J Appl Physiol. 2012;112(6):2313–2321.
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Bongers CCWG, Thijssen DHJ, Veltmeijer MTW, Hopman MTE, Eijsvogels TMH. Precooling and percooling (cooling during exercise) both improve performance in the heat: a meta-analytical review. Br J Sports Med. 2015;49(6):377–384.
Siegel R, Maté J, Brearley MB, Watson G, Nosaka K, Laursen PB. Ice slurry ingestion increases core temperature capacity and running time in the heat. Med Sci Sports Exerc. 2010;42(4):717–725.
Arngrímsson SA, Petitt DS, Stueck MG, Jorgensen DK, Cureton KJ. Cooling vest worn during active warm-up improves 5-km run performance in the heat. J Appl Physiol. 2004;96(5):1867–1874.
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Coelho LGM, Ferreira-Júnior JB, Williams TB, et al. Head pre-cooling improves 5-km time-trial performance in male amateur runners in the heat. Scand J Med Sci Sports. 2021;31(9):1753–1763.
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Disclaimer:
The content in this blog is for general information only and is not a substitute for professional medical advice, diagnosis, or treatment. Always speak with your doctor or allied health team before changing your diet, exercise, or taking supplements, especially if you have a health condition or take medication. Please use this information as a guide only. Aid Station doesn't take responsibility for individual outcomes.