In August of 2021, it hit 94.5℉ at the Tokyo Summer Olympics, surpassing previous record highs at Athens and Rio. The conditions were so brutal that many athletes dropped out, shattering lifelong dreams so they could stay safe.
But the Australian racewalking team was ready for the inferno. Before the Games, they completed about 7 weeks of heat acclimation training, diligently exercising in heated rooms to prepare for the slog. Spoiler alert…
It worked. Although they didn’t take home any medals, one member of the Australian team placed 8th in the men’s 50 km event—a personal record—and another placed 6th in the women’s 20 km event—nearly another personal record. Adapting to steamy conditions helped them perform in hellish conditions.
We’ve learned a ton from scientific studies on athletes, military personnel, and outdoor workers—I like to call them “industrial athletes”—who’ve undergone heat acclimation training. In this article, I’ll summarize many learnings from research that you can apply to your life, even if you’re not planning on walking 50 km as fast as humanly possible this summer.
What Is Heat Acclimation?
Heat acclimation entails exposing your body to heat over days or weeks. Controlled, intentional heat exposure promotes physiological changes that improve heat tolerance, reduce dehydration risk, and enhance physical performance in higher temperatures.
There are two types of heat acclimation training: active and passive. Active training involves walking, running, or cycling in hot conditions (often in a lab-controlled setting), while passive training includes modalities like hot baths and sauna. Many heat warriors (like the racewalkers we met earlier) combine active and passive regimens. Both methods work by elevating core body and skin temperatures, leading to heat-specific adaptations like increased sweating, improved core body temperature regulation, better fluid balance, and cardiovascular stability.
While the terms are often used interchangeably, heat acclimation is actually distinct from heat acclimatization. Heat acclimation entails intermittent (and oftentimes lab-controlled) exposure to heat over days or weeks. Heat acclimatization is a more gradual adaptation to a hot environment over a period of months or even years. For example, if you train in a sweltering jungle for two weeks, you’ll acclimate, but the benefits will begin to decay the moment you leave. But if you relocate permanently to a jungle hut, you’ll eventually acclimatize more permanently. You’ll still miss your A/C though.
Who Is Heat Acclimation Training For?
Heat acclimation can benefit anyone who needs to perform (or stay comfortable) in the heat. The most common beneficiaries of heat acclimation include:
- Endurance athletes like the Australian racewalkers
- Professional athletes like football players who compete in the heat
- Industrial athletes like construction workers, factory workers, landscapers, etc.
- Tactical athletes, like military personnel deployed in hot places
- Seniors living in warm climates
Seniors should train less vigorously than pro athletes (obviously), but research suggests that incorporating passive modalities, such as hot water immersion or sauna, can also bring benefits to many.
Benefits of Heat Acclimation
The case for heat acclimation has plenty of research behind it. Let’s review some science-backed benefits.
#1: Better heat tolerance
In the 1960s, the researcher C.H. Wyndham studied gold miners laboring in the fierce South African heat for 10+ hour shifts. He found that an 8-day heat acclimation protocol (step climbing for four hours per day in hot conditions) decreased cases of heat illness and improved work performance. Why? I’m glad you asked.
#2: Increased sweating and water conservation
Heat acclimation trains sweat glands to produce more sweat, allowing you to maintain a lower core body temperature and avoid heat illness. It also prompts the body to store more water, which allows a greater cooling capacity and prevents dehydration.
How much does heat acclimation increase sweat rate? It’s a tricky question because sweat rate is influenced by temperature, humidity, exercise intensity, aerobic capacity, body size, clothing, age, weight, hydration status, and other factors. There’s tons of variability between and within individuals.
Here are some figures summarized in a 2019 review paper from Sports Medicine:
- Some studies have found negligible increases in sweat rate after heat acclimation.
- One study found that 5 days of heat acclimation training increased sweat rate from 0.56 to 0.59 L/h (liters per hour).
- Another study found that 10 days of heat acclimation training increased sweat loss from 0.93 to 1.10 L/h.
- A 2007 study found that 10 days of active heat acclimation produced a 17% increase in whole body sweat rate.
- A 2008 trial found that a 10-day protocol of heated treadmill walking increased sweat rate by 6% in 8 men.
- A 2015 paper found that a 14-day heat acclimation protocol increased total evaporative heat loss (closely related to sweat loss) by 11%.
As you can see, these aren’t massive increases in sweating after heat acclimation. Still, the effect isn’t trivial. Athletes can lose up to 10 liters of fluids per day exercising in warm climates.
The bottom line? Based on the limited evidence, you’re looking at a 0–20% bump in sweat rate after acclimating to the heat—but your mileage may vary.
#3: Decreased sweat sodium concentration
As your body adjusts to the heat, you sweat more. Does that mean you lose more sodium too? Actually, it’s not so cut and dry. Heat acclimation decreases the sodium concentration in your sweat, helping you avoid the headaches, fatigue, and heat illness that can follow sodium deficiency or imbalance.
Here are a few studies quantifying this effect:
- In 3 people, a 1971 study found that passive heat acclimation (with hot baths) decreased sweat sodium concentrations by 51%.
- In 8 people, 10 days of cycling in the heat decreased sweat sodium concentrations on the arms and back by 40%.
- In 8 people, 10 days of hot treadmill walking decreased sweat sodium concentrations by about 34%.
The takeaway is that sweat sodium concentrations drop by approximately 30–50% after acclimating. So although you sweat more after heat training, you lose less sodium on a net basis. It’s a brilliant adaptation to retain this precious electrolyte.
#4: Reduced risk of dehydration
Heat acclimation also calibrates the thirst mechanism to match fluid needs better. When you reduce the risk of dehydration, you reduce the risk of cramps, headaches, and other dehydration symptoms.
#5: Improved athletic performance
A meta-analysis of 11 randomized controlled trials found heat acclimation training significantly improved time trial performance in athletes.
Yet, strangely, these researchers did NOT detect differences in corresponding physiological measures (maximal heart rate, maximum oxygen uptake, core body temperature, skin temperature, and others) for heat-acclimated athletes. “After heat acclimatization training,” the authors conclude, “athletes may be able to tolerate greater levels of thermal stress, but our analysis was unable to determine physiological markers of adaptation.”
This may suggest that some of the benefits are psychological. When you’re more comfortable in the heat, you can go harder for longer.
#6: Enhanced aerobic capacity
Another meta-analysis, however, did find a physiological attribute improved by heat acclimation: VO2 max. VO2 max, a crucial marker of endurance and longevity, is the maximum amount of oxygen a person can utilize during exercise.
According to the paper, heat acclimation can drive VO2 max improvements in both cool and hot environments, but more significantly improved performance in the heat.
What Heat Acclimation Protocols Look Like
Here are some characteristics from the protocols compiled in one meta-analysis:
- 4–14 consecutive days of running or cycling efforts in heated laboratory environments.
- 27–90 minute training sessions, with one study breaking sessions into three 10-minute intervals.
- The temperature in the sessions ranged from 86 F–107.6℉
- The studies usually calibrated desired exercise intensity with VO2 max. (For instance, exercising at 50–90% VO2 max equates to moderate-to-high intensity exercise. However, some studies used a percentage of maximum heart rate or other measures.)
How long should you train? One review paper suggests that “nearly” complete adaptation occurs after 7–10 days of training, with most benefits realized after 4–6 days. Longer protocols (14+ days) seem to have the most pronounced effect, but you can still see significant benefits at 4+ days.
How Long Does Heat Acclimation Last?
Heat adaptations start to decay the moment you stop training in a hot environment. One meta-analysis found that each “off day” results in a 2.5% decay in heart rate and core body temperature adaptations. This decay vexes some trainers because it’s challenging to complete a 10-day acclimation program precisely before the competition. Heat acclimation can be grueling, and elite athletes have other training and rest needs, not to mention travel-related obstacles. You’re lucky if you get free bananas at your hotel, much less a state-of-the-art heat acclimation lab.
But thankfully, athletes don’t have to push their 10 days of heat acclimation until the last minute. Since people are able to acclimate faster than their adaptations decay, one can re-acclimate to the heat. For example, one study found that after 10 days of heat acclimation and then 12 days of decay, it took only 2 days of re-acclimation training to reinstate full heat acclimation benefits. In other words, you can “top up” your heat acclimation faster than starting from zero.
Heat Acclimation and Hydration
After you acclimate, you sweat more—and that helps you keep cool. Consequently, you need to replace more fluids and electrolytes, especially sodium. (Add plenty of salt to your water bottle or use an electrolyte drink mix to accomplish this goal.)
Sure, sweat sodium concentrations decrease by about 40% after acclimation, according to one study, but it’s still possible to lose several grams of sodium per day. This sodium should be replaced to prevent the low energy, cramps, and neurological symptoms of hyponatremia—and just to keep you generally “feelin’ good.” Use the Beta version of our sodium intake calculator to estimate your sodium needs based on exercise duration, exercise intensity, temperature, humidity, dietary factors, and even heat acclimation status.
Your Heat Acclimation Goals
Before you dial in a heat-based training program, consider your goals. Elite athletes will have different goals than health-conscious knowledge workers, and younger folks will have different goals (and heat tolerances) than seniors.
If you’re willing to push your limits for a serious performance edge, a heated exercise protocol could be the ticket. It worked for the Australian racewalkers. But if you prefer a more gradual approach, that’ll work too. You can still adapt to the heat with gentler methods like sauna, hot yoga, and summer hikes. Whatever method you choose, remember to stay hydrated, stay salty, and listen to your body.