Exercise in the Heat Delivers Adaptations That Thermoneutral Training Cannot Match, Meta-Analysis Finds

As global temperatures climb, scientists are racing to understand how the human body can be better prepared for a hotter world. A new meta-analysis from researchers at The Chinese University of Hong Kong has tackled a deceptively simple question with major implications for athletes, military personnel, occupational health, and public health policy alike: when people adapt to heat through exercise, is the benefit coming from the hot environment itself, or simply from the act of exercising? The answer, published in the Journal of Sport and Health Science, is that both contribute, but the environment adds a distinct and measurable layer of adaptation that exercise alone cannot replicate.

The research team, led by Professor Stephen Heung-Sang Wong and Assistant Professor Eric Tsz-Chun Poon of the Department of Sports Science and Physical Education, conducted a systematic review and meta-analysis of 23 studies involving 413 participants. The work was designed to disentangle two sources of physiological stress that are usually bundled together in heat acclimation programs. Exercise in hot conditions, abbreviated ExH, combines environmental heat load with the metabolic heat generated by working muscles. Exercise in thermoneutral conditions, or ExN, generates only the internal heat of muscular work. By comparing these two training modes directly, the researchers could isolate the additional contribution of environmental heat exposure.

The analytical framework was deliberately comprehensive. Three complementary comparisons were performed: ExH versus ExN, pre- versus post-intervention responses following ExH, and pre- versus post-intervention responses following ExN. The primary outcomes spanned the key markers of heat adaptation used across the exercise physiology literature: aerobic performance under hot conditions, resting core temperature, exercise core temperature, exercise skin temperature, exercise heart rate, and sweat rate. This structure allowed the team to evaluate not only which training mode performed better against the other, but also whether each mode, on its own, produced meaningful physiological change.

The headline finding is unambiguous. Compared with thermoneutral exercise, exercise performed in the heat produced greater improvements in aerobic performance under heat stress, greater reductions in resting core temperature, greater reductions in exercise core temperature, greater reductions in exercise heart rate, and a greater increase in sweat rate. In other words, the classic constellation of heat acclimation adaptations, spanning both cardiovascular and thermoregulatory systems, was more fully developed when environmental heat was part of the training stimulus. The one exception was exercise skin temperature, where no significant difference emerged between the two training conditions.

These results carry particular weight because they address a long-standing ambiguity in the field. Exercise itself is a potent thermal stressor. Sustained skeletal muscle contraction dramatically increases metabolic heat production, and even in a temperate environment this internal heat load can elevate body temperature and strain the cardiovascular system. Some adaptations traditionally credited to hot-weather training might therefore be attributable, at least in part, to exercise-generated metabolic heat rather than to the ambient environment. The new meta-analysis confirms that this is true to a degree, but also demonstrates that environmental heat exposure provides an adaptive stimulus that metabolic heat alone cannot fully substitute for.

Indeed, the within-group analyses revealed that thermoneutral exercise was far from physiologically inert. Following ExN training, participants showed a significant reduction in exercise core temperature of approximately 0.14 degrees Celsius, alongside a decrease in exercise heart rate of approximately 5.6 beats per minute during subsequent exercise in the heat. These figures suggest that ordinary exercise training, performed without any deliberate heat exposure, can cultivate a modest degree of heat resilience, likely through the combined effects of exercise-induced metabolic heat production and general training adaptations such as improved cardiovascular function and expanded plasma volume.

Nevertheless, the magnitude and breadth of adaptation were consistently greater for exercise in the heat. Professor Wong emphasized the practical meaning of this pattern, noting that the findings indicate additional environmental heat exposure during exercise provides a meaningful adaptive stimulus beyond that produced by exercise alone, particularly for cardiovascular and thermoregulatory responses. For populations whose safety and performance depend on operating effectively in hot conditions, from elite athletes preparing for competitions in warm climates to workers in physically demanding outdoor occupations, the message is that deliberate heat exposure remains an essential ingredient of preparation.

The meta-analysis also identified two important moderators of the effect. The benefits of exercising in the heat were greater with longer interventions, indicating that heat adaptation is a dose-dependent process that accrues over time rather than a switch that flips after a few sessions. The benefits were also more pronounced among elite athletes, a finding that may reflect both the greater training loads such individuals can tolerate and their capacity to push thermoregulatory systems closer to their adaptive limits. For recreational exercisers, the adaptations from heat training may still be valuable, but the ceiling of improvement appears higher in highly trained populations.

Dr. Poon framed the dual message of the study, stating that the findings support the use of exercise in the heat as an effective strategy for enhancing heat tolerance and improving aerobic performance under heat stress, while simultaneously highlighting the potential contribution of exercise itself to the development of partial heat-related adaptations. This distinction matters enormously for the design of practical, safe, and individualized strategies. Heat acclimation carries inherent risks, including the potential for heat illness if sessions are poorly managed, so knowing that ordinary training confers some baseline protection, while targeted heat exposure delivers the full adaptation package, allows practitioners to calibrate programs to individual needs, schedules, and risk profiles.

The broader context gives this work its urgency. Global warming is substantially increasing the frequency, duration, and intensity of human exposure to high ambient temperatures, and heatwaves are projected to occur more often and last longer, exposing a growing share of the population to potentially harmful thermal stress. Elevated heat exposure is associated with increased risks of cardiovascular morbidity and mortality, heat stroke, cardiac arrest, and other heat-related illnesses. Current public health recommendations, including those from the World Health Organization, primarily emphasize minimizing heat exposure, keeping indoor environments cool, and maintaining adequate hydration. Yet complete avoidance of heat is often impractical given occupational demands, commuting, recreation, and routine physical activity. In that reality, strategies that build physiological resilience and enhance heat tolerance offer a complementary line of defense, and this meta-analysis provides a rigorous, evidence-based map of which training tools deliver which adaptations, and how much of each the human body actually requires.

Subject of Research: Comparative effects of exercise in hot versus thermoneutral conditions on heat-related physiological adaptations

Article Title: Where does heat adaptation come from: Environmental heat exposure or exercise itself?

Article References: Where does heat adaptation come from: Environmental heat exposure or exercise itself?. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: heat acclimation, exercise physiology, meta-analysis, thermoregulation, core temperature, sweat rate, aerobic performance, heart rate, global warming, heat stress, athletic training, public health