This episode of The Athletes Compass explores the role of heat training in endurance performance. Dr. Paul Laursen, Marjaana Rakai, and Paul Warloski discuss how controlled heat exposure leads to increased plasma volume, red blood cell production, and overall endurance gains—similar to the effects of altitude training. They also provide practical tips on integrating heat sessions into training, balancing active and passive heat exposure, and avoiding potential risks. If you're training for a hot race or simply looking for a new performance edge, this episode will challenge your perspective on heat adaptation.
We can't get too
we know this because just think about your body's made of protein and you don't wanna be like the egg on the fry pan, right? When you can actually see the proteins of that egg.
denaturing and turning to something else, right? That's the process. So we've got processes to slow us down so that we don't become the fried egg on the fry pan. We don't want to denature
Paul Warloski (:you
Hello and welcome to the Athletes Compass podcast where we navigate training, fitness and health for everyday athletes. So here's the question we'll get to today. We've done some episodes on heat adaptation and that got me thinking. If heat training helps blood adaptations for endurance athletes, all of us, should we be doing our indoor training without a fan and adding extra clothing, trying to create heat?
So let's start with Paul reminding us about the adaptations that come from heat training. What happens to our body when we adapt to the heat?
Paul Laursen (:yeah, that's a good one. So let's see. So when we exercise first and foremost, we remember it's an inefficient process, right? So just take the external heat out of the whole thing, right? Because we are focused on an indoor training session here. When your muscles move and generate energy, it's an inefficient process.
Only 20 % of that energy that's burning, right? And remember that burning is the food that we've consumed, burning in the mitochondria, and only 20 % goes to doing the work on the bike or the run or whatever you're doing. 80 % goes to forming heat. So we're warm-blooded animals, right? Feel your skin, the reason you've got warm skin and a warm body.
is because you're a warm-blooded animal and it's due to that inefficient process. So, of course, when you move your legs and start exercising more, you really start to kick up the system and that continues to spin and that 80 % really doesn't stop too much. It just speeds up and you develop more and more and more heat. And that's why you'll start to...
You see yourself turning red and sweating, So there's a process. We can't get too hot. we know this because just think about your body's made of protein and you don't wanna be like the egg on the fry pan,
And so the heat is sensed by the brain and we down-regulate our power accordingly.
All right, so, but of course we can get better and better at doing this. Actually the denaturing itself, you'll stop turning into the fried egg more and more the more adapted that you get. But also to Paul's point, the big adaptation that will come way before all of the heat shock protein stuff is this, is the fact that your body dehydrates and then it counters.
And it says, whoa, we got really dehydrated that time. We better do something about it for the next time. And the big hormone, one of the couple, but one of the big ones that gets secreted thereafter is called aldosterone. And aldosterone secreted by the kidneys cause for more sodium salt to be retained in the blood. And we always say that water follows sodium. Water follows that salt.
Paul Warloski (:you
Paul Laursen (:So that's why you, so you'll retain now more salt in the post exercise period. Then what winds up happening is the water component of your bloodstream opens up and expands, right? So you get more water relative to the amount of red blood cells that are in your blood. Now, so now you've got this system with more plasma volume, but a low level of red cell volume.
Paul Warloski (:You
Paul Laursen (:And then the new trick that happens here is your body starts to think, we're, it's like we're at altitude right now because it looks like we've got all this water and we've got really low red blood cell percentage relative to the water component. We'd better do something about that. So out comes EPO, right? And everyone's heard of EPO, all the...
Paul Warloski (:you
Paul Laursen (:the drugs doping scandals and all that sort of things. That's what gets injected. Your body makes more of its own EPO. And now because of that heat situation and the expanded plasma volume, you now start getting more red cells being produced in your bone marrow. So now it kicks out more of these red blood cells. And now you get the best of both worlds because you get both a, with heat training, you get both a plasma volume expansion.
and you get a red blood cell volume expansion. And this is why many out there are saying that, you know, heat training is the new, the poor man's altitude training. And indeed it is. So that's a very long-winded answer to your question, Paul, but the answer is yes. You want to get hot in your indoor training workouts. And by getting hot and being okay with that, you will
start to adapt from an altitude training sort of standpoint. You also get that as well. You get expanded plasma volume and an expanded red cell volume.
Whoa,
Marjaana Rakai (:haha
Paul Warloski (:What?
That was awesome. So, you know, we're all taught to get our big old fans out and everything, you know, get our, you know, giant body fans and keep ourselves cooled. mean, am I supposed to throw my fan out now?
Marjaana Rakai (:Get a space heater too.
Paul Warloski (:Yeah.
Paul Laursen (:Yeah, context is everything, right? So what matters are you doing, if you're doing a Zwift race and it's important that you, you know, they go well for your Zwift team or for your own ego, then yeah, get the fan out and kick ass and do what you can. But if you're in the context where I'm really working on getting hot for this race, then, or start getting hot for this session so that I can add a heat stimulus.
Paul Warloski (:Yeah, of course.
Paul Laursen (:to my person, then yeah, turn that thing off and just embrace the heat and know that it's all part of it. Right, MJ?
Marjaana Rakai (:Yeah, I love heat. I love it more now. Okay, let's say it that way. So let's give people some structure. Would one heat session a week be enough of a stimulus to get any benefits? Or does it have to be like two, three, four times a week?
Paul Laursen (:You love Heath. You love it more now.
Good question, actually. our friend, Julia Casadio, she would say that, you you want some sort of a regular cadence of this stimulus, right? This is a unique stimulus. And this is exactly what she looked at in her PhD work in New Zealand.
She was finding that two to three sessions in a week is usually enough for the body to remember that. you can, in her studies, she would kind of go through this heat adaptation phase where she'd probably get like, maybe string about five in a row, four or five in a row together, of classic. Because all the classic studies, they do like 10 days, 10 days in a row of heat.
And so she would do like, you know, probably five and then do some other parts of the training and then kind of come back and just holding that with two, maybe two sessions in the week, up two or three sessions in the week. And that was sufficient to hold on to a real adaptation to heat in well-trained individuals. So to your question, yeah, I think one or two in a week, I think you'll still get something from that. I think that's enough of a regular cadence.
Now if you do one every two weeks, I think you're probably gonna be missing some of that. That's probably gonna be almost too much of a time away to be sort of missing that. But I will just say for me, MJ, attending your velocity workouts on a cadence of about two per week, I'm feeling that they're a bit easier. I'm not using a fan here in my setup.
Marjaana Rakai (:Thank
Paul Laursen (:And I'm feeling like I'm getting less of a heat stress in those sessions. There's probably a training element there, but there's probably also that heat element there that's happening too. So I think context will totally depend on your situation. But I think, yeah, I think you will get something off two a week.
Marjaana Rakai (:So knowing that the heat stimulus actually, it's pretty hard, right? So it adds to the training load as a stress. So would you, if somebody wants to play around with this, would you recommend them to start with like an easy recovery session or just a regular aerobic session? Or would you do like smack two flies in the one go, do heat session?
Paul Laursen (:So yeah.
Paul Warloski (:you
Paul Laursen (:Hahaha
Marjaana Rakai (:in the heat.
Paul Laursen (:Yeah.
Yeah, unfortunately, it's always the depends the context one, right? Like, again, if you're a well-trained individual, you could totally do what we're doing, right? Or you're, I'm, you know, I'm doing a velocity workout with you and I'm getting, I'm getting hot on that. It's a combination of no, no fan and, and the hit session. So that's, that's great for me. I'm a, I'm a, I feel like I'm adapted to that. I don't have too many, too many worries.
Marjaana Rakai (:Right.
Paul Laursen (:But I think there's been some studies that have shown if you're adding in heat with that and doing HIIT sessions as well, it's a big stress on the body. So you gotta be kind of careful with that. So if you're a beginner, I would definitely start with the aerobic session and keeping the intensity of the exercise relatively low, but still adding, making it a little bit warmer in the room and then keeping the convection cooling element,
Paul Warloski (:you
Paul Laursen (:the wind flow that would go through your, over your skin. Keep that kind of on the, you know, yeah, on the lower side and just develop the ability to tolerate heat. And then in the more advanced, you can sort of add some of these and probably the advanced as well as being time crunched, right? Then it's kind of nice to have that, the combo hit, right? Combo hit plus heat.
Paul Warloski (:Thank
Marjaana Rakai (:Mm-hmm.
Yeah.
Paul Laursen (:So
yeah, I have not too many issues with doing that, although I'm a little bit out of it in online meetings thereafter.
Marjaana Rakai (:And what about doing a passive heat also in the mix, know, like sauna or a hot tub?
Paul Laursen (:Big time for me. That's personal favorite is the sauna work. yeah, just like last night, I'm very fortunate to have a wood fire sauna here in Revelstoke where it's minus 15 outside right now. And it's a really, it's a great thing to do to put on the wood fire sauna and then, know, had it up to 40.
Oh man, well, my body temp said it was on the watch that it was 47 last night. So I'm sure it's whatever that means on the watch kind of thing, right? Some combination of the ambient in the sauna plus my body temp. But I'm going back and forth between the minus 15 degrees bathing in the snow for five minutes, right? Which is also very, challenging. Like it's hard to stay in the snow in minus 15 degrees.
Paul Warloski (:you
you
Paul Laursen (:for five minutes as well, right? So there's a, but there's this contrast of the temperatures is just magic, I find. And yeah, so just kind of a going back and forth between the real extreme temperatures. I get a lot out of it. I don't, yeah. And I'm sure there's some science to support that as well, but yeah, I love it.
Paul Warloski (:Should people build up their tolerance to the heat in their training? For instance, start with a 20 minute aerobic session of their beginners and move up to 30 and 40 and 50. Should it be a gradual exposure like that?
Paul Laursen (:yeah. yeah. Big time. Yeah. Every with it's just another, it's a stress, right? Like it's just a, and it's a different, it's a different stress and your, your body will like, it's very interesting when you watch people join us in some of these sauna sessions, right? And you can just see that they're, they're not accustomed to these and they're, they need to leave. They need to exit very soon, right? After maybe five, 10 minutes, you can see they're already.
Paul Warloski (:another adaptation.
Marjaana Rakai (:you
Paul Laursen (:you know, getting a little stressed and, and, and they, they haven't had the regular exposure to it, right. Where I, you I can stay in there for an hour sometimes. It's no, know, and, yeah, it just totally like any stress, you need to go through a process of adaptation to become better and better at tolerating it. And that's the job of the body is to be able to, in the recovery period, to put the new processes in place to be able to.
to cover for that, those stresses that it's going to, you know, that might kind of come again in the future. That's what you're doing for the exercise training as well, right? You're trying to continue to slowly add on the resilience factors that are going to enable you to perform to the stress level that you're probably going to entail in the event that you're dreaming about.
Right, MJ?
Marjaana Rakai (:Hehehehehe
Paul Warloski (:You know, MJ was laughing
Paul Laursen (:Evil, evil laugh.
Paul Warloski (:about space heaters, but should we be using those? Like I've got an unheated garage. mean, should I break out the space heater on my endurance rides and hit sessions?
Marjaana Rakai (:I would say so, yeah.
Paul Laursen (:Yeah, yeah,
Paul Warloski (:Okay,
Paul Laursen (:for sure.
Paul Warloski (:yeah.
Paul Laursen (:Yeah, I've got one. I showed you my heat chamber room, right, MJ? So yeah, yeah, absolutely, for sure. if that's, yeah, like it's again, it's all about tweaking the individual stress or stimulus for yourself. So yeah, yeah.
Marjaana Rakai (:Yeah. Yeah.
Paul Warloski (:Okay.
Marjaana Rakai (:It's funny because this office doesn't have AC, like AC unit doesn't blow the air here. And I've been complaining about it gets so hot in here during the afternoon because the sun goes like that. but after the summer, I realized, hey, I guess I have my own heat jumper here. right in the office and I shouldn't be complaining about it, but embrace it because it's making me fitter.
Paul Warloski (:So how long do these heat adaptations, you know, with the blood and with the plasma volume, how long do they last if you don't continue heat adaptation work?
Paul Laursen (:Good question, Paul. I think probably, you you're going to start to see a reduction in their, you know, their levels probably after about four or five days, right? They're going to, yeah, they're going to, yeah, they're going to be quick. Yeah. You're going to, but they won't go, you know, to baseline. They'll just need a little bit of help to get them back up again. Right. And this is why.
Paul Warloski (:that quick.
Paul Laursen (:Yeah, it can be useful to get a little small bit of that same sort of stimulus to get them back up again, right? Marjaana you're pointing at the mind. What do you think?
Marjaana Rakai (:I'm pointing
at the mind, because even if the physical adaptations go away quickly, I think once you've done the heat training and you can see the benefits and the effect of it, it does something to your brain, because it's pretty amazing. So you can put yourself through the wringer again, knowing that you can adapt. And it probably happens a little bit faster without so much struggle.
Paul Laursen (:Mm-hmm.
Yeah. And you you're actually, you're making me reflect on one of the, probably the funnest research project that I ever got to be a part of. And I was, I was on my sabbatical working as a prof in Australia, but I got to come home to Vancouver and work with the UBC crew on a Himalayan altitude project. Cause the UBC team does all these
all these projects on Altitude with their respiration lab, Bill Scheele and others. So I came over and I did my postdoc and then I got taken over to the Everest area and the project we were gonna look at was basically this, what you're talking about, the adaptation and how long basically it, you you go without a stimulus and you still return to get that.
So what we did, and we were looking at this more in the altitude context, but it's going to be the same for heat as well. It's like, okay, Mountaineers were saying, you know, basically after 10 days, you're not going to get any more altitude benefit kind of, or sorry, that's what, sorry, the research said that you won't get anything after 10 days, but Mountaineers were saying, no, I come back down to Kathmandu and I go back up again. And the second time definitely feels better.
So we were trying to quantify this basically. we went to Kathmandu, went up to the Everest region and over 10 days took all these various blood measures, et cetera, all these wearable devices and whatnot to look at our adaptation, altitude sickness readings, et cetera, went back down to Kathmandu, hung out for 10 days and then repeated the exposure again. we had these gaps and
again, according to the research, nothing should have helped us in the second boat. But all of the markers showed that we adapted way quicker, altitude sickness levels way lower. So something, our mind, kind of like what you're saying, MJ, it knew and all the physiological processes, they are plastic and they remember and they move faster in the second time. So that is really the answer to your question is yes, maybe it won't be
Paul Warloski (:Hmm.
Paul Laursen (:spot on, ideal, but probably within 10 days, you'll still have the memory across the body on being able to adapt. It will pick back up much quicker the second time. yeah, and really this kind of comes back to whatever you can do to be able to get that stimulus is probably gonna work for you.
Marjaana Rakai (:So let's say somebody has a race that they know it's going to be in the heat. So they need some kind of a heat adaptation before the race. If they start with indoor heat training during the winter and then kind of try to keep that adaptation going over like several months during their buildup to the race,
they would do a couple of training sessions, active heat training, and then maybe one or two passive heat training. And that should nicely prepare them for the race.
Paul Warloski (:you
Paul Laursen (:Yeah,
Paul Warloski (:Thank
Paul Laursen (:is, yeah, couldn't really add much to that, MJ. That's exactly what I would recommend. So yeah, you know it's going to be hot wherever you've decided to race, then use both the active heat sessions. So the space heaters that Paul was saying, have those near your trainer. Get hot during the active sessions. And then at the same time, what is at your disposal that's practical that you can get passively hot in? Do you have a hot tub?
Paul Warloski (:Thank
Paul Laursen (:You know, get hot in that hot tub. Do have a sauna? Go use that or whatever it kind of might be, right? I think even I'm reflecting on the Magnus Ditlav. There's like basically like a video that accompanies his podcast, the podcast that I did that shows kind of a day in the life. And he's in Denmark in Copenhagen where it's pretty darn cold in the wintertime.
But the guy's got like, he's bundled up as much as I am when I go do my minus 20 downhill ski, he's bundled up just crazily and he is absolutely sweating up a storm, but he wants to be outside on his ride. So whatever you can do to sort of still get hot is valid, right? Because you know that you want that mind, the brain,
Paul Warloski (:you
Paul Laursen (:to be tolerating the heavy heat that is going to be occurring during the race itself.
Paul Warloski (:Is there an equivalent heat stimulus between training on the bike or dressing warmly and passive heat?
Paul Laursen (:I don't know if there's an equivalent stimulus, Paul, but again, I can remember having these conversations with high-performance sport New Zealand and lots of different coaches and stuff. at the end of the day, how can I practically feel hot? You want to feel kind of, unfortunately, uncomfortable. And yeah, and again, I'm reflecting...
on the podcast I did with Brad Culp and then the Norwegian method and Christian Blumenfeld, he just was so obsessed with winning the Tokyo Olympic race. And the guy did some insane heat sessions where he just really, like he really went to the next level. Like you gotta be careful, right? But the guy was obviously super obsessed.
And it, you know, he was measuring his core temperature and it was like, you know, like it was, it was, you know, around that 41, 42 kind of thing. And it was just under for quite a long duration, but he was, he just really knew that's what was, that is what it was going to kind of take to, win an Olympic medal that he had to have this super high overall body, mind tolerance of very high heat.
levels because the Tokyo Olympics was the hottest Olympics on record. that's what he did. I forget where they train, but it was just these insane heat sessions. So much so that when he actually did Tokyo one and then was interviewed by the reporter at the end, he said, actually, I wish the day was a little bit hotter, but it was a bit too easy for me.
Marjaana Rakai (:Kids don't do that at home.
Paul Warloski (:Yeah,
Paul Laursen (:No.
Paul Warloski (:I mean, are there risks associated with deliberately inducing this kind of heat stress during the winter training or anytime?
Paul Laursen (:100%. Yeah, for sure. Yeah, you've got to be cautious with all of this sort of stuff, right? Like we look at the stars and what they've done and these are, you know, like we've said before, these are special specimens. And, but yeah, we've, you've got to be mindful and you got to be, yeah, you got to be thinking about all these different things. You know, you could induce a dehydration or plasma volume lowering that causes you to black out.
right? And you could have an accident on the bike. You don't want to do that. know, we've, yeah, I mean, there is usually you will slow down before your, your body organs actually, you know, you you develop sort of that heat stroke and you have actual injury. That's the whole, that's the reason why the, mind and the brain will lower
Paul Warloski (:Hmm.
Paul Laursen (:the exercise output, the motor output, because the body has to do that, right? Like it has to protect itself. It knows at some level that the organs must be protected. But there are these cases where temperature has gotten too high and yeah, people have been hospitalized. So you really have to be careful, especially
se intensity, right? Sprint a:you've really got the ability to kind of like push out some big power and what comes with that is a massive load of heat and all of a sudden your core temperature is really high and you're frying that egg ultimately, right? You're potentially denaturing that protein. So that's kind of the most dangerous one. Usually in, you know, over the long sort of period of time, you will, you you'll have to kind of lower the exercise intensity.
That's why we see so many people walking in Kona, if you look at that. It's so hot there in that race. Even on a cool day, it's so hot. But yeah, people are walking, most like many times because their brain is telling them I'm getting too hot and that's about the exercise intensity I can sort of handle.
Marjaana Rakai (:Yeah, if you pay attention to that when you're, when you're doing the adaptation, you'll know when you've had enough, cause you just, like, it just starts to feel like really hard and then your pace starts to slow down and then you just end up walking. Just walk home, call it a day. And then you want to pay attention to that you recover from it. Cause if you're doing the heat adaptation and you're so tired that you can't do anything the day after then.
Paul Laursen (:yeah.
Marjaana Rakai (:Maybe that was a little too much of stress. Yeah.
Paul Warloski (:too much.
Paul Laursen (:Totally, totally.
Yeah, I'm sure you had some of those MJ when you were preparing. Yeah, and again, always the next session is the most important session, right? It's so hard to get right, but yeah, you wanna slowly kind of progress out that ability to tolerate the temperature in these sorts of sessions. But if it's too much, like you gotta know when to shut it down too. It's like, oh, that's a good enough stimulus today. you might feel almost like you're,
Marjaana Rakai (:Yeah.
Yeah.
Paul Laursen (:Your progress isn't great, but just kind of stick with it. There's a great blog from Cindy Maloney I'm reflecting on, on her heat adaptation. It's not a full focus, but she does mention it, I think in the blog and on her podcast that, that, that, that we did. And yeah, she sort of talks about the process that she used to go from cold Canada and Revelstoke where we are.
to having a personal best in Kona. And really, she said she didn't feel that hot in the race itself. And that's a real testament to her preparation and doing some of this combination of these active and passive sessions to prepare herself for that hot race.
Paul Warloski (:So here are some takeaways. Here are some things that I have learned today that heat adaptation creates more plasma volume and increased red blood cell count. Is that correct? Red blood cell count or just the size of the red blood cells or both?
Paul Laursen (:No, the actual,
like the number, the volume of red blood cells. So yeah, you'll get more red blood cells on top of that plasma volume. Plasma volume first, but then that's the altitude signal. And then the bone marrow will get to work and make more red blood cells. So red blood cell volume mass, I think, is the technical increased component.
Paul Warloski (:Okay.
That's so cool.
Okay.
That's so cool that that can happen. number two, if you are on the indoor trainer and you have a big race or something, then put your fan on. But number three, the most important takeaway for today is that if you want to get some adaptations in this, those two big adaptations, plasma volume and red blood cells, get yourself uncomfortably hot, in training, get the,
Paul Laursen (:It's amazing.
Paul Warloski (:Put away the fan, get the space heater out. You're looking to get one, two, three sessions per week should be enough. There should be a regular cadence of the stimulus, but if you are starting off as a beginner, as a beginner with this, if you're still a new athlete, start with aerobic sessions. If you are advanced, you can start with the HIIT sessions, but always graduate your efforts and your exposure.
Paul Laursen (:be careful with this extra stimulus, this extra stress, and use common sense. Common sense always prevails, and have a cooling rescue situation available. You know what I mean? And remember, like a cold bath, if you all of a sudden felt like you were too hot,
Paul Warloski (:Be careful.
Marjaana Rakai (:Thank you.
Paul Laursen (:A cold bath is going to be the quickest way, or you can jump out in the snow if you live in Revelstoke right now. But yeah, a cold bath is usually the quickest way that you can remove that body heat from your body. if you get too overheated, that will quickly cool you down. So just that disclaimer there.
Marjaana Rakai (:Ha ha!
Mm-hmm.
Yeah. So when you buy your space heater, also get yourself an ice bath. That's what I had in my garage. So yeah. yeah, it's basically just like a round bucket. So that's what I use when I went for these long runs outside and...
Paul Laursen (:Yes, that's an important add-on because you use it all the time. You're always sending me photos. I think it's hilarious. You've got a bucket.
Marjaana Rakai (:You know, I was pretty hot, so I would just jump in there or after actually their VO2 max sessions too. I like to just jump in there for half an hour and get myself cool.
Paul Laursen (:Mm-hmm.
Yeah, I'm so glad we brought that up because that's a really important thing. Like remember, we're talking here to the everyday athlete, right? And you got to get back to work. You're no use to your family, to your work colleagues, et cetera, if you can't function. Well, and Marjaana kind of found that out, right? And so she's doing all these various adaptations. The body's adapting accordingly, but then it's okay. Now I got to get back to business. So I get inside my garbage bin or whatever it is that's filled with ice water.
Paul Warloski (:Yeah, yeah.
Marjaana Rakai (:What?
Paul Laursen (:and I'm in there for five minutes. My body temperature is quickly brought down from 40 or whatever it is down to 37 again Celsius, 97 Fahrenheit and I'm back to business and I feel great, right? You feel awesome after it.
Paul Warloski (:Thanks for exploring the path to peak performance with us today on the Athletes Compass podcast. When you subscribe, you'll ensure you're always tuned in for the next journey into endurance mindset and performance. And when you share this episode with a friend, teammate or coach, you'll be helping them discover new ways to level up their training and their lives. Take a moment now, subscribe, share, and let's keep navigating this endurance adventure together.
For more information or to schedule a consultation with Paul, Marjaana or me, check the links in the show notes. For Marjaana Rakai and Dr. Paul Laursen, I'm Paul Warloski and this has been the Athletes Compass Podcast. Thank you for listening.