Hot conditions present a unique challenge for endurance athletes. When temperatures rise, your body’s cooling system competes directly with your working muscles for blood flow. This physiological balancing act forces your cardiovascular system to work overtime, driving up your heart rate and perceived exertion while accelerating fluid loss. 

The result? A significant drop in performance that can undermine months of dedicated training. Fortunately, strategic heat adaptation can transform this performance obstacle into a competitive advantage.

In a comprehensive 108-page paper in Physiological Reviews, Julien Périard and colleagues (2021) examine decades of research on exercising in the heat. In this article, I draw from their review — in particular, their section on heat acclimation — to apply their insights on how endurance athletes can prepare to race in hot conditions. 

Heat Adaptation and Its Benefits

Heat adaptation — the body’s process of adapting to heat stress — can occur through training in natural outdoor environments (traditionally called “acclimatization”) or in artificial settings like saunas or indoor environments (traditionally called “acclimation”). While researchers sometimes distinguish between these terms, they produce similar physiological benefits and I’ll simply use “heat adaptation” to cover both approaches in this article.

Heat adaptation induces a series of powerful physiological changes that collectively enhance your body’s efficiency in hot environments. These adaptations include:

• Expanded blood volume: Your blood plasma volume increases to support better cardiovascular function and cooling capacity.
• Enhanced sweating response: You begin sweating sooner, produce more sweat, and lose fewer electrolytes when exercising in the heat, which improves your body’s ability to cool itself. 
• Reduced core and skin temperatures: Your body maintains lower temperatures both at rest and during exercise, improving performance and comfort in the heat.
• Lower cardiovascular strain: Your heart rate decreases and stroke volume increases at a given workload, making exercising in the heat feel easier.
• Improved heat tolerance: Your perception shifts and you experience less discomfort in hot conditions.

These beneficial changes begin developing within the first week of heat exposure with adaptations such as plasma volume expansion and decreased heart rate. However, research indicates that “regimens of 14 days or longer are recommended to achieve maximal adaptations” (p. 1918). Keep in mind that individual variations, along with the type of adaptation protocol you use (frequency, duration, etc.), will impact how quickly you adapt.

What It Takes to Become Heat Adapted

Heat adaptation is not an overnight process—it requires consistent exposure and follows a predictable sequence: cardiovascular changes (like expanded plasma volume and reduced heart rate) develop first, followed by enhanced sweating responses and improved thermal comfort.

10-14 Days of Heat Exposure

Research indicates that 10-14 days of heat exposure are needed to develop significant adaptations. 

Keep in mind that heat training sessions create physiological stress, so factor them into your overall training load and recovery needs. If you begin heat training within a few weeks of your target race, be sure to account for the added training stress during your taper. 

If possible, start three weeks out from your target race. Accumulate at least 10 consecutive days of heat training sessions. Then schedule a maintenance session for every 2-5 days without one.

Or, you can start earlier in the season for an initial adaptation phase. Accumulate at least 10 consecutive days of heat training sessions. Schedule a maintenance session for every 2-5 days without one. You could then add another 10-day block of consecutive days within the last 2-3 weeks prior to your target race to maximize your adaptations.

Maintenance Sessions

Once developed, the adaptations do fade without continued heat exposure. Research indicates that about 2.5% of the adaptation is lost each day without a heat stimulus. This is why you’ll need regular maintenance sessions to keep the adaptations you’ve gained. 

As a general rule, once you’ve gone through an initial adaptation phase, schedule a heat session for every 2-5 days without one to maintain your adaptations. 

Research also indicates that “aerobic fitness and regular exercise in cool conditions contribute to maintain adaptations or reduce the rate of decay” (p. 1919), suggesting that maintaining fitness itself helps preserve some heat adaptation benefits.

Evidence-Based Protocols for Heat Adaptation

When choosing a heat adaptation strategy, endurance athletes face the dilemma of how to develop heat resilience without compromising the quality of their training. Your regular training sessions build the fitness foundation necessary for performance, while heat adaptation prepares your body for the specific challenges of a hot race. Balancing these needs is essential.

The protocols below are organized from least to most disruptive to your training quality, allowing you to select an approach that fits your circumstances. Each method has been demonstrated by research to produce physiological adaptations.

Post-exercise heat exposure leads this list as it allows you to maintain high-quality training while adding heat adaptation afterward. Passive heat exposure similarly preserves training quality by keeping the two elements separate. These two approaches, however, do require access to a dry sauna or hot tub. 

Simulated heat training begins to impact workout quality but maintains your normal training schedule; this can be done indoors by cranking up the heat or outdoors in normal ambient conditions by wearing extra layers. If you don’t have access to a dry sauna or hot tub, this may be your best option. 

If you live in a hot environment—or travel to train in one—then you don’t need to simulate training in the heat; you’re already training in the heat. Research indicates some strategies to help you control your exposure so you can adapt to the heat without letting it overwhelm your training.  

Post-Exercise Heat Exposure

Post-exercise heat exposure involves doing your regular training, then adding a passive heat session immediately after the workout. Research demonstrates that post-exercise heat exposure induces adaptations similar to those of more traditional heat acclimation protocols. 

Immediately after your training, spend time in either a dry sauna or hot tub. The key is to get into the sauna or hot tub while your body temperature is still elevated from your workout; this allows you to spend less time in the passive heat environment than if elevating your body temperature from its baseline.

Post-exercise dry sauna:

• Duration: 10-20 minutes
• Temperature: 158-194°F / 70-90°C

Post-exercise hot water immersion:

• Duration: 20-40 minutes
• Temperature: 104°F / 40°C

This approach has gained popularity since it allows athletes to complete high-quality training without the compromised performance that comes with heat stress, while still acquiring the adaptations associated with heat exposure. 

However, this method does require access to facilities like saunas or hot tubs, which may not be available to all athletes. It also extends the time commitment of each training session, potentially creating scheduling challenges. 

Passive Heat Exposure

Passive heat exposure involves regular sessions in hot environments without exercise beforehand. This method relies purely on external heat sources to elevate your body temperature and trigger adaptation responses. 

The research examines several approaches including hot baths (approximately 104°F / 40°C), environmental chambers (greater than 113°F / 45°C), or sauna sessions (158-194°F / 70-90°C) with exposure times ranging from 30 to 90 minutes depending on the medium used. Studies have shown that passive heat exposure produces many of the same physiological adaptations as exercise-based heat protocols. 

Since these sessions do not occur immediately after exercise when your body temperature is already elevated, you will need to spend a bit more time in either the dry sauna or hot tub to gain the benefits. 

Dry sauna:

• Duration: 15-30 minutes
• Temperature: 158-194°F / 70-90°C

Hot water immersion:

• Duration: 30-60 minutes
• Temperature: 104°F / 40°C

One of the primary advantages of passive heat exposure is that it doesn’t interfere with your regular training quality or schedule. However, the research notes that passive heat exposure may not develop the same sport-specific adaptations that contribute to improved performance in the same way that exercise-heat protocols do. Additionally, passive heat exposure requires a substantial time commitment beyond your regular training schedule, which may be challenging for athletes with limited time.

Simulated Heat Training

Périard and colleagues discuss a laboratory-based approach called “controlled hyperthermia, or isothermic heat acclimation,” which was developed to focus on the degree of body temperature elevation rather than the specific climate characteristics as the key stimulus for adaptation. This method involves manipulating both environmental conditions and exercise intensity to attain and maintain a target core temperature, typically around 101.3°F / 38.5°C.

While controlled hyperthermia protocols in research settings require specialized equipment to monitor core temperature, athletes can apply the underlying principle—that elevated body temperature drives adaptation regardless of how it’s achieved—through more accessible means. One practical approach is wearing additional clothing layers (e.g., puffy jacket and ski hat) during training sessions in normal ambient conditions to increase your body’s heat production and retention.

By adding insulating layers that elevate your core body temperature, you create an artificial heat stress that can trigger many of the same physiological adaptations that occur when training in hot environments. This method essentially creates a portable sauna effect during your regular workouts, stimulating your body to adapt without needing access to hot environmental conditions.

The primary advantage of this approach is its accessibility—it can be implemented anywhere, regardless of climate or season. This allows you to continue your regular training program without travel, potentially maintaining higher quality workouts than would be possible in genuinely hot conditions. Since you’re creating heat stress through insulation rather than ambient temperature, you can also more easily adjust the level of thermal challenge by adding or removing layers as needed during your session.

However, this method does have limitations. By focusing only on internal heat production rather than external heat stress, this approach may not perfectly replicate the combination of high ambient temperature and humidity that you’ll face during a hot race. It’s also difficult to replicate specific race conditions such as solar radiation and air flow that contribute to overall heat stress. 

Additionally, if not carefully monitored, excessive insulation could reduce training quality if overheating occurs. Make sure you are hydrating adequately, replacing electrolytes, and monitoring your heat stress during sessions that adopt this approach. 

Training in Hot Environments

Training in naturally hot outdoor environments is one of the most straightforward approaches to heat adaptation—but also the least controllable because it depends on the weather and your availability for training in that weather. But this approach does provide sport-specific adaptation by exposing you to the actual conditions you’ll face during competition.

Research shows that self-paced heat training—a strategy where you adjust your exercise intensity based on how you feel given the conditions—allows athletes to effectively induce adaptation while still self-regulating their effort according to their individual fitness level and the prevailing conditions. 

If you’re training in a hot environment, whether by choice or by virtue of where you live, remember that heat stress is another form of training stress. As the self-paced heat training strategy indicates, it’s important to moderate your intensity and duration when exercising in a hot environment—especially until you have gone through the requisite heat adaptation phase. 

Safety Considerations

Heat acclimation protocols involve deliberate heat stress, requiring careful monitoring and appropriate precautions to ensure safety. Here are some guidelines to keep in mind.

Hydration

• Pre-exposure hydration: Ensure euhydration before beginning any heat protocol
• During exposure: Consume fluids during active and passive heating protocols
• Post-exposure rehydration: Replace fluid losses with electrolyte-containing beverages
• Monitor hydration status: Use urine color, body mass changes, and thirst as indicators

Temperature

• Use a thermometer to verify water temperature for immersion protocols
• For sauna and hot tub use, begin at lower temperatures and shorter durations
• Cool down gradually after heat exposure—avoid immediate cold exposure
• Implement a buddy system for safety during home-based protocols when possible

Contraindications

• Recent illness or fever
• Cardiovascular conditions (consult healthcare provider first)
• Pregnancy (consult healthcare provider)
• History of heat-related illness
• Certain medications that affect thermoregulation

Other Precautions

• Exit immediately if experiencing dizziness, nausea, confusion, or headache
• Monitor heart rate during exposure and exit if it goes too high
• Avoid alcohol consumption for 24 hours before heat exposure
• Discontinue heat sessions if feeling ill or sleep-deprived

Conclusion

Remember, all of these heat training methods require accumulating at least 10-14 days to develop significant adaptations, with some benefits appearing within the first week but optimal adaptations taking two weeks or longer to develop. 

Consider your individual circumstances—available facilities, climate, training phase, and race goals—when selecting which method to use. You could even use a combination of these approaches, perhaps focusing on less disruptive methods during key training blocks and more intensive heat-specific training as race day approaches.

The bottom line is that integrating heat adaptation protocols into your training is essential if you will be racing in hot conditions. And even if you won’t be racing in the heat, heat adaptation training may hold benefits for your general conditioning as an endurance athlete. 

Reference

Périard, Julien D., Thijs M. H. Eijsvogels, and Hein A. M. Daanen. 2021. “Exercise under Heat Stress: Thermoregulation, Hydration, Performance Implications, and Mitigation Strategies.” Physiological Reviews 101(4): 1873–1979. https://doi.org/10.1152/physrev.00038.2020.