Racing in the Heat: What Your Body Does and How to Fight Back

Heat does not add time the way a long climb does. It throws off the systems that keep you moving. The damage builds slowly and quietly, and then it hits all at once. This guide is about race day, not training.

What Does Heat Actually Do to Ultramarathon Performance?

In the heat, your body's first job is to keep your core from getting hot enough to cause harm. To cool off, it sends blood away from your muscles and out to your skin. Your heart makes up for it by beating 5 to 10 beats per minute faster at the same pace. This is called cardiac drift. If you only watch pace, you won't see the cost until it's too late. A 2001 study found that when core temperature reached about 40°C (104°F), the brain sent weaker signals to the muscles. Your body slows you down to protect itself. (Nybo and Nielsen, Journal of Applied Physiology, 2001)

A 2011 study by Parise and Hoffman looked at 161 km races. It found something you might not expect: heat hurts faster runners more than slower ones. Fast runners spend more of their race in the hottest part of the afternoon, so they take on more heat. A 2019 review points to that same study. Runners at Western States were about 8 percent slower in the hot year of 2006 than in the cooler year of 2007. That's about an hour in a 13-hour race.

A 2026 study backs this up. Runners at Western States swallowed a pill that tracked their core temperature during the race. The researchers got race data from 23 finishers. The faster runners ran hotter. Seven of the 23 hit their highest temperature in the last 2 percent of the race. Only two went above 40°C (104°F), and not for long. (Mougin et al., "Gastrointestinal temperature of elite and recreational adult runners during a 100-mile ultramarathon under heat exposure," Physiological Reports, 2026)

How Should You Adjust Your Pace in Hot Weather?

On a warm day, plan to run 4 to 6 percent slower than your goal pace. On a truly hot day, plan for more. It works better to run by effort. In the heat, your heart rate is already higher at the same pace. If you chase your pace, you work harder than you mean to, and your core heats up faster. Run by feel and heart rate. Accept the slower splits.

The first third is where most races are lost in the heat. The morning start feels cool relative to what is coming. Back off 10 to 15 seconds per mile from what feels easy. The heat debt you avoid early cannot be repaid in the canyon miles. Abby Hall won the 2025 Western States running controlled through Foresthill while others slowed hard. Her crew reported she had spent months training in the heat of the Grand Canyon specifically for this reason. Her canyons felt cool because she had already been in worse.

What Are the Most Effective Ways to Cool Down During a Race?

The goal is reducing core temperature, not skin temperature. Cold water over your head feels good and has limited physiological effect. Good targets are the spots where big blood vessels run close to the skin: neck, armpits, groin, and wrists. Ice slurry ingestion (crushed ice in a drink) cools from the inside through the phase change from solid to liquid. Both are more effective than a bottle poured over your head.

• Neck: Makes you feel cooler fast, which helps you keep going.
• Armpits (axillary artery): Best during walking breaks at aid stations.
• Wrists (radial artery): Easy to do while moving through an aid station.
• Ice in the hat or buff: Sustained cooling for the next mile, not a brief spike.

Cool down before the start. Spend 10 to 20 minutes in the shade, put cold towels on your neck and armpits, and drink cold fluids. This gives your body more room to soak up heat before it gets into trouble. A study on ice vests worn during warm-up found lower body temperature and faster 5 km times in the heat. (Arngrímsson et al., Journal of Applied Physiology, 2004) Don't skip aid station stops to save time. In a hot race, 30 seconds spent cooling off at mile 30 pays you back with more than 30 seconds later. Keep cooling all the way to the finish, too. In that Western States study, almost a third of the runners got hottest in the last few miles.

How Much Should You Drink and Take in Sodium During a Hot Race?

Drink when you're thirsty, not on a schedule. If you drink plain water on a timer whether you need it or not, you water down the salt in your blood. This is called exercise-associated hyponatremia (EAH). In some ultras, up to 51 percent of runners have had it. It looks like heat exhaustion, but the treatment is the opposite. Mixing them up at the medical tent is dangerous.

You lose about 500 to 2,000mg of sodium per hour in sweat. Regular sports drinks can't keep up at the high end. Take salt tabs or electrolyte capsules all race long. The easy check is your pee. If it's very clear and you go often, you're drinking too much. If you haven't gone in several hours, you're not drinking enough. Our fueling guide covers electrolyte targets by race length.

How Do You Recognize Heat Illness Before It Becomes an Emergency?

Know the difference before you toe the line. One is manageable at an aid station. The other is a medical emergency.

The distinction is sweating. Heat exhaustion: still sweating. Heat stroke: stopped. If you see someone confused and dry-skinned in the heat, do not walk past.

In a hot year at Western States, the winning time can be an hour slower than a cool year. Give yourself permission to run the race in front of you, not the one you trained for.

How Do You Get Ready for Heat Before a Summer Ultra?

If your race is hot and you train somewhere cool, getting used to heat before race day can really help. There are two easy ways to do it. You can run in the midday heat for 10 to 14 days before you travel. Or you can soak in a hot bath or tub at 104°F for about 40 minutes after easy runs. Both raise your blood volume, lower your resting core temperature, and help you sweat better. Start at least a week before the race, and two weeks or more is better. Keep going until you arrive.

What the research says: In a 2016 study, runners soaked in 40°C (104°F) water for 40 minutes right after an easy run, for 6 days in a row. That was enough to adapt them to heat. Their 5 km time in hot weather got about 5 percent faster. A 2025 study by Jenkins and others went longer: 45-minute hot soaks, 5 days a week, for 5 weeks. The runners made more blood and their VO2max went up. (Zurawlew et al., Scandinavian Journal of Medicine and Science in Sports, 2016; Jenkins et al., Journal of Physiology, 2025)

What this means for you: You don't need to travel somewhere hot to adapt to heat. Hot baths after your easy runs, for at least a week before your race, can do much of the job. Controlled studies back this up. Soak for about 40 minutes, like the studies did.

What the research says: A 2025 study by Xu and others had trained runners do 20 heat sessions over 4 weeks. Afterward, when running hard in the heat, they burned 15 to 19 percent less carbohydrate. That saves stored carbs (glycogen) for later in a race. (Xu et al., Frontiers in Physiology, 2025)

What this means for you: Heat training also changes how you burn fuel. If you arrive at a hot race used to the heat, your body burns less sugar per mile. Some of the fuel you would have burned early is still there at mile 60. That's a second big reason to heat train.

People often say a sauna works just as well. The proof is weaker than you might hear. A 2025 controlled trial (Ahokas et al., Frontiers in Sports and Active Living) tested short infrared sauna sessions in female team-sport athletes: 10 minutes, three times a week, for six weeks. They added very little beyond normal training. Muscle growth was the same, with small gains in jumping and sprinting. One catch: these were short infrared sessions. Longer regular saunas (20 to 30 minutes, hotter) make more sense in theory for raising blood volume. But there are few good studies on them for ultra runners. If you use a sauna, treat short sessions as recovery, not heat training.