A Deep Dive on Heat Training for Runners

Badwater 135

Running in the heat is no fun. You already know that. Runs that feel twice as hard at half the pace, and leave you wondering if it gets better, and what you'll do if your race is this hot.

If you don't have a hot race on the calendar, you mostly don't need to do anything deliberate about the heat, just let your body adapt — and there's a decent chance it's worth the misery.

But if you've got a hot race coming, heat is one of the few training levers that can truly work fast, and it's worth doing right!

Why Heat Hurts

When you run in the heat, your body has two priorities at once: power your muscles, and shed heat before you end up medium rare in your own skin. Blood that would otherwise be delivering oxygen to your legs gets routed to your skin to dump heat. Your core temperature climbs anyway. Your heart rate drifts up at a pace that felt easy an hour ago. The same effort results in less output, because of physics, not weakness!

Slowing down in the heat is not a mindset problem. You can be tough and well-trained and it doesn't matter — the physiological ceiling is real, and it shows up really early.

Performance starts to suffer around 59°F, well before it actually feels hot.

So the single most important thing you can do on a hot day is also the simplest: run by effort, not pace. If you're going by feel, the adjustment happens automatically, without thinking. You back off, your body stays at a sustainable effort, and you stop fighting a fight you'll never win. If you even think about pace on a hot day, you're holding yourself back.

Does that make it feel less hot? Of course not. But it does help your body tolerate the heat better, and stop the rising stress and frustration from tracking that slowing pace.

But Might Be Worth the Misery

Those brutal summer runs may be doing more for you than just making you slower, but the real answer, as usual, is that it's complicated.

Train in the heat consistently and your body adapts. The first changes are about cooling — more blood plasma, an earlier and bigger sweat response, a lower heart rate at the same effort. Those are real and they show up pretty quick. Then there's a second, slower mechanism that has seen some hype. In some studies, weeks of consistent heat exposure raised hemoglobin mass — your oxygen-carrying protein.

But real talk. That blood-side change reliably helps you in the heat. Whether it does anything for you in cool conditions is a genuine open question. Some studies show a benefit. Others raised hemoglobin mass but found no performance improvement in cool conditions at all. So if you're banking on heat training as a cheap altitude camp for a fall race in great weather, the jury is still out on whether you're actually going to get adaptations that cross over.

That doesn't mean "it's doing nothing." But if you don't have a hot weather race coming up, you really don't need to target any of this. It's extremely marginal gains that come with real caveats we'll discuss later.

If you're already grinding through a hot summer because you have no choice, this is permission to stop worrying about it. First because you're not losing fitness, just taking a temporary output (and maybe a sanity) hit. But really, you're getting those hot-weather adaptations for free, and there's at least a chance of a little more.

Heat Training for Runners

Alright you've got a steamy race coming up. Or maybe 'it's just a dry heat.' Doesn't matter. Point is, you need to be able to execute your race plan on a day that's likely to be hotter than your normal training.

Here's how to prepare. We won't get into cooling methods on race-day – that is a whole other article!

First, there are two main mechanisms related to heat adaptation. I'm putting blood plasma in the Heat Tolerance category because the timelines match.

Heat Tolerance

These are the cooling adaptations: lower core temperature, increased blood plasma volume, earlier sweat response, lower heart rate in the heat. This is really what will keep you from falling apart on a hot day. Adaptations happen pretty quickly, in anywhere from a few days to a couple weeks, BUT, they fade just as fast.

Blood Adaptations

Hemoglobin changes happen much more slowly. They need weeks of consistent heat exposure, and these are the ones with that contested cool-weather payoff. But for a hot race it stacks on top of heat tolerance, and it's worth having if you can!

All the recommendations that follow are just different routes to one or both of these.

Passive Heating

Passive heating happens after your runs — time when you would be updating your Strava, posting a photo or doing your mobility work. (Right?!) Passive heating is great because it has fewer costs in terms of slowing down your runs or increasing recovery needs.

Here's what we're chasing regardless of method: raise your core temperature meaningfully and stay in the heat as long as you can reasonably tolerate. We'll talk dose and timing later.

Hot Water Immersion

This one is first because lots of people have a bathtub, and they can start tonight. It's also one of the best-studied methods in actual runners, not an extrapolation from lab cyclists.

The original protocol comes from Zurawlew and colleagues: an easy run in normal conditions, then straight into a hot bath, around 40°C (104°F), for up to 40 minutes, repeated six days in a row. The run pre-warms you so the bath finishes the job instead of starting from normal body temp, which is why "immediately after" matters. When it's genuinely too hard to bear, get out!

This will get you lower resting and exercising core temperature and better running performance in the heat, which are sticky for around two weeks after. Worth knowing: in those studies the short six-day block produced the cooling adaptations but not the hemoglobin ones. That's probably a dose thing, not a bathtub thing — more on that in timing. But for most people heading into a hot race, the cooling side is the main goal anyway.

No real equipment needed here — just hot water and a tub. The run should be one of your easy runs you were doing anyway, so the added training cost is close to zero. Evenings are slightly better, since your core temperature is already higher later in the day, but it's not worth flipping your schedule around for if you're a morning runner.

If you have access to a hot tub, that's can be better: a bath cools the moment you drop your heat-sink of a body into it, so a hot tub held at a steady 104°F gets you to the temperature that triggers adaptation faster, and keeps you there. Circulation also prevents your body from cooling the water immediately next to your skin.

Some athletes will track heart rate (David Roche used 100bpm as his target prior to winning Javelina 100mi) but you probably don't need to get quite that specific, and your number would differ.

Sauna Post-Run

If you have free and convenient access to a sauna, it's useful — you can generally tolerate longer sessions right away in the dry heat of a sauna, and you may be able to multi-task. Post-run strength in the sauna anyone?

The protocol is the same: 20 to 30 minutes in a dry sauna at 160 to 180°F sometime after your run. If you postpone rehydration and cooling a little, there are some anecdotal reasons to think it might improve results. In that case, rehydrate gradually and take a hot shower instead of a cold one. But if time is tight, or you're not sure, don't force those diminishing returns — the priority is still recovery from your session.

Active Heating: Ways to Suffer for the Cause

Active heat means you bring the heat to your run instead of a tub. However, active heat has real costs that the passive methods don't, so I urge caution.

A hot easy run is not free the way a post-run bath is. It taxes the run itself, and your recovery from both that run and your harder efforts around it. You'll need to dial back other runs and workouts because this will stress your body like a workout.

When possible, it's important to keep your real quality work in cool conditions where you can be economical, and let your easy running be the part that carries the heat.

Overdressing

This is the easy, but not-so-fun option. No sauna, tub or hot climate, and you can do it on tomorrow's easy run. Just add extra layers of dark clothing on a low-intensity effort on a high-sun day. You're building a microclimate — a heat chamber around your own skin. Waterproof layers may do a little more for their weight if you have them.

If you choose to do this, don't wimp out. It has to be genuinely, uncomfortably hot or it's not a real stimulus. If you're not clearly overheating and sweating hard at the easiest of efforts, it isn't working for you.

Run When It's Hottest

This one isn't accessible to everyone, and may be limited to runners working from home, the self-employed, or those whose off hours already align. But it's simple: just run during the hottest hours of the day, in direct sun if possible. For most areas of the country that means between the hours of 2pm and 4pm.

Dose, Timing and Decay

Alright now the nitty gritty, and this stuff matters. The different physiological changes from heat training adapt and decay at different rates, and that affects how and when you do them.

Blood Adapts Slowly

If you want the deeper hemoglobin adaptations, you need around five weeks of consistent exposure. The protocol matters less, but should be something you can do consistently. The original primary study was on cyclists, and studied only active heat training sessions. However a recent study by Jenkins et al. (2025) showed that five weeks of 5x45min hot water immersion sessions also increased hemoglobin as well as VO2max. It's important to note that this was a small study of just n=10, but well-designed, and a former gap in the literature.

Heat Tolerance and Plasma Volume Adapt Quickly

The related heat tolerance factors, as well as blood plasma volume, all adapt quickly, sometimes in as little as four sessions.

However, they also decay quickly, meaning you lose those adaptations nearly as fast as you gained them (some research shows about a day's loss of adaptation for every two days without heat stimulus.)

The good news is that those four sessions of heat exposure gets you the basic survival adaptations, enough to not blow apart. The following week or two is what turns survival into actual performance.

So even if you only have a few days before a hot race, a short block is still worth doing, but if you have more time, use it!

Taper Constraints

Finishing a heat block within a few days of the race runs straight into normal taper logic, which says shed accumulated fatigue starting around two weeks out. The active methods are the biggest problem, since an overdressed run is exactly the kind of stress you're supposed to be cutting in race week. Your race-week sessions should all be passive—at least if you have any choice in the matter!

Two Positives

Decay is real but it isn't the whole story. The fitter you are, the longer you'll maintain the adaptations. It's not a lot longer, but every bit helps! Best of all, re-acclimation is several times faster than the first time around, which means maintenance is easier. Practically: the blood-side gains have held on with as little as three heat sessions a week, and heat tolerance can be propped up with roughly 20 to 40 minutes in a sauna or hot tub every other day.

Dehydrated Training

You'll run into people deliberately restricting fluid during heat sessions, the idea being that mild dehydration adds an extra push to the plasma-volume response on top of the heat itself. There's some data behind this, but the risks are real. A few small studies found fluid restriction during short heat blocks raised plasma volume a bit more than staying hydrated. But the evidence is thin and split, the studies are small, and experienced practitioners who do this for a living mostly don't use it.

The reason is recovery: training dehydrated on purpose blunts how well you bounce back, and that cost lands hardest on exactly the people most drawn to optimizing, and on masters athletes, who already regulate temperature and thirst less well. For a small handful of elites deep in a hot-race build it might be worth the tradeoff. For almost everyone reading this, it isn't.

In general, I'd drink to thirst during any active training session and during most passive heat training sessions too. Any potential increased adaptations are limited enough that it probably isn't worth the added stress.

Keep It Safe

Hopefully it's obvious, but this isn't medical advice. If you've got cardiovascular history or anything that makes deliberately spiking your core temperature a bad idea, please talk to a doctor before you start.

Heat training works by pushing core temperature into a stress zone on purpose. The way most people get hurt isn't heat stroke – it's doing too much, too soon. Tons of heat sessions, accumulating stress and recovering poorly. Ramp up slowly. Treat it like any other hard stress, because it is one!

Two specific groups get some additional notes:

Masters athletes don't regulate core temperature as well, and often have a lower sweat rate, lower efficiency, and sometimes a blunted thirst drive, which together mean higher dehydration risk at the same effort. Masters athletes should start even slower and allow a little more time for adaptations to accumulate.

Female athletes typically adapt more slowly, get less out of short protocols, and sweat later in the session. I'll note though: the research on female response is thinner than it should be. My advice is female athletes should prioritize the longer adaptation time frames if possible, and not judge their progress on the typical timeline.

The most important factor to managing risk is just like managing overall risk while training, and that's modulating your stress, while allowing time for recovery. Don't put active heat sessions next to your hardest workouts, since the hot easy runs are nearly as stressful as workouts. Don't use active methods deep into your taper expecting to also show up fresh. Don't do long passive heat sessions after hard workouts, or in the few days right before a race.

Finally, individual variation here is real, and the science is inconclusive in several areas: how often you actually need exposure, how the adaptations decay over longer stretches, how much of the female-response picture we're missing. Like all training science, this is a starting point.

One Protocol To Start Them All

Recommending a specific protocol requires knowing a lot about you and your training, so consider this a starting point.

I recommend 4 sessions/week of passive post-exercise heat for 20-40 min per session. Start six weeks out from your race, and maintain until 3 days prior to your race. Sauna, hot bath or hot tub — it's your call. Pick what's convenient or vary them throughout the week. Be sure to ease into the session duration, and consider starting on a down/cutback week for added safety. If you have less than six weeks, just use whatever you can!

The Bottom Line

Heat is coming whether you plan for it or not. If you don't have a hot race on the calendar, you don't need a protocol. Run by effort with effective training, and let any adaptation be a bonus.

If you do have a hot race, now you've got real tools and enough time to use them well. Respect that the heat is doing real work on your body, for you and against you, the whole time.

Either way, the heat isn't wasted. Run smart in it and it'll give you something back.