Heat training is having a moment. With record-breaking springtime temps across the West and emerging research suggesting the benefits extend far beyond simply running in the heat, we’ve got elites setting up home hotboxes, weekend warriors training with core temp sensors, and hot tubs everywhere transformed from a means of leisure to torture.
It’s true, your new habit may be making you faster—or it may be actually slowing you down.
The basics of heat training are fairly simple: Intentionally elevating your core temperature while training or at rest helps your body expand blood plasma volume, increase sweat rate, and reduce sodium loss. The result? You run more efficiently, and faster. But the devil is in the details.
For years, the standard heat training protocol was 10-14 days leading into a race. But that narrative seems to be is shifting to longer, more intense, and more involved regimens. Call me a cynic—it wouldn’t be the first time—but I’m skeptical of any advice based on the premise “more is better,” especially when it involves pushing your body to the limit. Not to mention, if you’ve dabbled in heat training, you probably know that sometimes it makes you feel like complete crap.
So, I started to wonder what was real and what was pseudoscience. At what point do the risks outweigh the gains? What techniques truly work, and which ones should I avoid?
I had the opportunity to have all of these questions answered and then some when I joined Des Linden and Magda Boulet for heat training and lab testing before we all shipped over to the Sahara for Marathon Des Sables. There, I spoke with a pair of experts who have spent their careers studying the line between positive heat adaptations and heat stroke:
Roxanne Vogel holds a PhD in exercise physiology with a focus on nutritional strategies in heat and is the director of research and education at GU Energy Labs. Daniel Snape holds a PhD in sport and exercise physiology and specializes in environmental factors like heat. He consults with Olympic marathoners like Linden and Boulet, pro triathletes, and champion ultrarunners, including Ruth Croft and Tom Evans.
Des Linden undergoes lab testing and heat training before Marathon Des Sables. Author Andy Cochrane spoke with the scientists working with her to distill fact from fiction. (Photo: Andy Cochrane)
What’s the Point of Heat Training?
Heat training induces a wide range of adaptations that improve both your body’s ability to regulate temperature and lower your heart rate, especially while running in the heat. Ultimately, that ups your fitness. It’s also beneficial for general wellness, boosting your heart health, sleep quality, and immune system.
If done correctly, initial heat adaptations can take place in a week or less. Full acclimatization takes two weeks or longer, depending on the individual. However, heat training comes with risks, ranging from acute exposure like heat stroke to a general malaise, often from dehydration. As with any part of training, there is a fine line of stressing your body in a healthy way and overloading the system.
The Right Way To Heat Train
For a runner preparing for a hot race like Marathon des Sables, the Western States 100, or a summer marathon, heat training is critical, but varies considerably for the individual. The needs of each person are different, so experts recommend you get tested for your individual sweat rate and sodium concentration. In lieu of that, here are some general guidelines to follow.
Snape says the standard heat protocol aims to raise your core temperature to 38.5 degrees Celsius (101.3 degrees Fahrenheit) and your skin temp to 36 degrees Celsius (96.8 degrees Fahrenheit) and maintain that for 30–45 minutes. This can be done during a workout or right after using a sauna or hot bath.
“The passive strategy is actually the best bang for your buck,” Snape says, “because it lets you focus on keeping your core and skin temperature in the ideal range. The minimum dose is four sessions over four consecutive days, although that only allows you to survive in the heat, while the next few weeks help improve performance.”
Heat training should be done at least five days a week and needs to be built into your training plan in a thoughtful manner; avoid doing it around your hardest sessions so that you don’t overcook yourself.
“As long as it isn’t negatively impacting your training or recovery, the goal is ten sessions or about two weeks,” Vogel says. ”The common mistake people make is doing too much too soon, which results in an accumulation of fatigue, lack of motivation, and poor recovery. We’re seeing really good results with simulated environments, which allow you to heat train anywhere in the world. This can even be heaters in a small room.”
What To Expect During Heat Training
If you’re heat training on a run, it’s common to initially see a higher heart rate (typically around 5-10 beats per minute) at a familiar pace. Over time your heart rate will align more closely with your baseline, although it will likely never be exactly the same. Therefore, you should start heat training at a lower intensity for the first few days.
To help your body adapt, Vogel says carb loading like you would the day before a hard session or race is useful. “We burn carbs differently in hot conditions, relying more on glycogen that’s onboard versus consumed carbs. To avoid GI distress, it’s smart to pull back your [mid-run] carb intake around 10-15% per hour in the heat.”
Because of the current hype around high-carb fueling, we’re seeing a lot more GI issues, Snape says. “You need to accept that we can oxidize less in the heat, or you’ll shit your pants. The best way to avoid that is the right hydration plan before a race and staying sensible during it.”
It’s worth noting that female athletes typically take longer to adapt, get less of a benefit on shorter protocols, and see a later sweat onset than males. Likewise, most masters athletes don’t manage core temperature as well as younger runners due to reduced sweat rate and efficiency, coupled with a diminished thirst drive, putting them at higher risk of dehydration.
As heat pulls blood from the brain to the skin for cooling, decision-making is impacted as well. “It’s an inverted U-shape relationship,” Snape says. “Cognitive function increases as core temperature rises to 38 degrees Celsius, then drops after that.”
Pro Tips For Heat Training
Des Linden undergoes heat training, lab testing, and personalized fueling plans ahead of racing Marathon des Sables. (Photo: Andy Cochrane)
Runners have long used trash bags or layers for active heat training. This works, if you have the right layers, Snape says. “It needs to be uncomfortably hot or it’s a weak stimulus. The thermal sensation should be almost unbearable with lots of sweat.”
Vogel agrees, especially for people who don’t have easy access to a sauna or a hot place to train. “Overdressing is basically creating a microclimate, your own heat chamber around your skin. It’s uncomfortable and looks funny, but the science shows it is an effective way to acclimatize.”
Thanks to the rise in popularity of heat training, sweat testing kits are readily available. These at home tests aren’t perfect, but they are accurate enough for those without access to a lab. “Lab equipment is still the gold standard,” Snape says, “but I do think at home kits are useful, because they help you record a lot more data and see a pattern.”
The main goal of a sweat test is to determine your sweat sodium concentration, or how many milligrams of sodium you lose per liter of sweat, and your sweat rate, which is the total fluid volume you lose per hour under specific conditions. The former doesn’t change a ton while training, but the latter can vary wildly based on temperature, humidity, and your training. Advice typically suggests around 500mg per hour, but individuals range from as little as 200mg to over 2,000mg.
Both Vogel and Snape believe personalization is the way forward with heat, much like other facets of training. “Wearables are getting more accurate everyday,” Vogel says, “and they are accessible in terms of price, too. Core temperature sensors, which provide a layer of safety, are just under $300.”
The Big Heat Training Myths To Avoid
Permissive dehydration—intentionally training without fluids—is growing in some circles, despite a ton of hard data behind it. “I tend to avoid dehydration training with athletes,” Snape says, “because you often aren’t able to recover the same way. It might work for a few elites, but not for most people”
Likewise, there is a lot of hype around using ice vests for “pre-cooling” before races to delay the rise in core temperature. While there’s a little more evidence behind it, pre-cooling is far from the silver bullet some make it out to be.
“Some literature says that pre-cooling can lower core temperature by 0.3 or 0.4 degrees Celsius, delaying the drop in performance due to heat later on,” Snape says. “However, starting colder means your sweat glands are delayed, which comes with risk, too.”
Pre-cooling has minimal impact later on in long races, when it’s the hottest part of the day. “For ultras, you’re often starting early in the cold anyway and using more ice later in the race, as the day heats up” Vogel says. “It’s effective for shorter races or intermittent sports when you’re on and off the field, but the heat sink created by pre-cooling doesn’t last long.”
Last but not least, there’s a new trend of sodium loading before and after runs, which likely hurts more than it helps. “We’re seeing a lot of new products with lots of sodium,” Vogel says. “But the higher you go with sodium, the more water is drawn into the gut, which risks GI issues. In most cases there isn’t a need to go crazy.”
“If your sweat rate is high, just season to taste,” Snape says. “These massive amounts are just creating really expensive urine.”
Caleb Olsen cools down en route to winning the 2025 Western States 100. While pre-cooling may have minimal impact during longer racing, mid-race cooling certainly does. (Photo: Peter Maksimow)
Personalizing Heat Training for You
A recent paper showed heat training in 80% relative humidity elevated plasma volume slightly more than training in a drier climate. That’s because of the sweat response, but it also means that depending on where you live, it’s easy to overdo it.
“A hot-humid environment feels harder because the sweat isn’t evaporating off your skin, which is what cools us,” Vogel says. “This makes heat plus humidity a stronger stimulus, but could mean you’re not able to train as long. There are tradeoffs to both.”
Time of day is also a factor, although not how I expected. Research shows that if you elevate your core temperature in the evening and then cool off, it helps you fall asleep faster and stay asleep longer. “Your core temperature rises throughout the day,” Snape says. “Which means evening passive sessions are easier to get to 38.5, just remember to rehydrate before going to bed.”
Once a runner has acclimated for a race, the physiological gains don’t last as long as I suspected. “The decay of heat acclimation is pretty fast,” Vogel says. “In two weeks, heat adaptations are significantly diminished, but you can do a heat maintenance protocol to keep your grains. Twenty to 40 minutes in a sauna or hot tub every other day can prolong the adaptations for a long time.”
