As global temperatures continue to climb, runners face a brutal new normal: training and racing in extreme heat.
Forecasts suggest that the summer of 2025 is on track to be the hottest on record worldwide. But for athletes chasing PRs, Boston Qualifiers, or finish-line redemption stories, quitting isn’t an option.
The real question becomes: how can we outsmart the heat?
Over the last decade, research has increasingly shown that heat, especially when combined with humidity, isn’t just an inconvenience. It’s a powerful limiter of endurance performance, with the capacity to derail even the best-laid race plans.
But with proper acclimation and smart planning, runners can adapt, survive, and even thrive under blistering conditions.
This guide draws from the latest sports science and real-world data to provide a comprehensive strategy for training and racing in extreme heat.
Why Heat Is the Silent Killer of Performance
Most runners intuitively know that hot races feel harder.
But the reasons go deeper than just discomfort. Heat stress triggers a cascade of physiological changes that degrade endurance capacity—even before you’ve started sweating buckets.
The core issue is competition for blood flow. When you run, blood must be directed to the muscles to deliver oxygen. In the heat, your skin also demands blood to help release heat through sweat evaporation. As more blood is diverted to the skin, your heart must work harder to maintain the same pace, resulting in a higher heart rate and earlier fatigue.
A 2008 study published in the Journal of Applied Physiology confirmed this “protective slowdown” mechanism. Runners in hot conditions reduced their pace before their core temperatures even reached dangerous levels—a subconscious, neurologically-driven response aimed at survival, not performance.
And the problem escalates with humidity. High humidity inhibits sweat evaporation, which is the body’s main cooling mechanism. Without effective cooling, core temperature rises faster, increasing the risk of heat exhaustion or even heat stroke.
What the Data Says: Heat and Race Outcomes
Newly published research from a large-scale analysis of 3,891 marathon performances across 754 races confirmed what many seasoned runners have long suspected: performance suffers significantly in hot and humid conditions.
The study, conducted by John Davis at RunningWritings.com and published in April, identified the ideal temperature range for marathon performance as 35–55 F (2–13 C). Once the temperature exceeds 65 F (18 C), runners begin to slow significantly—by as much as 8% in hot and humid conditions.
To put that in perspective, a 3:00 marathoner could see their time balloon by 15–20 minutes simply due to weather.
Interestingly, humidity alone had little effect below 60 F—but became a major factor when paired with higher temperatures. In fact, the Heat Index (which factors in both temperature and humidity) is a better predictor of performance outcomes than temperature alone.
This tracks with Road Runners Club of America guidance, which recommends that runners avoid running outside altogether if the temperature exceeds 98.6°F (37 C) and humidity surpasses 70–80%.
Can You Train Your Body To Handle the Heat?
Yes—and this is where things get encouraging. The body is remarkably adaptable, and just as you can train for hills or altitude, you can condition your body to perform better in heat.
This process is known as heat acclimation, and it brings about significant physiological changes, including:
- Lower resting and exercising heart rate
- Increased blood plasma volume
- Earlier onset of sweating
- Improved sweat efficiency (better sodium retention and cooling)
- Lower core body temperature during exercise
- Improved VO₂ max and lactate threshold
According to a 2019 meta-analysis published in Frontiers in Physiology, most adaptations occur within 7–14 days of consistent heat exposure. Elite athletes, or those with a strong aerobic base, may adapt slightly faster.
Two Paths to Heat Acclimation: Active vs. Passive
1. Active Heat Acclimation
This is the most direct and effective method: train in the heat. That could mean:
- Running outdoors during warm parts of the day
- Wearing extra layers to simulate heat stress (especially in cooler climates)
- Running indoors on a treadmill in a non-air-conditioned room
One runner, preparing for the notoriously hot Canyons 100K, described running on a treadmill in a hoodie inside his 65 F home, simulating race-day conditions. While not glamorous, the technique worked—and he completed the event without succumbing to the heat.
Start with short, easy runs and progressively increase duration or intensity. Be cautious with intervals: keep them short and focus on perceived effort over pace. Hydrate aggressively before, during, and after.
2. Passive Heat Acclimation
Don’t have access to outdoor heat? You can still stimulate adaptations by exposing your body to heat at rest.
- Soak in hot baths (104–105 F) for 20–40 minutes after workouts
- Use saunas or steam rooms post-run
- Layer up post-workout to stay warm for longer
A 2019 study in Frontiers in Physiology found that hot baths post-exercise increased plasma volume and reduced core temperature during subsequent workouts—demonstrating tangible gains from passive methods alone.
Preparing for Race Day: When Acclimation Meets Strategy
Even with weeks of heat training, race day execution matters. Here’s how to avoid a meltdown:
Adjust Your Pacing
According to pacing models based on heat index, for every 10 F increase above 55 F, runners should slow by 10–20 seconds per mile. Trying to “tough it out” at goal pace can backfire quickly. Build this into your race plan.
Hydrate Smarter
Start hydrating the day before—not just during the race. On race morning, sip fluids with electrolytes, and continue sipping steadily through the event. Many elite athletes favor two bottles: one frozen (for later miles) and one cold.
Carry fluids in a hydration pack or handheld if aid stations are sparse. For long races, consider sodium tablets or sports drinks to avoid hyponatremia—dangerously low blood sodium caused by overhydrating with plain water.
Cool Your Body Proactively
- Start with an ice vest or ice packs under your shirt
- Soak a bandana in ice water and tie it around your neck
- At aid stations, douse your head and shirt with cold water
- Wear light-colored, loose-fitting, sweat-wicking clothing
One study published in the International Journal of Sports Physiology and Performance found that pre-cooling strategies reduced core temperature and improved time-to-exhaustion by 16% in warm conditions.
When It’s Too Hot: Know the Red Flags
Even with the best preparation, there are limits.
Warning signs of heat exhaustion:
- Nausea or dizziness
- Headache
- Muscle cramps
- Cool, clammy skin
- Rapid heart rate
Warning signs of heat stroke (a medical emergency):
- Confusion or disorientation
- Cessation of sweating
- Red, hot, dry skin
- Seizures or loss of consciousness
If you experience these, stop immediately, get into the shade, cool yourself with water, and seek medical attention. Every year, races like Chicago 2007 and New York 2019 are stark reminders of what happens when heat is underestimated.
Train for the Weather, Not Just the Course
Too many runners treat the weather as an afterthought. But as our climate shifts, adapting to environmental stressors may be the most important kind of training there is.
If you’re targeting a summer or early fall race, build in heat acclimation the same way you would a taper or long run. Train smarter—not harder—and give your body the tools it needs to survive, adapt, and ultimately, perform.
It might be the hottest summer on record. But it doesn’t have to melt your goals.
