Heat training improved performance more than cooler training
- Heat acclimation
- Heat training
- Running performance
Heat can make endurance racing considerably harder, so runners and coaches often use heat acclimation before hot events. This study asked whether adding environmental heat to otherwise similar training actually produces extra adaptation.
Reference: Chen et al. Exercise in hot or thermoneutral conditions? Comparative effectiveness on aerobic performance and physiological adaptations in the heat: A meta-analysis. Journal of Sport and Health Science (2026) DOI: https://doi.org/10.1016/j.jshs.2026.101168.
Study snapshot
A quick, practical summary for runners and coaches.
Quick answer
This meta-analysisA meta-analysis quantifies the overall effect size of a treatment by compiling effect sizes from all known studies of that treatment. combined 23 controlled studies comparing exercise in hot and thermoneutral conditions over 4 to 15 days. Training in the heat produced moderately greater improvements in aerobic performance and several markers of heat adaptation. The main caution is that the certainty of the evidenceCertainty of evidence tells us how confident we are that the published results accurately reflect the true effect. It’s based on factors like study design, risk of bias, consistency, directness, precision, and publication bias. High certainty means that the current evidence is so strong and consistent that future studies are unlikely to change conclusions. Whereas, low certainty means more doubt and less confidence, and that future studies could easily change current conclusions. was low to very low.
Key takeaways
- Heat training improved performance and several heat-adaptation markers more than matched training in thermoneutral conditions.
- The certainty of the evidence was low to very low, and most participants were male.
- Heat acclimation may be worth considering when preparing for endurance competition in hot conditions.
How confident should we be?
Evidence confidence: Low
The controlled comparisons and meta-analytic methods strengthen the evidence. But the included studies varied in quality and methods, and the authors rated every comparison as low or very low certainty.
Bottom line
If your target race is likely to be hot, planned heat training may provide useful adaptations beyond normal endurance training. The evidence supports considering it, not turning every easy run into a tropical survival test.
Read the deep dive below to get a practical interpretation, the wider evidence, some actionable decisions, my thoughts, my rating of perceived scientific enjoyment, and the full study details (research question, study design, participants, methods, data integrity, results, and the strengths & limitations).
The deep dive
The details behind the headline result, including the practical meaning, full findings, limitations, and my interpretation.
Practical meaning
Practical meaningWhat does this research mean for runners and coaches?
The practical interpretation is fairly narrow. Training in the heat appears to provide additional heat-specific adaptations beyond completing the same exercise in more comfortable conditions. That matters most when the event itself is likely to impose substantial heat stress.
For runners
If you are preparing for a hot marathon, trail race, or ultra, a planned heat-acclimation block may be worthwhile. The study does not prove that every runner needs heat training, nor does it identify 1 perfect running-specific protocol.
Most of the included studies used cycling rather than running, so the evidence is more directly about endurance exercise than running itself. The laboratory protocols also cannot reproduce everything that happens in a long road, trail, or mountain race.
The useful bit is that the environmental heat seemed to add something beyond training alone. But ordinary exercise was also followed by some heat-related changes, so runners who cannot train in hot conditions are not starting from scratch.
For coaches
The findings support treating heat as a specific training stimulus when athletes are preparing for hot competition.
That does not mean adding heat indiscriminately. Heat increases physiological strain, so any heat-acclimation block needs to fit around the athlete's important training and recovery.
The exploratory analyses suggested that longer interventions and higher training status may influence the size of the response. Those findings are interesting, but they are not strong enough to become rigid coaching rules.
Evidence in context
Evidence in contextWhat does the wider evidence say?
Heat acclimation is already well established as a way to reduce physiological strain and improve endurance performance in hot conditions. Repeated exercise in the heat typically lowers exercise heart rate and core temperature, increases sweating, and can improve performance, with expert recommendations generally favouring repeated exposures over about 1 to 2 weeks (Racinais et al. 2015). Previous meta-analysis also suggested that heat acclimation can improve aerobic capacity beyond exercise training alone, particularly when performance is tested in the heat (Waldron et al. 2021).
How this study fits: This study agrees with that wider evidence and adds a more direct comparison between matched training performed with and without added environmental heat.
Practical decision
Practical decisionShould runners change anything?
Maybe. This study makes heat acclimation more convincing when the goal is performing well in hot conditions. It does not justify adding heat training to every endurance programme.
Consider this if
- Your target race is likely to be hot or humid.
- You have enough time to introduce heat exposure gradually without compromising key sessions.
Do not overreact if
- Your target races are mostly cool.
- The extra heat would interfere with training quality, recovery, or safety.
A sensible next step
If heat is likely to matter on race day, consider a planned heat-acclimation block rather than randomly making workouts hotter. That is a practical interpretation of the evidence, not a protocol tested directly by this meta-analysis.
TIP: Never make any major changes to your training or lifestyle habits based on the findings of one study, especially if the study is small or provides low-quality evidenceA low quality of evidence means that, in general, studies in this field have several limitations. This could be due to inconsistency in effects between studies, a large range of effect sizes between studies, and/or a high risk of bias (caused by inappropriate controls, a small number of studies, small numbers of participants, poor/absent randomisation processes, missing data, inappropriate methods/statistics). When the quality of evidence is low, there is more doubt and less confidence in the overall effect of an intervention, and future studies could easily change overall conclusions. The best way to improve the quality of evidence is for scientists to conduct large, well-controlled, high-quality randomised controlled trials.. Check whether other trials confirm the findings. If there is a meta-analysis on the topic, look at the effect sizeA standardised measure of the magnitude of an effect of an intervention. Unlike p-values, effect sizes show the size of the effect and how meaningful it might be. Common effect size measures include standardised mean difference (SMD), Cohen’s d, Hedges’ g, eta-squared, and correlation coefficients., the variability between studies, and the quality of evidence.
TIP: Never make any major changes to your training or lifestyle habits based on the findings of one study, especially if the study is small or provides Expert interpretation
Expert interpretationMy thoughts
I like the question behind this paper. Heat acclimation clearly changes physiology, but how much of that comes from the hot environment and how much simply comes from repeatedly exercising and producing lots of metabolic heat?
This review gets closer to answering that than many earlier summaries because the comparison groups performed matched training in different environmental conditions. The heat condition came out ahead for aerobic performance, exercise heart rate, core temperature, and sweating when people were later tested in the heat.
That makes the findings useful. They're also reasonably actionable for athletes preparing for a hot race.
But I'd keep both eyebrows somewhere near their normal position. The evidence was low to very low certainty, most participants were male, most studies involved cycling, and some temperature findings changed when individual studies were removed.
So the force is fairly strong with this one, but Yoda probably isn't rewriting your training plan just yet. Heat acclimation looks worth considering when the race demands it. The more interesting question now is how much heat exposure runners actually need to capture most of the benefit without making the rest of their training worse.
My Rating of Perceived scientific Enjoyment
RPsE: 6/10
I experienced moderate scientific enjoyment because the review was preregisteredPre-registration is when a detailed description of a study plan is deposited in an open-access repository before collecting the study data. It promotes transparency and accountability and boosts research integrity. Without preregistration, it is easier for scientists to change outcomes after seeing the data, selectively report “exciting” results, or run many analyses and only show the ones that work, which can introduce bias and weaken the trustworthiness of the findings., searched several databases, assessed bias, publication biasPublication bias in a meta-analysis occurs when smaller or less favorable studies are probably missing. This means that studies with satistically significant results are more likely to be published than those with non-significant findings, leading to distorted conclusions. This bias can inflate effect sizes and misrepresent the true effectiveness of interventions., and certainty, and asked a genuinely useful question for endurance athletes. The low to very low certainty of the evidence and limited runner-specific data took some shine off the pint glass.
Read on for further details about the methods, results, data integrity, strengths, and limitations.
Research question
Research questionWhat did the researchers ask?
The authors wanted to know whether exercising in hot conditions produces greater improvements in aerobic performance and heat-related physiology than completing the same training in thermoneutral conditions.
They also explored whether factors such as training status, exercise intensity, intervention duration, training frequency, and the heat-acclimation method influenced the response.
Study design
Study designWhat type of study was this?
This study was a systematic reviewA systematic review answers a specific research question by systematically collating all known experimental evidence, which is collected according to pre-specified eligibility criteria. A systematic review helps inform decisions, guidelines, and policy. and meta-analysis.
The review included both randomizedRandomization means assigning people to different parts of a study (e.g., groups in a randomised controlled trial) by chance, not by choice. This helps make the groups similar at the start and reduces bias, so any differences you see are more likely due to the treatment, not background differences. In a crossover study, randomization usually decides the order in which each person gets the treatments (for example, Treatment A first then B, or B first then A). This way, order effects—like learning, fatigue, or simple time passing—are less likely to skew the results. and non-randomized controlled studies. A meta-analysis can combine results across multiple experiments and provide a better overall estimate than relying on 1 small study, but its conclusions still depend on the quality and similarity of the studies being pooled.
That matters here because the authors rated the certainty of every comparison as low or very low.
Participants
ParticipantsWho took part?
The review included 23 studies and 413 participants. The eligibility criteria covered healthy adults aged 18 to 59.
The authors reported 316 male participants and 30 female participants. Another 67 participants came from studies that included both sexes, but the male and female breakdown within those samples was not reported.
The participants ranged from sedentary adults to highly trained athletes. Around half were trained or highly trained, and most studies used cycling rather than running.
Methods
MethodsWhat did the researchers do?
The researchers searched PubMed, SPORTDiscus, the Cochrane Library, Scopus, and Web of Science up to November 20, 2025. They preregistered the review in PROSPERO and reported following the PRISMA guidelinesPreferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) reporting is a set of guidelines that helps researchers clearly and completely report a systematic review or meta-analysis. It gives them a checklist and flow diagram to show what they searched, what they included, and how they made their decisions. This makes the review easier to understand, assess, and trust..
The review included studies comparing exercise in a hot environment with the same training performed under thermoneutral conditions. Passive heat-acclimation methods such as hot-water immersion, sauna-based approaches, and clothing designed to increase heat strain were excluded from the main comparison.
The hot conditions ranged from 30°C to 45°C (86°F to 113°F), averaging about 37°C (99°F). The thermoneutral conditions averaged about 19°C (66°F).
Individual training sessions lasted from 18 to 180 minutes, averaging around 69 minutes. The interventions lasted 4 to 15 days, with an average duration of just over 8 days.
The researchers assessed aerobic performance in the heat plus resting core temperature, exercise core temperature, skin temperature, heart rate, and sweating.
Data integrity check
Do the numbers add up?
Minor integrity concern; closer inspection may be warranted
The main numbers look broadly plausible and agree across the paper, with no obvious red flags in the effect sizes, confidence intervalsA measure of uncertainty used in Frequentist statistics. The 95% confidence interval is a plausible range of values within which the true value (e.g., the true treatment effect) would be found 95% of the time if the data were repeatedly collected in different samples of people. If this range of values (the confidence interval) crosses zero, there is little confidence that the average value is the true effect. If the confidence interval does not cross zero, we can be confident that the average value is the true effect., p-valuesA p-value is a statistical measure that indicates the probability that the result is at least as extreme as that observed if the null-hypothesis was true. If P is small, the observed difference is big enough to disprove (reject) the null hypothesis. In very basic terms, P equals the probability that the effect could be explained by random chance, and a P-value of less than 0.05 means the results look so promising that there’s only a 1-in-20 (or 5%) chance that they would have occurred if the treatment had no effect at all. Common thresholds for statistical significance are 0.05, 0.01, and 0.001., or physiological results. One reporting issue deserves attention: some reference numbers used to describe the included studies are inconsistent, including a study appearing in both the randomized trial list and the non-randomized trial list. But this looks more like a citation or editing mistake than a problem with the underlying data.
Main findings
Main findingsWhat did the study find?
Overall, training in the heat produced greater changes in aerobic performance and several markers of heat adaptation than matched training in thermoneutral conditions.
Aerobic performance was assessed across 13 studies and 21 effect estimates. The advantage of heat training was statistically significantEvidence that a result is unlikely to be due to chance under a “no effect” model (or null hypothesis). Statistical significance is often judged by a p-value below 0.05 to flag that “something” is going on, but not how big or important that “something” is. One statistically significant result doesn’t mean proof; replication is needed. And, a statistically significant result doesn’t necessarily indicate clinical significance. and moderate in size. In practical terms, people tended to perform better in hot conditions after training in the heat than after equivalent training in thermoneutral conditions.
Resting core temperature was assessed in 9 studies. It fell about 0.17°C (0.31°F) more after heat training than after thermoneutral training, representing a moderate effect.
Exercise core temperature was assessed in 11 studies. It fell about 0.09°C (0.16°F) more after heat training, also representing a moderate effect. The resting and exercise core-temperature findings were less stable than some of the other results because removing individual studies could make them statistically non-significant.
Exercise heart rate was assessed in 11 studies. It fell about 6 beats per minute more after heat training than after thermoneutral training, a moderate effect. That is consistent with less cardiovascular strain during the standardized exercise tests.
Sweating was assessed in 10 studies. Heat training produced a statistically significant, moderate increase in sweat rate compared with thermoneutral training.
Skin temperature was assessed in 6 studies. The primary analysis did not find a statistically significant difference between conditions.
The thermoneutral training condition also changed some outcomes. Exercise core temperature fell by about 0.14°C (0.25°F), a small standardized effect, while exercise heart rate fell by about 6 beats per minute, a moderate standardized effect. Aerobic performance, resting core temperature, skin temperature, and sweat rate did not change significantly in the main thermoneutral analyses.
Those within-condition findings deserve more caution because they do not isolate the effect of training as cleanly as the direct comparison between the hot and thermoneutral conditions.
The exploratory subgroup analyses suggested that longer interventions and training status may influence the response to added heat. The authors warned that these analyses were based on limited data and should be interpreted cautiously.
The authors concluded that added environmental heat during exercise produced greater improvements in aerobic performance and several physiological markers of heat adaptation than thermoneutral exercise, while thermoneutral exercise alone could still produce modest changes in some outcomes.
Study strengths and limitations
What helps my confidence in the findings?
What helps my confidence in the findings?The strengths
- The review included 23 controlled studies rather than relying mainly on uncontrolled before-and-after comparisons.
- The comparison conditions completed the same training protocols, which helped isolate the additional effect of environmental heat.
- The authors searched 5 databases and preregistered the review in PROSPERO.
- The statistical methods accounted for multiple related effect estimates coming from the same studies.
- The authors assessed risk of biasRisk of bias in a meta-analysis refers to the potential for systematic errors in the studies included in the analysis. Such errors can lead to misleading/invalid results and unreliable conclusions. This can arise because of issues with the way participants are selected (randomisation), how data is collected and analysed, and how the results are reported., publication bias, sensitivity of the results, and certainty of the evidence.
What limits my confidence in the findings?
What limits my confidence in the findings?The limitations
- The authors rated all comparisons as low or very low certainty because of risk of bias, imprecision, indirectnessWhen assessing the certainty of evidence in a meta-analysis, indirectness tells us how well the participants, interventions, and outcomes match what we’re interested in knowing about the topic., and related concerns.
- The review included 9 randomized studies and 14 non-randomized studies, and several studies had methodological concerns.
- The evidence was overwhelmingly male, while the sex breakdown of 67 participants from mixed-sex studies was not reported.
- Most interventions used cycling rather than running, limiting direct relevance to marathon, trail, and ultra runners.
- Post-intervention testing occurred at different times, ranging from less than 24 hours to as long as 5 days in the studies that reported it, while 4 studies did not report the interval. Some core-temperature findings were also sensitive to the removal of individual studies.
Funding and conflicts
Funding and conflictsWho funded the study?
The study was supported by the Research Grants Council General Research Fund of the Hong Kong Special Administrative Region, China, under grant 14618825.
The authors declared no competing interests. The paper reports no commercial funding or financial conflict related to a heat-training product or service.
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FAQ
Does heat training improve running performance in hot weather?
This meta-analysis found that heat training produced a moderate improvement in aerobic performance compared with equivalent thermoneutral training when performance was tested in the heat. Most studies used cycling, though, so the evidence is not specifically about running races.
How long does heat acclimation take?
The interventions in this review lasted 4 to 15 days, averaging just over 8 days. Wider guidance commonly recommends repeated heat exposures across about 1 to 2 weeks.
Does normal endurance training improve heat tolerance?
Possibly. Thermoneutral exercise was followed by reductions in exercise core temperature and heart rate, although it did not significantly improve several other heat-adaptation outcomes.
Should runners train in the heat before a marathon?
It may be worth considering if the marathon is expected to be hot. This study supports the principle of heat acclimation, but it does not establish 1 optimal marathon-specific protocol.
Is heat training more useful for highly trained runners?
The exploratory analyses suggested larger additional benefits in some highly trained participants. The evidence for this subgroup finding was limited, so it should not yet be treated as a firm rule.
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