Heat does not add time to your finish the way a long climb does. It destabilizes the systems your body depends on to stay moving, and it does so in ways that build quietly before they become obvious. This is not about training protocols. This is about race day.
performance drops begin
the same pace
in hot editions of WS
for heat stroke
What Heat Does to Your Body
Your body's priority in the heat is keeping core temperature from damaging tissue. To do that, it redirects blood from working muscles toward the skin for cooling. Your heart compensates by beating faster, climbing 5 to 10 bpm at the same pace — what exercise scientists call cardiac drift. If you are running by pace, the cost is invisible until it is not. Research has identified roughly 40°C (104°F) as the threshold where your central nervous system begins suppressing voluntary muscle activation. The slowdown is not weakness. It is a hard biological ceiling.
A 2011 study by Parise and Hoffman on 161km ultramarathons found something counterintuitive: faster runners are more severely impacted by heat than slower ones. Because faster runners complete more of the race during peak afternoon temperatures, they absorb a greater heat load. A 2020 review in Sports Medicine put the performance penalty for hot editions of Western States at approximately 8 percent — roughly an hour on a 13-hour race.
"There were times too, I'm struggling in the heat just like everyone else, around Green Gate, through those five-mile splits on Auburn Lake Trails and Quarry Road's tough times." — Jim Walmsley, 2021 Western States champion interview, iRunFar
Pacing
Plan for 4 to 6 percent slower than your goal pace on a moderate heat day. More on a genuinely hot one. The more reliable frame is effort-based: in the heat, your heart rate is already elevated at the same pace, so chasing a pace target pushes effort higher than intended and accelerates core temperature rise. Run by feel and HR. 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.
Cooling
The goal is reducing core temperature, not skin temperature. Cold water over your head feels good and has limited physiological effect. The four targets with the highest return are the areas where major blood vessels run close to the surface: neck, armpits, groin, and wrists. Ice or cold water applied here cools blood before it circulates. 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 (carotid arteries): Highest impact per second spent.
- 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.
Pre-cooling before the start — 10 to 20 minutes in shade, cold towels on the neck and armpits, cold fluid ingestion — creates additional heat storage capacity. Research on ice vests during warm-up found a 2.6°C average core temperature reduction and performance improvements of 5 percent or more. Do not skip aid station stops to save time. In a hot race, 30 seconds of effective cooling at mile 30 returns more than 30 seconds of running.
Your race vest matters in the heat. Our race vest reviews cover which packs have the capacity for ice at aid stations.
Hydration and Sodium
Drink to thirst, not on a schedule. Drinking plain water at fixed intervals regardless of need dilutes sodium in the blood, causing exercise-associated hyponatremia (EAH). Research in ultramarathons puts EAH incidence at up to 51 percent in some events. Symptoms mimic heat exhaustion. The treatments are opposite. Getting this wrong at the medical tent is dangerous.
Sodium loss in sweat ranges from roughly 500 to 2,000mg per hour. Standard sports drinks do not replace it at the high end of that range. Supplement with salt tabs or electrolyte capsules throughout the race. The practical check is urine color and frequency: very clear and frequent means overhydrating, no urine for several hours means under-hydrating. Our fueling guide covers electrolyte targets by race length.
Recognizing Heat Illness
Know the difference before you toe the line. One is manageable at an aid station. The other is a medical emergency.
Heat exhaustion: Heavy sweating, rapid but weak pulse, pale or cold skin, nausea, dizziness, cramps. The athlete is still responsive and sweating. Move to shade, apply ice to the neck and armpits, give fluids if the athlete can swallow.
Heat stroke: Core temperature above 104°F, hot and red skin, little or no sweating, confusion, possible loss of consciousness. This is a medical emergency. Call for help immediately. Begin aggressive cooling while waiting. Do not give fluids to someone who is not fully conscious.
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.
Pre-Race Heat Acclimatization
If your race is in a hot environment and you train in a cool one, acclimatization before race day can meaningfully reduce heat stress. The most practical protocol for runners: heat training runs (running in mid-day heat for 10-14 days before travel), or hot water immersion sessions (10-20 minutes in a hot bath or tub at 104°F after easy runs). Both raise plasma volume, lower resting core temperature, and improve sweat rate efficiency. Start 10-14 days before the race and continue through arrival.
What the research says: A 2025 study by Jenkins et al. in the Journal of Physiology found that post-exercise hot water immersion sessions triggered heat shock protein upregulation and cardiovascular adaptations equivalent to those seen in athletes attending hot-climate training camps. The immersion did not need to occur during exercise itself — soaking for 20 to 40 minutes in water at approximately 40°C (104°F) after a normal training run produced the same physiological signal as exercising in a hot environment. (Jenkins et al., Journal of Physiology, 2025)
What this means for you: You do not need to travel somewhere hot to trigger heat adaptation. A hot bath after your easy runs, starting two weeks before your race, can produce real cardiovascular adaptation on its own. This is not just anecdote — it is now backed by controlled research showing equivalence with environment-based acclimatization. The protocol already mentioned above (post-run immersion at 104°F for 10 to 20 minutes) has stronger evidence behind it than most runners realize.
What the research says: A separate 2025 study by Xu et al. in Frontiers in Physiology found that athletes who completed 10 days of active heat acclimatization reduced carbohydrate oxidation by 15 to 19 percent during submaximal exercise at the same intensity, preserving glycogen for the demanding back half of the race. (Xu et al., Frontiers in Physiology, 2025)
What this means for you: Heat training does not just help you tolerate the temperature. It changes your metabolism. Athletes who enter a hot race heat-acclimatized are running on a more fuel-efficient engine — their bodies burn less sugar per mile, leaving more glycogen available for the back half. The glycogen you would otherwise burn at mile 30 is still available at mile 60. This gives heat acclimation a second, underappreciated reason to do it beyond comfort alone.
Sauna use is commonly discussed as a substitute, but the evidence is more limited than often presented. A 2025 randomized controlled trial (Ahokas et al., Frontiers in Sports and Active Living) found that brief infrared sauna sessions -- 10 minutes, three times per week for six weeks -- produced minimal physiological adaptation beyond training alone. No significant benefit for body composition or most performance markers was observed. The caveat: this protocol used short-duration infrared sessions. Longer traditional sauna sessions (20-30 minutes, higher temperatures) have better theoretical support for plasma volume expansion, but RCT evidence in ultra-specific contexts remains thin. If you are going to use sauna for heat acclimation, treat brief sessions as recovery tools, not acclimatization protocols.