Hot weather racing season is approaching. If you aren’t used to competing in hot, humid conditions, the heat can really take a toll on performance—which is why many athletes utilize heat acclimation training.
But what exactly is heat acclimation training? What’s the science behind it and does it work? We’ll dive into these questions here, including protocols to utilize heat training to your advantage.
What Is Heat Acclimation Training?
Heat acclimation training refers to artificially exposing yourself to heat—such as riding a bike indoors in a heated room or wearing a sauna suit. The goal is to trigger specific physiological adaptations that ultimately increase performance in hot environments while reducing the risk of heat illness.
Does Heat Acclimation Work?
The research is clear that heat acclimation training works. Athletes who utilize heat acclimation protocols report that hot-weather exercise feels easier, which leads to better performance. They also say that exercise in the heat feels more comfortable, which helps them stay motivated and focused.
Interestingly, some research suggests that heat training may even improve performance in cooler environments. While the evidence is mixed, there’s no indication that it negatively affects performance in those conditions.
Fun fact: The Olympics medical department published guidelines recommending heat training for athletes traveling to compete in hot conditions like those expected in Paris 2024.
The Science Behind Heat Acclimation Training
Exercising in hot environments puts strain on the body. It causes faster dehydration due to higher sweat rates, higher thermal load, and increases the body's demand for cardiac output because more blood is needed in both the muscles and the skin to help regulate body temperature.
Heat acclimation training helps the body adapt to these challenges by initiating the following physiological changes that improve heat tolerance and support sustained performance:
Lower Core Temperature
Heat training reduces core temperature by improving skin-blood flow, which allows the body to dissipate heat better.
This is important for performance because, as your core temperature rises, your body diverts blood flow away from muscles and to the skin. As a result, your muscles compete for blood and performance suffers in hot conditions.
Earlier and Increased Sweating
One of the primary adaptations to heat training is that your body starts sweating sooner—and produces more sweat overall. Since sweating is the body’s main way to cool itself, this early and enhanced response is crucial for thermoregulation.
The sweat glands also become more fatigue-resistant, allowing higher sweat rates even in humid conditions.
Sodium Conservation
Although heat-acclimated athletes sweat more, they don’t necessarily lose more sodium. That’s because the body adapts by conserving sodium and making sweat less salty. This adaptation helps maintain electrolyte balance and supports proper hydration.
Improved Cardiovascular Efficiency
Heat training triggers an increase in plasma volume—the liquid part of your blood. This allows your heart to pump more blood with each beat. As a result, your resting heart rate and heart rate during moderate exercise decrease, making endurance efforts feel easier.
The rise in plasma should also lower hematocrit, or the concentration of red blood cells. In response, your body starts producing more red blood cells. Over the course of a few weeks, this can lead to a higher total red blood cell count, which improves oxygen delivery to your muscles.
Reduced Cardiovascular Strain
Improved thermoregulation and lower skin temperatures mean less blood is redirected to the skin for cooling circulation. As a result, more blood remains in circulation and core-to-skin temperature gradient improves, which reduces cardiovascular strain and improves performance.
How to Do Heat Acclimation Training
There are two key ways of doing heat acclimation training: active and passive. With both methods, it’s important to follow specific protocols to get results, and time the sessions so the body has time to adapt but without over-stressing the body before key events.
Active Heat Training
Active heat acclimation training involves performing endurance exercise, such as running or cycling, in hot environments. For active heat training to be effective, athletes should follow these guidelines:
- Duration: Sessions should last 60-90 minutes, as this much time is needed to induce a high sweat rate response and elevate core body temperature high enough.
- Number of sessions: Research shows men need 5 sessions and women need 9 to achieve the majority of adaptations. However, some sources recommend more sessions than this for both men and women.
- Intensity: Typically at a heart rate close to the first lactate threshold.
- Temperature: The conditions or temperatures need to be hot enough to induce profuse sweating. You can also aim to mimic race conditions as closely as possible, especially with regard to humidity.
- Pre-hydration: Successful heat training involves inducing a high sweat rate, so starting the session adequately hydrated is important to ensure your body can sweat effectively.
- Sodium: Because heat training involves sweating, you may need to consume more sodium to sustain fluid-electrolyte balance. For most, 1.5-3.2 g of sodium should be efficient.
Passive Heat Training
Passive heat training involves subjecting the body to hot temperatures without exercise. For example, taking a very hot bath or sitting in a sauna. According to Dr. Tim Podlogar, passive heat training doesn’t work on its own.
“You need to connect it with training. While you are still hot from training, go into the sauna or take a hot bath. This works pretty well, but you’ll need 15-30 minutes of heat exposure.”
Note that passive heat training methods can be used after any training session, not just active heat training.
How Long Do Heat Training Benefits Last?
Heat acclimation benefits typically last for about 3 weeks in hot conditions, and up to 4 weeks in cooler conditions. After this period, you will need to re-adapt the body with more heat training sessions.
Because of this short window, you should ideally do heat training close to the event. However, heat training is also stressful on the body and can interfere with recovery.
Dr. Tim Podlogar suggests starting with heat training sessions 2-3 weeks before the event, and to have a maximum of 2 sessions in the week before the event. This approach allows you to prevent excessive fatigue.
Note: Research shows that once you’ve done some heat training, you can re-acclimate faster. So, it may be beneficial to regularly work some heat training into your schedule.
Risks of Heat Acclimation Training
While heat acclimation training has proven benefits, it also comes with these risks.
Heat Stroke
Heat stroke can have a rapid onset and lead to organ failure or death, so the risk needs to be taken seriously. It typically occurs when core body temperature goes above 40°C (104°F). However, we often see these core temperatures in professional athletes without heat stroke.
For example, in the 2016 UCI Road World Championships in Qatar, research found that three cyclists had core temperatures ranging from 40.8°C to 41.5°C (105.4°F to 106.7°F), yet didn’t collapse. In fact, their team won a medal.
This research indicates that trained athletes’ bodies operate differently than those of untrained individuals and body temperature alone isn’t a good indicator of heat illness. Instead, you will need to look out for other symptoms.
Stop heat training if you have these symptoms:
- Dizziness
- Disorientation
- Nausea
- Confusion
- Altered mental state
Note that some athletes are more prone to heat illness. For example, athletes with higher body volumes compared to skin surface area (higher amounts of muscle or fat) will have a harder time dissipating heat, so will overheat faster and need to be extra careful.
You should also avoid doing heat training while sick. As Dr. Francis O’Connor told NPR, underlying infections, such as colds, gastroenteritis, sinus infections, or periodontal disease, can elevate body temperature. Adding heat training on top of this increases the risk of heat illness.
Impaired Training and Recovery
Heat also puts the body under stress, which can impact recovery and your ability to train properly. If not done carefully, you could end up hindering performance instead of helping it.
To ensure effective heat training, follow these guidelines:
- Start slowly: Start with low-to-medium intensity workouts.
- Spread sessions out: Instead of doing heat training on consecutive days, spread it out over longer periods. For example, only doing heat sessions on days where you have easy training the next day, so the body can recover.
- Limit sessions before key events: Do a maximum of 2 sessions in the week before a key race or event to avoid excessive fatigue.
The Bottom Line
Research shows that heat acclimation training works for improving performance and reducing the risk of heat illness during exercise. However, it’s important to follow protocols to make sure the body has time to adapt and that the sessions don’t interfere with training.
