Heat Training Is a Free Performance Upgrade

Miles: Hey, welcome back to Full Spectrum Fit. I'm Miles. Amara: And I'm Amara. And Miles, I have to say, this week's episode might be one of my favorites we've done. Miles: High bar. What makes you say that? Amara: Because we're talking about heat training, and I know you had a rough week with it. Miles: Rough is generous. I ran 6 miles in 90 degree humidity Tuesday and my body filed a formal complaint. Amara: Same. I did a post-lift sauna session Thursday in January. I genuinely question my decisions as a person. Miles: But here's the thing, and this is the whole point of today's show. That suffering, it's actually doing something. Amara: Okay, so get this. Most athletes treat summer heat like an obstacle to manage. The research says you're leaving a serious performance edge on the table if you're just surviving it. Miles: And we're not just talking about heat race prep. Frontiers in Physiology just published a study on four weeks of heat. of heat acclimation showing real metabolic shifts, less carbohydrate oxidation at the same effort, better aerobic capacity, Amara: Right. Plasma volume expansion, improved sweat efficiency, and EPO spike. The Journal of Applied Physiology has data on this, too. Your body gets better in the cold because you trained in the heat. Miles: Which sounds backwards until you understand the physiology, and we'll break all of that down. We'll also get into what an actual two-week acclimation block looks like. Outdoor running versus post-workout sauna, we'll debate that one. And then we're going into hydration because drink more water isn't the answer here. Amara: Sodium, that's the story. Plot twist, water might actually be the problem. Miles: Okay, let's get into it. We start with the check-in. Okay, I need to open with a confession. Amara: Already? Miles: Already, Saturday long run, I slept in, which fine, but that meant I didn't start until 10 AM instead of 7 and Amara, I was absolutely destroyed. Amara: How bad? Miles: I started mentally pricing out treadmills around mile four. Amara: No! Miles: It was maybe 83 degrees, humid. Mid, I've run in worse, but something about hitting that heat without being adapted to it, I felt like I was working twice as hard for two-thirds the output. Amara: Okay, so get this. Tuesday I had a track session and it was 85 degrees when I got there and my pace was slower than a normal tempo, like noticeably slower, but my perceived effort off the charts. Miles: Right, right. Amara: And here's the thing. I wasn't frustrated. I was kind of... fascinated, because that gap between what you expect your body to do and what it actually does in the heat, that's not a failure. That's a signal, Miles: Hmm. I was not feeling fascinated. I was feeling humbled. Amara: which is the same thing, just with more suffering. Miles: Fair enough. Amara: But no, seriously, National Geographic published a piece on this recently, and it makes the point that most runners treat heat as something to just just white knuckle through. Miles: Survive and get back to real training in the fall. Amara: Exactly. But the research says the opposite. Regular heat exposure forces real adaptations, better sweat rate, better blood volume, less cardiovascular strain over time. Miles: So what happened to me on Saturday? Amara: Was actually your body telling you it hasn't adapted yet, which means there's room to change that. Miles: So here's where it gets interesting, because that felt terrible, but you're saying terrible is kind of the point. Amara: The stress is the stimulus. Your body doesn't adapt to easy. Miles: I mean, I've seen this play out in other contexts. The athletes who train smarter in the off-season are the ones who show up different in October. Amara: And that's exactly what we're getting into today, because the question isn't just why heat makes your run harder. The real question is what's actually happening inside your body while you're suffering out there, and whether you can turn that into an advantage. Miles: So the body is already working against you in the heat. Heart rate climbs, blood gets shunted to the skin for cooling, and your muscles are suddenly competing for blood flow they'd normally have all to themselves. Amara: Right, so what does consistent heat exposure actually do to fix that? Miles: Okay, so get this. The first thing that happens is plasma volume expansion. Your blood literally gets more fluid in it. Hold on, more fluid in the blood. What does that actually feel like to someone pushing through mile three of a 5K? Amara: I love that framing. So in practical terms, more plasma volume means your heart can push out more blood per beat, stroke volume goes up, heart rate stops drifting as badly, you just feel less cooked at the same pace. Miles: That's the cardiovascular piece, and according to National Geographic's reporting on this, heat training also improves how early and efficiently you sweat, right? Amara: Yes, and the numbers from a 2025 Frontiers in Physiology study on... The untrained runners are where it gets really interesting. Four weeks of heat acclimation produced a 21% increase in sweat rate and a 4% increase in plasma volume. Miles: 21%? That's not a rounding error. Amara: Not even close, but the number that made me stop and reread the study was the erythropoietin spike, 13%. Miles: Okay, for people who haven't heard that word from a while, what is erythropoietin? Amara: So erythropoietin, or EPO, is a hormone your kidneys produce that tells your bone marrow to make more red blood cells. More red blood cells means more oxygen delivery to working muscle. Miles: Which is exactly what altitude training is supposed to do. Amara: Exactly. Heat stress creates a similar signal. Your kidneys sense that the blood is getting diluted and a little... Little oxygen deprived, and they respond by pumping out more erythropoietin. You're getting a fraction of the altitude effect without leaving sea level. Miles: Hmm, and this carries over to performance in cool conditions too. That part surprises me every time I bring it up to people. Amara: That's the piece most people miss. The Journal of Applied Physiology published a study where 10 days of heat acclimation raised VO2max by 5% in cool conditions and 8% in hot ones. Miles: Wow, time-trial performance improved by 6-8%, so you suffer through summer to get faster in fall. That is actually a compelling pitch. Amara: I mean when you put it that way. But yeah, the No Limits Endurance Team talks about this specifically. The adaptations don't disappear when the temperature drops. Your expanded plasma volume, your improved sweat response, that carries forward. Miles: So the June misery pays October dividends. Amara: Basically, you're building a better cardiovascular engine, not just toughening up mentally. Miles: Which raises the obvious question- How do you actually get those adaptations without ending up on the side of the road? Amara: Right, because there's a wrong way. Miles: There is very much a wrong way, and that's exactly where the protocol question gets interesting because most people just wing it and call it heat training. Amara: Speaking from experience? Miles: 10 a.m., 83 degrees, no plan, I think we've established that. Amara: The research has a lot to say about structure here, and some of it runs counter to what most coaches actually tell their athletes. Miles: So the big question now is how you actually structure this without turning yourself into a heat casualty. Amara: Which one of us nearly did a few weeks ago. Miles: No comment. But here's what a real two-week block looks like for someone working a nine-to-five. You're not going out at noon. That's not the plan. Amara: Good. Please don't. Miles: You pick your morning window, right? Maybe 6 or 7 a.m. when it's already warm but not brutal. First three sessions, you back off. Lower intensity, shorter duration than normal. Perceived effort is your guide, not pace, not power. Amara: That's the part people skip. They want to hit their usual numbers and then wonder why they feel demolished by day four. Miles: Right, right, right. And the point is, the heat is the stimulus. You don't need to also crush a tempo run. The body's already working hard just keeping core temp in check. Amara: So how long until something actually starts happening? Miles: Sooner than most people expect. According to Science Review, some adaptations kick in within several days, Amara: Wow. Miles: but you need that 8 to 14 day window to really lock in the plasma volume gains and the cardiovascular stuff we talked about. Amara: Eight to 14 days of consistent exposure. That's the medium-term sweet spot the research keeps landing on. Miles: And the timeline shifts depending on whether you're doing active acclimation, which is literally just running outside in summer. or Passive Post-Exercise Exposure, meaning Sauna after a normal training session. Amara: Okay, this is where I want to push back a little. Miles: Go ahead. Amara: Outdoor running is convenient, and yeah, it works, but Sauna after training is more controllable. You know the temperature, you know the duration, you can actually dose it. Body Science Review looked at a 2025 meta-analysis on post-exercise passive heat exposure. And the evidence is solid, not perfect, but solid. Miles: I hear you. My argument is that the Sauna is the barrier. Not everyone has access. If you're training in July and it's already 85 degrees out, you're getting your heat dose. Amara: Fair, and both approaches work, but if you do have access, sauna after training gives you something outdoor running can't. You finish your session cool, you recover, then you add the heat stress intentionally. Finally, your body handles both signals better. Miles: That's actually a good point. The training quality stays higher. Amara: Exactly. Fifteen to 30 minutes right after the workout while core temp is still elevated, you're stacking stimuli. Miles: And you can track progress with it too. Your heart rate in the sauna should drift down session by session as you adapt. Amara: Yes, that heart rate drop is the adaptation showing up in real time. Same heat, easier response. That's your body changing. Miles: So for the nine to fiver doing this in summer, the practical version is... And Amara: Outdoor run outside Monday, Wednesday, Friday. Back off intensity weeks one and two. If you have a gym with a sauna, add 20 minutes after the easy sessions. Miles: if you don't? Amara: The outdoor heat counts. Just keep week one lower key than you think you need it to be. Your Perceived effort is telling you something. Miles: I've seen athletes trash week two because they felt great on day five and pushed... push too hard. Day five feels deceptively good. Amara: The Miles: Day eight tells the real story. Amara: research is pretty clear on that. The adaptations are building even when the session feels manageable. Miles: And here's where most people immediately go wrong once the sessions are in place. They nail the heat exposure and completely forget about what they're losing while they're sweating through it. Amara: Fluids. Miles: Fluids. Specifically, it's not just about drinking water. And that's a much bigger conversation than most people realize. So the sweat losses are happening whether you track them or not, and here's where most people actually blow it. Amara: The research is clear on this: plain water during a hard heat session is not enough. You're losing sodium with every drop of sweat, and if you just pour water back in without replacing it, you're diluting your blood sodium concentration. Miles: Right, and that has a name. Amara: Hyponatremia: exercise-associated hyponatremia and look, most cases are mild, symptoms feel like bonking, nausea, a foggy head, but people assume they're just dehydrated so they drink more water and it gets worse. Miles: I've coached athletes who came off a long run looking wrecked, convinced they hadn't drunk enough; they'd actually over hydrated. Amara: Classic and sodium is the electrolyte that drives fluid into your cells. and keeps plasma volume stable. Without it, the water you're drinking isn't really staying where you need it. Miles: So how much fluid loss are we actually talking about before performance tanks? Amara: ACSM puts it at 2% of body weight. For a 150 pound person, that's roughly three pounds of fluid before you start seeing real degradation. Miles: Three pounds. You can hit that in under an hour on a hot day. Amara: Easily. Miles: So my practical rule when I'm working with athletes is to put my fluid in a Speaker 3: tank. Miles: Athletes Before a Hot Morning Session Eat something salty the night before; have a small sodium source with your pre run drink (I stopped recommending plain water for summer runs years ago). Amara: That tracks-and here's the part that surprises people: according to research on heat acclimation, your body actually gets better at conserving sodium as it adapts over a two week training block, so your electrolyte needs shift across the block. Miles: Wait, so week two you need less sodium than week one? In week one? Amara: You're losing less sodium per liter of sweat; yeah, the kidneys get more efficient, but total sweat volume goes up because you're sweating earlier and more freely. Miles: So the math gets complicated. Amara: Right; the short answer: don't ditch the electrolytes just because you feel more adapted; your sweat rate is higher, so total losses are still significant. Miles: So the practical takeaway is electrolytes the whole block. Block, not just the first few sessions. Amara: Exactly. Sodium during, and honestly, food is your best recovery tool. The salt in a real meal postrun is doing a lot of work. Miles: So the bro science avoid sodium thing is genuinely backwards here. Amara: For endurance athletes training in heat, one hundred percent backwards. Sodium is not the enemy. Miles: And if there's one thing you do before a hot session, it's not a fancy supplement. And it's just making sure sodium is in the picture. Amara: That's the whole message. Water is good. Sodium-inclusive hydration during heat training is better. Miles: All right, so we've covered the heat stimulus, the protocol, and what you're... drinking. The last piece is how all of this fits into everything else you're already doing because heat training adds load and recovery has to account for that. So here's where it gets practical. Heat adds real physiological load on top of your training load. I've seen this play out a hundred times. Athletes stack a heat block right on top of peak volume and then wonder why they're wrecked two weeks in. Amara: Right. Recovery has to scale with it. Sleep quality, easy day intensity, caloric intake, all of it needs adjusting, not just the hard sessions. Miles: And the risk side matters too. Anyone coming back from illness or new to structured training, those people need a much slower ramp. Heat illness isn't a maybe, it's a when if you ignore the ramp. Amara: Right. The body hasn't built the machinery yet to handle that stress. Miles: So what does this change for you on Monday? Honestly, if you're in a high volume week, don't bolt a heat block on top. Wait for a lower volume week, then add it. Amara: Okay, but here's the part I love, especially for anyone listening in January and May. In Minnesota, No Limits Endurance makes this point clearly. Fifteen to thirty minutes in a sauna after your regular workout can sustain heat acclimation adaptations all Miles: So, Amara: year. Miles: your cold climate people, you're not off the hook. Amara: You really aren't, and we talked about sauna in our last episode, so this actually connects. The sauna isn't just recovery. Used right, it's a year-round heat stimulus. without touching your training quality. Speaker 4: A misery is optional, the adaptation is not. Amara: That might be the most honest thing you've said all episode. Miles: All right, that's a wrap on heat training. Amara: And honestly, my favorite episode in a while. Miles: I mean, we opened with me pricing out treadmills at mile four, so the bar was set early. Amara: It was, but the reframe is what I keep coming back to. That gap between what you expect your body to do in the heat. Miles: That's the signal, not the failure. Amara: Mm Miles: Exactly. Amara: -Hmm. Miles: Amara: The big takeaway is heat stress is a training tool. Your body adapts when you give it a real stimulus. Miles: And the sauna angle means you don't even need a brutal August to make it work. Amara: If this episode shifted how you think about summer runs, share it with someone who needs the nudge. Miles: Drop us a review, tag us at Full Spectrum Fit, and new episodes land every Tuesday. Amara: Thanks for listening, everybody. Miles: We'll see you next week.