Ultrarunners are extremists by nature. I mean that as an observation, not a criticism, but I think that statement is well-accepted and one I personally identify with. The willingness to embrace discomfort and push well past what most people consider reasonable is part of what draws us to the sport. But that same instinct – if something is good, then more must be better – causes real problems when it migrates from race day into training and race preparation, including training volume, strength training and carbohydrate intake. And in recent years, as the conversation around heat preparation has gone mainstream when heading into races like Western States 100, we see it with heat training.
The logic goes like this: if heat acclimation improves performance in the heat, I should do as much heat training as possible before heading into a race. This way of thinking leads to athletes training in heat suits for long efforts, stacking thermal stress on top of training stress and grinding through degraded workouts in the name of being heat-adapted. Some athletes get overcooked and show up to the start line under-recovered. Everyone else just did far more than they needed to and likely sacrificed some training.
The information below is a pushback on that approach – not on heat acclimation itself, which is well-supported by the evidence and meaningfully beneficial for races conducted in hot environments, but on the maximization mindset. There is a minimum effective dose for heat adaptation, and it is lower than most athletes think. It can be achieved without disrupting training quality and it requires no miserable long runs in a heat suit.
The argument is simple: fitness is your biggest performance lever while heat adaptation is a component of specificity. Specificity should never come at the cost of fitness and when it does, you have optimized the wrong variable.
Fitness First, Everything Else Second
Before we get into the physiology of heat adaptation, we need to establish a hierarchy. In the framework I use with my athletes, training is the biggest hammer in the toolbox. When looking at other factors like heat acclimation, altitude exposure, excessive strength work or any marginal gain strategy, nothing is even close to the return on investment that comes from consistent, high-quality training.
That hierarchy matters when making decisions about how to structure your preparation. Heat acclimation is a component of race specificity, and specificity sits below fitness in the hierarchy. This means an athlete should pursue specificity in ways that translate fitness to race day demands.
This is where a lot of athletes go wrong with heat training. They add heat stress on top of training stress without accounting for the compounding load. The result is degraded workout quality, increased recovery demands and sometimes, non-functional overreach, and they arrive at race day with excellent heat tolerance and a fitness level that is lower than it should be. The adaptation they maximized was not the one that mattered most.
What Heat Acclimation Actually Does
Understanding why heat acclimation matters starts with realizing what happens to your physiology in the heat. When you run in hot conditions without prior adaptation, your cardiovascular system is placed under competing demands: working muscles need blood for oxygen delivery, and your skin needs blood for thermoregulation. That competition elevates heart rate, reduces cardiac output available for locomotion and accelerates the onset of fatigue. Core temperature rises faster, sweat onset is delayed and the perception of effort is magnified at any given pace.
Heat acclimation addresses each of these problems through several well-documented adaptations, summarized below (Casadio et al., 2018; Périard et al., 2015):
- Plasma volume expansion. This is the most performance-relevant adaptation. More blood volume means better cardiac filling, reduced cardiovascular strain and improved oxygen delivery to working muscles.
- Reduced core temperature at rest and during exercise. This effectively extends the time before core temperature becomes a limiting factor.
- Earlier onset of sweating and increased sweat rate. The evaporative cooling this provides is the primary mechanism for dissipating heat during exercise.
- Reduced sweat sodium concentration. The body conserves sodium more efficiently, which reduces the risk of electrolyte depletion and, for athletes like most Western States competitors who are already sodium-conscious, supports better fluid balance. This is not an argument to reduce sodium supplementation on race day, it is simply one more way the adapted body manages heat stress more economically.
- Reduced exercising heart rate and perceived effort. At any given work rate, heat-adapted athletes have a lower heart rate and report the effort as easier. This is partly cardiovascular and partly central. The brain’s tolerance for heat stress improves alongside the body’s capacity to manage it.
All of these adaptations compound. The athlete who arrives at Western States heat-adapted is not just more comfortable in the heat, they are physiologically better equipped to sustain output across what is already a long, demanding day. The adaptations are real, meaningful and worth pursuing. The question is not whether to pursue them, but how.
The Minimum Effective Dose
The research on heat acclimation has a clear message that many athletes never hear: you do not need much.
The landmark work comes from Zurawlew et al. (2016, Scandinavian Journal of Medicine and Science in Sports), who examined whether post-exercise hot water immersion over six consecutive days would induce heat acclimation in non-acclimated males. The protocol was simple: a 40-minute submaximal treadmill run at 65% VO2max in temperate conditions, immediately followed by 40 minutes of hot water immersion at 104 degrees. That’s it – normal training, followed by a hot bath.
After six days, resting core temperature and exercising core temperature dropped in temperate conditions and hot conditions, and a 5k time trial performance improved in both environments. These are the hallmarks of heat acclimation, achieved without a single workout in the heat.
Follow-up work from the same group found that these adaptations persisted for at least two weeks after the final session – a critical finding for taper planning. And a 2022 study in the American Journal of Physiology (Walsh et al.) directly compared post-exercise hot water immersion to conventional exercise-heat acclimation over 12 days. The finding was counterintuitive: post-exercise HWI (hot water immersion) produced more complete heat acclimation than exercising in the heat, while neither protocol increased overreaching risk compared to temperate exercise.
Let that land. You can achieve equivalent or superior heat adaptation by doing normal training and jumping in a hot bath afterward, without the interference of training in a heat suit or deliberately executing workouts in harsh conditions.
Sauna studies tells a similar story. Scoon et al. (2007, Journal of Science and Medicine in Sport) showed that approximately three weeks of post-exercise sauna bathing – averaging about 13 sessions at roughly 194 degrees for 30 minutes – increased plasma volume by 7.1% and improved time to exhaustion by 32% in competitive male runners, equivalent to roughly a 1.9% improvement in a time trial.
The mechanism is straightforward. The post-exercise body is already thermally elevated from training, which means less time is required to accumulate the heat stimulus that drives adaptation. Standalone heat protocols conducted without the exercise warmup require the athlete to build from a cold baseline – while post-exercise exposure skips that ramp entirely. You arrive at the bath or sauna already partway there, and the adaptive signal accumulates faster with less total exposure time required.
Two Phase Protocol
Based on the evidence and practical application with athletes, I recommend thinking about heat preparation in two phases rather than as a single extended block, when preparing for an extremely hot race like Western States.
Phase one is a build phase that can begin as far out as 6-8 weeks before the race, which is exactly what I have my Western States athletes doing right now. This is where you establish the foundational adaptations: plasma volume expansion, reduced resting core temperature and earlier sweat onset. Target approximately 6-10 consecutive or near-consecutive sessions to drive full adaptation. A hot bath at 104-108 degrees for 20-30 minutes works, and a sauna in the 175-195-degree range for 20-30 minutes also works. Both are passive and neither requires you to run in them.
Between phase one and phase two, maintain the adaptation with one session every 3-4 days, or twice a week. You are not building at this point; you are keeping the signal alive. The decay literature shows that adaptations from short-term protocols are typically retained for about seven days but begin fading by day 14 without continued exposure (Daanen et al., 2018; Zurawlew et al., 2019). One session every few days is enough to slow that decay and carry the foundational adaptations forward without accumulating meaningful additional fatigue.
Phase two corresponds with your taper, beginning 14-21 days out from race day. Run daily or near-daily sessions in the first week of taper, when training volume is already dropping and the recovery pool is opening up. One additional session early in race week is reasonable if readiness allows but skip it if sleep, HRV or subjective feelings are trending in the wrong direction. Plan your last session no closer than three days out.
In both phases, the non-negotiable rule is that passive heat exposure happens after training, not instead of it and not in a form that corrupts the training session itself. The workout comes first and the heat exposure is the addition.
Not every athlete has reliable access to a sauna or a dedicated HWI setup, and the two-phase model is not a requirement. A single build phase of 6-10 consecutive or near-consecutive post-exercise sessions in the 2-3 weeks before the race is a legitimate minimum effective dose and will produce meaningful adaptation. A bathtub works. The modality matters less than the consistency and the timing – post-exercise, passive and protected from interference with training quality.
The Decision Framework
Heat training is worth doing if you are preparing for a race that has meaningful heat exposure as a performance variable. Western States clearly qualifies, and so do other summer races held in the American Southwest, parts of Europe in July and August and any event where temperature is likely to be a primary physiological stressor.
If you decide to do it, the decision framework is this:
- Training quality is non-negotiable and the highest priority. If the heat protocol cannot be layered onto training without degrading it, the protocol is wrong, not the training.
- Passive exposure post-exercise is the correct mechanism, not sacrificing quality or long sessions with a heat suit.
- Two phases: approximately 10 consecutive or near-consecutive sessions in phase one to establish full adaptation, followed by bridge maintenance sessions every 3-4 days, and then phase two corresponding with your taper – daily or near-daily sessions in the first week of taper, and one session early race week if recovery allows.
- Two phases are ideal. If access is limited, a single phase of 6-10 consecutive or near-consecutive sessions in the 2-3 weeks before the race is a legitimate, minimum effective dose.
- Time your last session no closer than three days before the race to allow clearance of acute heat load while retaining the adaptations you have built.
- Monitor recovery as you would with any additional training stress. Heat exposure is a stressor and draws from the same recovery pool as your training. If your sleep, HRV or subjective readiness is trending in the wrong direction, scale the heat exposure back before scaling back the training.
Simple Does Not Mean Easy
The version of heat preparation I am describing is not dramatic. There is no suffering or aggressive protocol that impresses people on social media.
That simplicity is not a compromise; it is the point. The evidence says this is enough and that post-exercise passive exposure is at least as effective as conventional heat training and more effective than exercising in the heat on most adaptation markers.
More heat exposure does not make you more heat-adapted beyond a certain threshold. It makes you more fatigued and that is not a trade worth making.
References:
Zurawlew MJ, Walsh NP, Fortes MB, Potter C. Post-exercise hot water immersion induces heat acclimation and improves endurance exercise performance in the heat. Scand J Med Sci Sports. 2016;26(7):745–54.https://doi.org/10.1111/sms.12638
Zurawlew MJ, Mee JA, Walsh NP. Post-exercise hot water immersion elicits heat acclimation adaptations that are retained for at least two weeks. Front Physiol. 2019;10:1080. https://doi.org/10.3389/fphys.2019.01080
Zurawlew MJ, Mee JA, Walsh NP. Post-exercise hot water immersion elicits heat acclimation adaptations in endurance trained and recreationally active individuals. Front Physiol. 2018;9:1824. https://doi.org/10.3389/fphys.2018.01824
McIntyre RD, Zurawlew MJ, Mee JA, Walsh NP, Oliver SJ. A comparison of medium-term heat acclimation by post-exercise hot water immersion or exercise in the heat: adaptations, overreaching, and thyroid hormones. Am J Physiol Regul Integr Comp Physiol. 2022;323(5):R601–R615. https://doi.org/10.1152/ajpregu.00315.2021
Scoon GSM, Hopkins WG, Mayhew S, Cotter JD. Effect of post-exercise sauna bathing on the endurance performance of competitive male runners. J Sci Med Sport. 2007;10(4):259–62. https://doi.org/10.1016/j.jsams.2006.06.009
Casadio JR, Kilding AE, Cotter JD, Laursen PB. From lab to real world: heat acclimation considerations for elite athletes. Sports Med. 2018;47(8):1467–1476. https://doi.org/10.1007/s40279-016-0668-9
Casadio JR, et al. Passive heating: reviewing practical heat acclimation strategies for endurance athletes. Front Physiol. 2018;9:1851. https://doi.org/10.3389/fphys.2018.01851
Daanen HAM, Racinais S, Périard JD. Heat acclimation decay and re-induction: a systematic review and meta-analysis. Sports Med. 2018;48(2):409–430. https://doi.org/10.1007/s40279-017-0808-x
Périard JD, Racinais S, Sawka MN. Adaptations and mechanisms of human heat acclimation: applications for competitive athletes and sports. Scand J Med Sci Sports. 2015;25(Suppl 1):20–38. https://doi.org/10.1111/sms.12408