Heat Acclimation Training

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I've done quite a bit of Running in the Heat, from North Carolina's brutal summers, to winning the 2010 Keys 100 or pacing Chris Moon at the Badwater 135.

Heat acclimation training improves performance in hot conditions, may improve performance in cool conditions, and protects against heat injury. It is also genuinely dangerous. People have died doing what this page describes, and the equipment that makes heat training practical at home is the same equipment that killed them. Please read the safety section before anything else.

Summary

  • Heat training has two quite different goals. Preparing for a hot race takes 1–2 weeks. Using heat to improve performance in cool conditions takes at least 5 weeks and the evidence for it is weaker.
  • The core of every protocol is the same. Raise core temperature to about 38.5c/101.3f over roughly 30 minutes, hold it for 60–90 minutes, and repeat daily. Never exceed 39.0c/102.2f.
  • You probably cannot measure your core temperature. Ear, forehead, oral and armpit thermometers are not merely inaccurate during exercise, they read low, which is the dangerous direction. Plan your safety around time limits and symptoms, not numbers.
  • The danger is real and cannot be eliminated. A heat suit or heat chamber works by preventing your body from cooling itself, so if you get into trouble you keep getting hotter after you stop.
  • Never use this to lose weight. Sweat-based weight cutting is what has actually killed athletes.

Danger of Death and Serious Injury

Heat training can kill you. This is not a disclaimer to limit my liability; it is the single most important thing on this page. A dedicated athlete can push themselves hard enough to raise core temperature to fatal levels.[1] The techniques below deliberately disable your ability to lose heat. Start far more gently than you think necessary, build up over months rather than weeks, and stop at the first sign of trouble.

What has actually happened

In November and December 1997, three previously healthy collegiate wrestlers died in three different states. Each was trying to lose weight rapidly by restricting food and fluid and maximising sweat loss, wearing vapour-impermeable suits under warm-up suits while exercising vigorously in hot environments. One had a rectal temperature of 42c/108f at the time of death, with hyperthermia given as the cause; another showed rhabdomyolysis and myoglobinuria.[2] The NCAA subsequently banned vapour-impermeable suits, saunas, steam rooms and hot rooms above 26c/79f for weight management.[2]

Their intent was different from yours and their fluid restriction made things far worse. The physics did not care. If you take one thing from this page, take this: never combine heat training with deliberate dehydration or weight cutting.

What can go wrong

  • Exertional heat stroke. Potentially fatal, and the main risk. See below for recognition and first aid.
  • Collapse while enclosed. In a heat suit you continue to gain heat after you stop or collapse, because the suit is still working. In a heat chamber you are inside the heat source. This is the risk that most distinguishes home heat training from simply running on a hot day.
  • Kidney injury. Muscle-damaging exercise before running in the heat produced greater inflammation and kidney stress, with half of the participants in one small study meeting criteria for acute kidney injury.[3] Do not schedule heat sessions on legs already damaged by a hard long run, a downhill session or a race.
  • Gut damage. The rise in core temperature during exercise correlates strongly with increased intestinal permeability, and a core temperature above 39c/102.2f was associated with increased permeability in every study reviewed.[4] This is a direct, independent argument for the 39.0c ceiling.
  • Rhabdomyolysis, which featured in the wrestler deaths above.[2]

Who is at higher risk

Cancel the session, rather than modifying it, if any of these apply. Recognised risk factors for exertional heat illness include sleep loss, dehydration, fever or current illness, poor conditioning, inadequate heat acclimatisation, high intensity of exercise, certain medications including antihistamines, diuretics and antihypertensives, and a reluctance to report problems or report feeling unwell.[1]

That last one deserves emphasis, because it is the one that applies to committed athletes. The mindset that gets you through a hard interval session is the mindset that gets you hurt in a heat chamber.

Reducing the risk

The risk can be reduced substantially. It cannot be eliminated, because the entire method depends on deliberately impairing heat loss.

  • Have someone who knows what you are doing. Someone who knows you are in the chamber or suit, roughly when you expect to finish, and who will check on you. This is the single most effective mitigation against the collapse scenario, because an unconscious person in a sealed suit does not cool down.
  • Keep the exit free. A chamber door that opens from the inside, nothing blocking it, and a phone within reach.
  • Cap the session by time as well as temperature. If you cannot measure core temperature reliably, and most people cannot, time is your primary control.
  • Build up over months. Start with short, low-intensity sessions in mild heat. Add duration before you add heat.
  • Never do interval or high-intensity work in the heat. Intense work can raise core temperature faster than you can detect or reverse it.
  • Prepare cooling before you start, not while you need it. See first aid below.
  • Do not restrict fluid. Drink to thirst and include sodium, as described under hydration below.
  • Never use heat training to lose weight. It does not work, and it is dangerous. The weight you lose in a heat session is water, not fat; it comes back within a day or two as soon as you rehydrate. Meanwhile, deliberately not replacing that fluid compounds every risk on this page, and dehydration is one of the recognised risk factors for heat illness.[1] Heat training should be done fully hydrated, with the aim of adaptation rather than any change on the scales.
  • Stop on symptoms, not on the clock.

Symptoms: when to stop

Stop, get out of the heat and start cooling if you notice any of the following. You do not need to be certain.

  • Confusion, disorientation, odd behaviour or difficulty with simple tasks. This is the most serious sign. Central nervous system disturbance is the defining feature of exertional heat stroke.[5][6]
  • Dizziness or loss of coordination.
  • Nausea or vomiting.
  • Headache. Having had headaches from heat, dehydration and low blood sugar, I find the heat headache is distinctly more intense.
  • Unusual muscular weakness.
  • Burning cheeks that the air temperature does not explain. Blood flow to the skin, and the face in particular, increases substantially to dissipate heat,[7] so a hot, flushed face during exercise is entirely normal and is not in itself a warning. What I have found useful is the mismatch: cheeks that feel like they are burning when the air around me does not justify it. I am not aware of any research establishing this as a recognised early sign, and facial flushing has too many other causes to be relied upon, so treat this as my personal experience rather than evidence.
  • Panting. My personal experience is that when my core temperature rises I start breathing much faster and deeper than the exercise intensity would explain, and this is my earliest reliable warning. This is a personal observation and I am not aware of research establishing it as a recognised early sign.

A practical problem with the confusion symptom is that the person least able to notice it is the person experiencing it. This is why having someone check on you matters more than any thermometer.

Recognising heat stroke

Exertional heat stroke is defined by central nervous system disturbance together with hyperthermia, usually above 40c/104f rectal.[5] A core temperature of 40c on its own is not sufficient to make the diagnosis,[5] and UK pre-hospital consensus similarly treats CNS dysfunction as the key diagnostic criterion, because the temperature at which individuals develop symptoms differs.[6]

However, do not wait for confusion to appear before acting. Treatment should not be delayed because a rectal temperature cannot be obtained, or because the reading is somewhat below the diagnostic threshold.[6] If your temperature is excessive, cool down. Cooling someone who turns out to be merely hot costs nothing. Delaying cooling in a developing heat stroke is what causes deaths and lasting organ damage.

First aid

Cool first, diagnose second.

  • Cold water immersion is the treatment of choice. Whole-body cold water immersion produced a 100% survival rate in 274 runners diagnosed with exertional heat stroke over 18 years at one road race, cooling at an average of 0.22c per minute.[8]
  • Colder is better, and immerse the trunk and limbs. Cooling is more effective when water is at or below 10c, with torso and limbs immersed; there is not enough evidence to support immersing only the forearms and hands.[9]
  • Speed is what matters. Consensus for Tokyo 2020 was whole-body cooling to 39c within the first 30 minutes of collapse as the critical requirement for a good outcome.[10]
  • If you have no tub, tarp-assisted cooling with oscillation, where the person lies on a tarpaulin holding water that is rocked around them, achieved adequate cooling rates and is a workable alternative.[11]
  • Otherwise, move to shade, strip, spray with water and fan continuously and aggressively. Cooling can stop at 37.5c or when symptoms fully resolve.[6]
  • Call emergency services, and cool while waiting rather than waiting to cool.

In practice this means a cold bath, or a dustbin or builder's trug filled with cold water, prepared before the session rather than improvised during one. In the UK, tap water in winter will do; in summer you will want ice.

Measuring Core Temperature

This section exists because the honest answer is that most people cannot measure their core temperature usefully, and the advice on this page is designed around that fact.

Methods to avoid

Oral, armpit, ear (tympanic or aural), temporal and forehead measurements did not accurately reflect core temperature in athletes exercising in the heat outdoors,[12] and the same held during controlled indoor laboratory exercise.[13] Aural thermometry specifically is not accurate for diagnosing exertional heat stroke and should not be used as a substitute for rectal measurement.[14]

These devices are not merely imprecise. They generally read lower than true core temperature, which is the dangerous direction: they will reassure you at exactly the moment you need warning. An earlier version of this page recommended an in-ear infrared thermometer. That was wrong, and I have removed it.

Ingestible pills

Telemetric pills are valid and are what most research uses, but they are impractical for most people: the pill must be swallowed some hours in advance, readings can be affected by food and cold fluid, and the cost per session is high.

Rectal measurement

Rectal temperature is the practical reference standard, and is genuinely useful for occasional spot checks. Two practical problems are worth understanding.

Insertion depth changes the reading. Depth affects measurement bias, and 15cm from the anal sphincter is the most valid depth.[15] At rest, a reading at 4cm was about 0.5c lower than at 16cm; the shallow reading responds fastest and rises most, while deeper readings are more stable, and depths beyond 10cm show no systematic differences between them.[16] A cheap digital thermometer inserted a short way gives both a low reading and a variable one. If your readings are not repeatable, inconsistent depth is the most likely explanation, and it is fixable.

The thermometer may malfunction in a hot chamber. Rectal temperature itself is largely independent of ambient temperature, so the measurement is sound in principle. The device is the problem. I have found that when chamber air is close to body temperature, some thermometers terminate the reading before insertion and report the room temperature instead. With fans running, the noise masks the completion beep, so the failure is easy to miss. The body of the thermometer also becomes very hot, which these devices are not designed for. The only solution I found was to leave the chamber and take the reading outside.

A further limitation of spot checks is that they miss what happens afterwards. Core temperature can continue to rise after you stop exercising.

Wearable estimates: the CORE sensor

The CORE sensor estimates core temperature from skin temperature, heat flux and heart rate. I used one for a long time and initially found it agreed with an internal thermometer to within half a degree Celsius. After a few months of training, and once I moved into a heat chamber, it underestimated my temperature by 1.0–1.5c, which is far too much to be useful.

That experience matches the published validation work:

  • Against a rectal probe during cycling, roughly half of all paired measurements differed by more than the 0.3c threshold the authors set for validity, and the authors concluded the results did not support the manufacturer's claim of validity. They specifically cautioned that the device underestimates elevated core temperatures above 39.5c.[17] This study was funded by the European Union and the Slovenian Research Agency, and states that greenTEG had no role in it.
  • In elite field hockey players during heat training, agreement with an ingested pill was poor in all sessions, and the underestimation grew larger as core temperature rose.[18]
  • During a 5km running time trial at 30c, the mean absolute difference from a gastrointestinal pill was 0.47c, and the device did not capture the cooling effect of cold water ingestion, missing a 0.26c decline that the pill recorded.[19]

The pattern is consistent and it is the worst possible one: the device is least accurate when you are hottest.

I still think it has value as a trend tool. The Heat Strain Index, which combines skin and core temperature, I found more useful than the core temperature estimate itself: a hard run in cool weather takes my core temperature to 38.4c with no noticeable heat stress, while 38c with high skin temperature in the chamber is extremely unpleasant. My heart rate to power ratio works as a rough proxy for accumulating heat strain, with the ratio dropping as heat stress builds. But I would not use any of these as a safety limit.

The manufacturer's site at corebodytemp.com publishes heat training protocols which some people may find a useful starting point. Bear in mind that it is a commercial source with an interest in the device, and treat its protocol recommendations accordingly.

Is It Worth The Risk?

Having read the section above, the reasonable next question is whether any of this is worth doing. The honest answer depends entirely on which of the two goals you have, because the risk is similar in both cases and the benefit is not.

If you are racing in the heat: probably yes

Here the benefit is substantial and the risk calculation is unusual, because heat training reduces a risk you are going to face anyway.

If you line up for a hot race unacclimatised, you will be exposed to heat stress regardless, you will be exposed to it while racing hard, and you will be exposed to it without the adaptations that protect you. Acclimatisation produces lower core temperature and heart rate at a given intensity, earlier and more profuse sweating, and expanded plasma volume,[20][21] all of which make heat illness less likely during the race itself. Inadequate heat acclimatisation is itself a recognised risk factor for exertional heat illness.[1]

The performance case is also strong and not seriously disputed: two weeks of acclimatisation allowed trained cyclists to complete a long time trial in the heat in a similar time to cool conditions, whereas unacclimatised they suffered a marked drop in power.[22]

So you are choosing between controlled, progressive heat exposure where you decide the dose and can stop at any moment, and uncontrolled heat exposure in a race where you cannot. The risk does not disappear, but it is far better managed. For most people preparing for a hot race, this is a sensible trade.

If you are chasing cool-weather performance: much less clear

This is a marginal gain, and it should be treated as one.

The attraction is real: heat training offers some of what altitude training offers, at a fraction of the cost and inconvenience. You do not need to travel, take time off, or rent a house at 2,000m. Long heat training blocks reliably raise haemoglobin mass by roughly 2–4%.[23]

But look carefully at what that buys. Across the studies, haemoglobin mass goes up consistently while performance improvements are smaller, less consistent, and frequently not distinguishable from a control group.[24][25] The evidence that heat acclimation improves performance in cool conditions is contested, with the studies that included proper control groups mostly failing to replicate the positive result.[26][27][28] And it is a five-week commitment of five sessions a week before you find out.[29]

Before taking this on, ask honestly whether you have exhausted the cheaper and safer improvements first. Consistency, total training volume, sleep, iron status, nutrition, strength work, and simply training more appropriately will almost always offer more than heat training will, at none of the risk. Heat training for cool-weather performance makes sense for an athlete who has genuinely worked through all of that and is looking for the last fraction of a percent.

That is your decision to make, not mine. But make it deliberately, with the safety section in mind, rather than drifting into it because heat training is currently fashionable.

Practical Heat Training

Skip the session today if...

Run through this list before every heat session. Any one of these means no heat training today. This is not a session to push through: the entire method depends on your body regulating its temperature well, and all of these impair exactly that.

  • You are dehydrated. Dark urine, a body mass well below your normal, or simply knowing you have not drunk enough.
  • You are ill or have a fever. Current illness and the presence of a fever are both recognised risk factors for exertional heat illness.[1] A fever means your body is already defending a raised temperature.
  • You have had vomiting or diarrhoea recently. See below, because this one needs more than a day.
  • You slept badly. Sleep loss is a recognised risk factor.[1]
  • You are taking medication that affects thermoregulation or hydration, including antihistamines, diuretics, antihypertensives and stimulants.[1]
  • You did a hard or damaging session recently, particularly downhill running, a race, or anything that has left your legs sore. Muscle-damaging exercise before exercise in the heat produced greater inflammation and kidney stress, with half the participants in one small study meeting criteria for acute kidney injury.[3]
  • You have had a previous episode of heat illness and have not been cleared to return.[1]
  • You have drunk alcohol recently, which worsens both hydration and blood pressure regulation in the heat.
  • You will be alone and uncontactable. See the safety section.
  • Something feels off and you cannot say why. This is a legitimate reason. The cost of skipping a session is close to zero.

After vomiting or diarrhoea

This deserves separate treatment because it is the risk most likely to be underestimated. Vomiting and diarrhoea cause fluid and electrolyte losses that can persist for days after you feel better, and heat training then adds a further one to three litres of sweat loss on top of a deficit you may not know you still have.

Guidance for athletes recovering from gastrointestinal infection is to resume training when you are afebrile, adequately rehydrated, able to tolerate solid food and free of residual gastrointestinal symptoms, or when at least 48 hours have passed since the last episode.[30]

I would treat that as the threshold for returning to ordinary training, not to heat training. My own rule, which is judgement rather than evidence, is to meet those criteria, then complete two or three normal training sessions comfortably, eating and drinking normally and with body mass back to baseline, before going back into the heat. In practice that usually means four to five days after the last episode. If you are at all unsure, wait longer; a few days of lost heat training costs very little, and the adaptations you already have decay slowly enough that you will not lose much.[31] This is not a theoretical risk; I have seen a runner's heart fail and need to be revived multiple times in a race. They had vomiting and diarrhoea, but recovered 2 days before the race, and obviously their electrolytes had not recovered.

Choosing a protocol

Decide which of the two goals you are pursuing, because they need different protocols.

Goal A: preparing for a hot race

This is the well-evidenced use of heat training.

  • Duration: 10–14 days of daily exposure.
  • Session: raise core temperature to about 38.5c/101.3f over roughly 30 minutes, then hold it for 60–90 minutes.
  • Method: the controlled hyperthermia or "isothermic" approach, where you adjust effort to hold the target temperature, rather than holding a fixed workload. Reaching and maintaining a target internal temperature is difficult with a fixed-workload protocol, which is precisely the problem the isothermic approach solves.[32]
  • Expect most of the benefit early. Most adaptations develop within the first week and more slowly over the following two weeks, and adaptation is up to twice as fast in highly trained athletes.[33] Athletes benefit from only a few days, but may need 6–10 days for near-complete cardiovascular and sweating adaptation, and around two weeks to optimise aerobic performance.[33]

Goal B: heat training for temperate performance

This is the newer and less certain use. See the science section for what the evidence does and does not show.

  • Duration: at least 5 weeks.
  • Session: 5 sessions per week of about 50 minutes, at low power output, in a heat chamber or wearing a heat suit in ordinary conditions.[25][29]
  • Maintenance: 3 sessions per week maintained the adaptation.[29]
  • Intensity: low. In the published heat suit protocol, rectal temperature averaged 38.9c in the first week, settling to 38.5c thereafter, with fluid losses of around 1.6–1.7 litres per session.[25]

Progression

  • Start with low intensity in mild heat, for a short period.
  • Add duration before you add temperature.
  • Expect to feel disproportionately tired. I found the first month or two profoundly tiring, and it took longer than that before I was not wiped out afterwards.
  • Reduce your other training to compensate. The heat makes sessions far more fatiguing than the power numbers suggest.
  • Consider alternating heat sessions with cooler sessions, so that you can still train hard somewhere.

Hydration and fuelling

  • Stay hydrated. Maintaining hydration during acclimation produced increased sweat rate, reduced skin temperature and improved time-trial performance in the heat, while an otherwise identical dehydrated protocol did not.[34] This study was funded by Aspetar Orthopaedic and Sports Medicine Hospital, a hospital rather than an industry sponsor.
  • Include sodium. I use about a quarter of a teaspoon of salt per litre. Sweat losses in a chamber session can be substantial, and plain water in that volume risks diluting blood sodium.
  • Drink to thirst. This is the general consensus for avoiding both dehydration and over-drinking. There is a tension here worth being aware of: the acclimation evidence favours maintaining euhydration,[34] and with the very high sweat rates of a chamber session thirst may lag behind losses. Weighing yourself before and after sessions will tell you whether thirst is keeping up for you.
  • Drink cool fluids, not ice cold. This is a personal hypothesis rather than an established recommendation. My reasoning is twofold. First, cold drinks genuinely lower core temperature, which reduces the thermal stimulus you are training for; one study recorded a 0.26c decline in core temperature after cold water ingestion.[19] Second, if the drink is pleasantly cooling, I find it encourages drinking for the cooling rather than for the fluid, which risks over-drinking. I am not aware of research testing either point in the context of heat acclimation specifically.

Iron

Heat training that increases haemoglobin mass requires iron to build the extra red cells. There is no heat-specific guidance that I am aware of, but by analogy with altitude training, where haematological adaptation depends on adequate iron stores and ferritin above about 50 ng/mL is recommended beforehand,[35] it is worth having iron status checked before a long heat block. Treat that as a reasonable extrapolation rather than established practice.

I eat black pudding regularly, aiming for 50–100% of my RDA from that source, because iron is hard to absorb from plant sources and supplements. I have had blood tests through heat training, and as expected my red blood cell concentration dropped as blood volume increased, then returned to normal. That transient dip has since been documented: haemoglobin mass fell from day 4 of heat training before returning to baseline by the end.[36]

Maintenance and decay

  • About 2.5% of the adaptations in heart rate and core temperature are lost for each day without heat exposure.[31]
  • Re-acclimation is much faster than decay, inducing adaptations in heart rate and core temperature 8 to 12 times faster than they are lost.[31] This makes short top-up sessions during a taper practical.
  • Training in the heat every five days maintained adaptation for at least a month.[37]

What not to do

  • No interval or high-intensity work in the heat. Core temperature can spike faster than you can respond.
  • No weight cutting. See the safety section.
  • Do not combine heat and altitude in the same session. Combining them does not appear to enhance temperate sea-level performance beyond heat or altitude alone.[38] Using them sequentially is a different question and looks more promising; see the science section.
  • Do not train in the heat when ill, feverish, sleep-deprived or dehydrated.[1]

Methods

Active heat training

Exercising in the heat, whether outdoors, in a chamber, or by overdressing. This produces the most complete adaptation and is the basis of most research protocols.

Passive heat

Sitting in heat after training, rather than exercising in it. This is far more practical for most people and the evidence has improved considerably:

  • Six days of hot water immersion at 40c for up to 40 minutes after a temperate run produced acclimation, including lower resting rectal temperature and lower end-exercise rectal temperature in both temperate and hot conditions.[39]
  • Post-run sauna roughly three times a week for three weeks improved VO2max and speed at 4 mmol/L lactate more than training alone.[40] It worked in both women and men.[41]
  • A recent meta-analysis concluded that passive heat acclimation is a practical and effective alternative to active heat acclimation.[21]

A hot bath is the most accessible version of this and requires no equipment at all. It is cheap and easy, but it is not risk free.

Passive heating carries its own hazard: you can pass out, and in a bath that means drowning. If you deliberately drive your core temperature up in a bath, you are combining hyperthermia with immersion and with a drop in blood pressure. Hypotensive syncope has been identified as a cause of sudden death by drowning during hot bathing, developing from roughly four minutes after immersion as sympathetic tone falls.[42] Animal work suggests hyperthermia itself contributes to sudden death during hot-water bathing.[43] In Japan, where whole-body immersion in water at 41–45c is the norm, bath-related deaths run to many thousands a year, concentrated in older adults.[43]

That risk falls overwhelmingly on elderly people rather than fit endurance athletes, and alcohol is a major contributor, so the absolute risk to you is low. But it is not zero, and the consequence is drowning in your own bathroom. Keep the water at a sensible temperature rather than as hot as you can stand, keep sessions short, tell someone you are doing it, and get out at the first sign of light-headedness rather than trying to finish the duration.

Saunas carry the same caveat with a better escape route, since you are not in water. In a review of sauna-related injuries, the most common causes were slips and falls, followed by dizziness and syncope, which accounted for about 30% of cases.[44] Fainting typically happens just after the sauna, as blood pressure falls.[45] Very few sudden deaths occur in saunas, and alcohol is the single biggest contributor to those that do.[45] Stand up slowly, do not drink alcohol anywhere near a session, and do not use a sauna alone if you are pushing your temperature deliberately high.

Mixed-method heat acclimation

The most practical approach for anyone with an existing training plan. Six international triathletes completed eight sessions over two weeks, five active and three passive: high-intensity outdoor sessions were followed by 30–60 minutes of hot water immersion, while low-intensity sessions were moved into a hot chamber. Exercising heart rate fell and sweat sodium concentration dropped at both measured sites, and this was achieved without modifying training load and with no apparent detrimental effect on health or training stress markers.[46] This study was funded by Hong Kong Baptist University.

This directly addresses the main practical complaint about heat training, which is that it displaces the training you were going to do anyway.

Heat suit

A heat suit creates a hot microclimate against your skin, allowing heat training in ordinary or even cold conditions, with no special facility needed. It also removes your ability to cool down, including after you stop, which makes it the most dangerous method described here. See Fellrnr Heat Suit for construction and use.

Heat chamber

A heat chamber is a heated enclosure, typically used with an indoor bike trainer. It gives the most control over conditions and is the most comfortable option for long sessions, at the cost of needing space, power and construction work. See Fellrnr Heat Chamber for how I built mine.

Heat Training as Low-Impact Cross-Training

Heat training seems to provide a strong cardiovascular stimulus with very little impact or mechanical stress, which makes it useful as cross-training and potentially valuable for maintaining fitness while injured. The evidence here is worth unpacking, because the obvious explanation turns out to be wrong.

The intuitive version is incorrect. It is tempting to assume that because heat raises heart rate at a given power, you get cardiovascular stress without stressing the muscle. In fact, at a matched work rate, heat increases metabolic stress in the muscle. Cycling at 40c compared with 20c produced lower post-exercise creatine phosphate, higher creatine and ammonia, increased muscle glycogenolysis and higher muscle lactate, with no difference in oxygen uptake.[47] Raising muscle temperature alone increased glycogenolysis, glycolysis and high-energy phosphate degradation.[48] Working the other way, blunting the rise in body temperature reduced muscle glycogen use.[49] So the muscle is not spared metabolically; it is working harder.

The heart rate inference is also only partly sound. During passive heat stress, cardiac output rises substantially with much of it going to the skin, and this rise is driven mainly by heart rate.[7] So at the low intensities used in heat training, a raised heart rate does reflect real extra cardiovascular work. But during intense exercise combined with heat stress or dehydration, cardiac output, stroke volume, blood pressure and blood flow to brain, skin and working muscle all fall.[7] At that point a rising heart rate signals a falling stroke volume, not a larger stimulus.

What does hold up is the mechanical argument. Heart rate at fixed blood lactate concentrations of 2, 3 and 4 mmol/L and at ventilatory thresholds was not significantly different between temperate and hot environments, despite significantly reduced power output in the heat.[50] In other words you reach the same internal intensity at meaningfully lower mechanical power. That is exactly the trade worth having if your limitation is impact or tissue load rather than cardiovascular capacity.

Muscle damage is not increased. After a football match at 43c compared with 21c, creatine kinase was elevated after both but the response in the heat was lower, myoglobin did not differ, and the authors concluded that heat stress does not aggravate the recovery response.[51]

There is direct evidence for the rehabilitation use. A review proposes incorporating repeated heat exposure into rehabilitation to minimise skeletal muscle and cardiovascular deconditioning, noting that passive exposures can help maintain cardiovascular fitness from the onset of injury provided they raise body temperature, circulation and sweating sufficiently.[52] A controlled trial applying heat therapy through immobilisation and rehabilitation reduced muscle atrophy and maintained plantarflexor strength.[53]

One caveat worth keeping in view: some of the benefit attributed to heat may simply be the exercise that accompanies it, a training effect rather than a heat effect.[54]

As a runner I like using a bike for heat training. Once my temperature is up, the effort needed is very low, often only 100 watts, which would be too slow to be a useful run. A good saddle and bib shorts matter more than you would expect for sessions of this length; I settled on an ISM PR 3.0 saddle, and bib shorts rather than waisted shorts to avoid waist compression and the GI problems that come with it.

Personal Thoughts

These are my own observations and hypotheses, clearly separated from the evidence above.

  • Adaptation continued far longer than the literature suggests. The science says acclimation is largely complete in a couple of weeks. I found a slower adaptation continuing for months, and my ability to train in the heat was much better at six months than at three. This is at least consistent with the finding that longer regimens produce greater adaptation.[55]
  • It disrupted my sleep for several months. I do heat training in the afternoon, and balancing recovery from the morning run against finishing early enough to sleep well was difficult.
  • Judging the right level of heat stress is hard, even after years of training through North Carolina summers.

Hypothesis: heat training teaches earlier recognition of overheating

My hypothesis is that heat training teaches an athlete to recognise the onset of overheating earlier, and that this reduces the performance cost of mild overheating. The failure mode I have in mind is not heat illness but something milder and much more common: going out too hard, overheating slightly, and then being forced to slow substantially. Easing off earlier and holding a more modest effort should produce a better overall result.

I looked for evidence, and this one has more support than I expected, though with a genuine complication.

The problem is real and measurable. In outdoor 43.4km time trials at about 37c, cyclists started the first 20% at a similar power output regardless of climate or acclimatisation state. Unacclimatised in the heat, they then suffered a marked drop in power. That drop was partly attenuated after six days of acclimatisation and further reduced after fourteen. Final rectal temperature was essentially identical across all three hot trials at about 40.2c.[22] That is precisely the pattern I describe: start at your usual effort, hit a thermal ceiling, and pay for it with a forced slowdown.

Experience alone, not just physiology, seems to help. Athletes doing a second time trial in the heat changed their pacing profile from positive to even, a change not seen in temperate conditions. The trials were deliberately separated by about 11 days to prevent physiological acclimation, and the authors concluded that altered perceptions of heat from familiarisation, rather than acclimatisation itself, may drive the initial change in pacing and performance. They recommend that athletes without time for full acclimatisation at least familiarise themselves with hot conditions.[56] This is close to direct support for my hypothesis.

But the proposed mechanism is doubtful. Improving thermal perception artificially, using a menthol spray that made participants feel cooler and more comfortable without changing their actual temperatures, did not alter pacing strategy or performance. The authors concluded that thermal perception is not a primary driver of early pacing in trained cyclists.[57] If conscious thermal perception is not what sets early pace, then "learning to notice you are overheating" may not be the route by which familiarisation helps.

My conclusion: the effect looks real, but my explanation for it may not be. Something about prior experience in the heat improves pacing, and it may be a largely subconscious recalibration rather than conscious recognition of warning signs. Either way the practical advice is the same, and it is worth stating: if you are racing in heat you are not used to, expect to need a lower starting effort than feels right, and get some exposure beforehand even if you have no time for a full acclimation block.

Recovery from Heat Training

If you set your heat sessions by heart rate, as most people do, a heat session is not simply a harder version of the same session done cool. To hold a given heart rate in the heat you have to produce less power, so you get less mechanical work and more thermal work. That changes what recovery is needed, in both directions.

In the one study that compared matched heart rates directly, 90 minutes at the same heart rate in 33c rather than 18c produced 17% lower power, 14% lower energy expenditure and 19% lower carbohydrate oxidation, with roughly double the sweat rate.[58] So the metabolic and muscular cost of a heat session is lower than the temperate equivalent, not higher, while the fluid cost is substantially higher. The familiar claim that heat increases carbohydrate use is correct, but only at matched power.[59]

The harder parts are central and autonomic. Running in the heat produced greater low-frequency force depression and greater cardiac vagal withdrawal than running in temperate conditions across 72 hours afterwards, even though the heat trial was 53% shorter, while maximal force, voluntary activation and HRV were back to baseline at 24 hours in both conditions.[60] That is a plausible basis for the common experience of feeling unusually flat after a heat session without being sore.

There is no evidence for a fixed recovery penalty such as an extra 24 hours after every heat session. Three weeks of heat training matched by heart rate produced the same perceived training load and well-being scores as the same programme in temperate conditions.[61] The risk that is real is cumulative: five consecutive days of high-intensity heat acclimation impaired cycling capacity in well-trained athletes, and it had not recovered a week later.[62]

For the full treatment, including what to monitor the morning after and how to fit heat sessions around hard training, see Recovery from Heat Training. For readiness monitoring generally, see Training Readiness.

The Science of Heat Acclimation

Why heat impairs performance

Exercise becomes harder as temperature rises. Blood vessels in the skin dilate to aid cooling, adding to the demand on the cardiovascular system, and sweating produces evaporative cooling which supplies the great majority of heat loss in dry conditions and rather less in humid conditions.[63] Fluid loss through sweating can cause dehydration, which impairs performance in addition to the effect of the heat itself.[63]

Exhaustion in the heat appears to be driven by core temperature itself. In a classic study, exhaustion coincided with core temperature reaching 39.7c, with acclimation progressively lengthening the time taken to reach that level, and the authors concluded that high core temperature per se, rather than circulatory failure, was the critical factor.[20]

Core temperature targets

Published protocols that specify a target core temperature rather than an ambient temperature cluster tightly, which is where the 38.5c figure comes from:

  • 38.5c/101.3f for 90 minutes, with the target reached over about 30 minutes.[64][65][66][67]
  • Just over 39.0c/102.2f for two 50-minute bouts with 10 minutes recovery,[68] or for 2 hours.[69]
  • 38.5c or 39.0c for 90 minutes, with similar results from both.[70]
  • Passive exposure at 38.5c to 39.0c for 2 hours.[71]
  • 60 minutes daily at about 38.5c in a short- and medium-term isothermic protocol.[32]

One study set a limit of 39.7c without giving a rationale.[72] That is close to the temperature at which exhaustion occurred in Nielsen's work[20] and uncomfortably close to heat stroke territory, and I would not go near it.

Adaptations

Heat acclimation produces:

  • Increased plasma and blood volume.[20]
  • Sweating beginning at a lower core temperature, and more profuse sweating.[20][21]
  • Sweat containing less sodium and chloride.[21]
  • Reduced core temperature and heart rate for a given exercise intensity.[20]
  • Improved thermal comfort and reduced thirst sensation.[21]

The magnitudes, from a meta-analysis of 135 studies: acclimation improves subsequent exercise performance and capacity in the heat with an effect size of 0.7, reduces resting core temperature (−0.6) and heart rate (−0.5), and increases sweat rate (0.4), with effect sizes lower than previously reported and considerable heterogeneity between studies.[21]

Time course

Most improvements in heart rate, skin and core temperature and sweat rate occur in the first week, with slower gains over the following two.[33] However, adaptation is greatest when regimens run longer than 14 days,[55] which fits my own experience of continuing improvement over months.

Performance in the heat

This is not seriously disputed. Two weeks of acclimatisation allowed trained cyclists to complete a 43.4km time trial in the heat in a similar time to cool conditions, whereas unacclimatised they suffered a marked drop in power.[22] Another study in the same programme found a 16% improvement in time trial performance in the heat.[27]

Performance in cool conditions

This is contested, and the page previously overstated it.

For: ten days of heat acclimation increased VO2max by 5% and time trial performance by 6% in a cool 13c environment, with a matched control group training in the cool showing no change in any measure.[73] This study was funded by the NIH.

Against: in trained subjects, only that study reported a VO2max improvement; others observed no change.[26] Two weeks of heat acclimatisation did not transfer to improved aerobic capacity or outdoor time trial performance in cool conditions in well-trained cyclists.[27] In a longer study, time trial performance at 14c improved in the heat group but improved similarly in the control group.[28] Acclimation with controlled heart rate did not appear to significantly increase VO2max in cool conditions.[34]

My reading: the effect in trained athletes is unproven. The Lorenzo result is the outlier rather than the consensus, and the studies with proper control groups mostly fail to replicate it.

Haemoglobin mass

The more interesting recent development, and the reason for the longer Goal B protocol.

  • Five weeks of heat training raised haemoglobin mass in elite cyclists, but this did not produce a greater VO2max increase than the control group.[24]
  • A heat chamber and a heat suit both raised haemoglobin mass by around 2.5%; pooled, they improved a composite performance index more than control, and three sessions per week maintained the gain.[29]
  • In elite cross-country skiers, heat suit training raised haemoglobin mass and red cell volume more than control, but with no detectable difference in endurance performance.[25] This study reports no external financial support and no conflicts of interest.
  • In elite female and male cyclists, five weeks raised haemoglobin mass by 4.0% and 3.5% respectively and improved lactate threshold, VO2max and 15-minute power in a thermoneutral environment.[74]
  • A 2025 review notes that every study of at least five weeks published to date reports meaningful haemoglobin mass expansion of roughly 2–4%.[23]
  • Haemoglobin mass fell transiently from day 4 of heat training before returning to baseline, while plasma volume rose in both heat and temperate training.[36]

Two caveats. Almost all of this work comes from one research group in Norway, and the samples are small. And the pattern across studies is consistent: haemoglobin mass goes up reliably, while performance improvements are smaller, less consistent, and often not statistically distinguishable from control.

Heat and altitude

  • Combining heat and altitude in training does not appear to enhance temperate sea-level performance more than heat or altitude alone, though using them sequentially warrants investigation.[38]
  • Altitude may impair heat adaptation when the two are combined.[75]
  • Sequentially, however, three weekly heat sessions after a three-week altitude camp maintained haemoglobin mass, while the control group lost 3.3%.[76]

Other observations

  • Younger athletes cope better with heat than older athletes, but training can offset this, and acclimation is faster in fitter athletes.[77]
  • Up to 75% of acclimation is reached after 5 days.[78]
  • A sauna produces heart rate and cardiac responses similar to 60–100W of exercise.[79] My experience matches that for heart rate, though my breathing rate is much higher than that comparison would suggest.

See Also

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 DJ. Casa, JK. DeMartini, MF. Bergeron, D. Csillan, ER. Eichner, RM. Lopez, MS. Ferrara, KC. Miller, F. O'Connor, MN. Sawka, SW. Yeargin, National Athletic Trainers' Association Position Statement: Exertional Heat Illnesses, J Athl Train, volume 50, issue 9, pages 986-1000, September 2015, doi 10.4085/1062-6050-50.9.07, PMID 26381473
  2. ↑ 2.0 2.1 2.2 Centers for Disease Control and Prevention, Hyperthermia and dehydration-related deaths associated with intentional rapid weight loss in three collegiate wrestlers - North Carolina, Wisconsin, and Michigan, November-December 1997, MMWR Morb Mortal Wkly Rep, volume 47, issue 6, pages 105-108, February 1998, [1]
  3. ↑ 3.0 3.1 Z. Li, et al., The combined effects of exercise-induced muscle damage and heat stress on acute kidney stress and heat strain during subsequent endurance exercise, Eur J Appl Physiol, volume 122, pages 1239-1248, 2022, doi 10.1007/s00421-022-04919-1. Partially funded by the Graduate and Professional Student Association and the Exercise Physiology Laboratories at the University of New Mexico.
  4. ↑ W. Pires, JDC. Veneroso, SP. Wanner, et al., Association between exercise-induced hyperthermia and intestinal permeability: a systematic review, Sports Med, volume 47, issue 7, pages 1389-1403, July 2017, doi 10.1007/s40279-016-0654-2, PMID 27943148
  5. ↑ 5.0 5.1 5.2 William O. Roberts, Lawrence E. Armstrong, Michael N. Sawka, Susan W. Yeargin, Yuval Heled, Francis G. O'Connor, ACSM Expert Consensus Statement on Exertional Heat Illness: Recognition, Management, and Return to Activity, Current Sports Medicine Reports, volume 22, issue 4, 2023, pages 134-149, doi 10.1249/JSR.0000000000001058
  6. ↑ 6.0 6.1 6.2 6.3 R. Hemingway, F. Stourton, T. Leckie, D. Fitzpatrick, et al., Faculty of Pre-Hospital Care Consensus Statement on the Pre-Hospital Management of Exertional Heat Illness, November 2024, [2]
  7. ↑ 7.0 7.1 7.2 CG. Crandall, J. González-Alonso, Cardiovascular function in the heat-stressed human, Acta Physiol (Oxf), volume 199, issue 4, pages 407-423, 2010, doi 10.1111/j.1748-1716.2010.02119.x
  8. ↑ JK. DeMartini, DJ. Casa, R. Stearns, L. Belval, A. Crago, R. Davis, J. Jardine, Effectiveness of cold water immersion in the treatment of exertional heat stroke at the Falmouth Road Race, Med Sci Sports Exerc, volume 47, issue 2, pages 240-245, February 2015, doi 10.1249/MSS.0000000000000409, PMID 24983342
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