Running Hot: The Emerging Science of Thermal Tolerance and Why Some Athletes Thrive Where Others Fade
For decades, the prevailing logic in sports physiology held that rising core body temperature was essentially a countdown timer — a biological constraint that, once triggered, moved inexorably toward fatigue, impaired motor function, and, in extreme cases, heat illness. Coaches and sports scientists responded accordingly, designing cooling protocols, mandating hydration schedules, and treating thermal management as a universal performance variable to be minimized. Yet a compelling and increasingly well-supported line of research complicates that picture considerably.
Some athletes, it turns out, do not merely tolerate higher core temperatures during intense exercise — they appear to perform at or near their peak within thermal ranges that would demonstrably impair their competitors. Understanding the mechanisms behind this divergence, and whether it can be cultivated through training, represents one of the more consequential frontiers in applied exercise physiology.
What Core Temperature Actually Does to the Body
To appreciate why individual variation in heat tolerance matters, it helps to understand the physiological cascade that elevated internal temperature sets in motion. As core temperature climbs during sustained exercise, the cardiovascular system is forced into a difficult balancing act. Blood must simultaneously supply working muscles with oxygen and fuel while being redirected toward the skin to facilitate radiative and evaporative cooling. This competition for circulatory resources — sometimes called cardiovascular drift — progressively reduces the volume of blood returning to the heart, which in turn diminishes cardiac output and muscular perfusion.
Beyond the cardiovascular strain, rising brain temperature has been linked to alterations in neurotransmitter activity, particularly involving serotonin and dopamine pathways associated with perceived effort and motivation. The central governor model of fatigue, developed by exercise physiologist Tim Noakes and subsequently refined by others, proposes that the brain may actually impose a protective brake on physical output before true muscular failure occurs — and thermal load appears to be one of the primary signals driving that protective response.
Critically, however, the temperature at which these processes begin to meaningfully impair performance is not fixed. It varies — sometimes substantially — between individuals.
The Genetic Architecture of Heat Tolerance
Research into heat stroke susceptibility and, conversely, exceptional heat endurance has identified a number of candidate genes that appear to influence how efficiently the body manages thermal load. Variants in genes encoding heat shock proteins — molecular chaperones that protect cellular structures from heat-induced denaturation — have been associated with differential resilience to thermal stress. Athletes carrying certain polymorphisms in the HSP70 family of proteins, for example, may mount a more robust cellular protective response during periods of thermal challenge, preserving muscular and neural function at temperatures that compromise less thermally endowed competitors.
Additionally, variation in the genes governing sweat gland density, sweat rate, and the electrolyte composition of sweat itself contributes meaningfully to how efficiently an individual dissipates heat through evaporative cooling. An athlete who sweats earlier, more copiously, and with greater sodium efficiency relative to fluid loss is, all else being equal, better positioned to maintain thermal homeostasis during prolonged output in warm conditions.
These genetic factors do not operate in isolation. They interact with body surface area, body composition, acclimatization history, and aerobic fitness in ways that make thermal tolerance a genuinely complex, multifactorial trait rather than a simple binary distinction between those who handle heat and those who do not.
Heat Acclimatization: The Training Adaptation Most American Athletes Ignore
While genetic endowment sets a baseline, the research on heat acclimatization makes clear that thermoregulatory capacity is substantially trainable. Controlled exposure to exercise in warm environments over a period of roughly ten to fourteen days produces a well-characterized suite of physiological adaptations: plasma volume expansion, earlier onset of sweating, reduced sweat sodium concentration, lower resting core temperature, and a decreased cardiovascular strain response to a given thermal load.
What is less commonly appreciated is that these adaptations do not merely allow athletes to perform in hot conditions — they appear to confer performance benefits even in temperate or cool environments. The plasma volume expansion associated with heat acclimatization, for instance, functions similarly to altitude-induced increases in red blood cell mass, improving oxygen delivery capacity and reducing cardiovascular strain at any ambient temperature. Several studies have documented meaningful improvements in time-trial performance in cool conditions following heat acclimatization protocols, an effect size that rivals more logistically complex interventions like altitude camps.
Despite this evidence, heat acclimatization remains underutilized in American athletic preparation. Many recreational and even competitive athletes view deliberate heat exposure as a risk management problem rather than a performance-enhancement opportunity, a framing that the research increasingly suggests is misguided for those with appropriate health clearance and supervised implementation.
The High-Temperature Performers: What Makes Them Different
Perhaps the most intriguing aspect of the emerging thermoregulation literature involves athletes who appear to sustain high-intensity output at core temperatures that would ordinarily predict significant performance decrements. Studies examining elite endurance competitors during marathon and ultramarathon events have documented rectal temperatures exceeding 40°C (104°F) in athletes who nonetheless maintained competitive pace and showed no clinical signs of heat illness.
Several mechanisms have been proposed to explain this phenomenon. One centers on a recalibrated central fatigue threshold — essentially, a brain that has learned through repeated thermal exposure to tolerate higher internal temperatures before invoking protective fatigue signals. Another involves superior peripheral adaptations: more efficient local blood flow regulation in working muscles that maintains oxygen delivery even as central cardiovascular resources are being taxed by thermoregulatory demands.
There is also evidence suggesting that highly trained athletes exhibit a more favorable ratio of heat production to heat dissipation during exercise — not because they produce less heat, but because improvements in movement economy reduce the metabolic cost of a given pace, thereby generating less thermal byproduct per unit of work performed. In this framing, the athlete who runs hot and still performs is often also the athlete who runs efficiently.
Rethinking Universal Cooling Protocols
The implication of this research for practical athletic preparation is not that cooling strategies should be abandoned — heat illness remains a genuine and serious risk, particularly in high-humidity environments where evaporative cooling is impaired. Rather, the evidence suggests that blanket, one-size-fits-all cooling protocols may be suboptimal for athletes who have developed meaningful thermal tolerance.
Pre-cooling interventions such as ice vests, cold water immersion, and menthol application have demonstrated performance benefits primarily in athletes who have not undergone systematic heat acclimatization, or in conditions where thermal load is genuinely excessive relative to the athlete's adaptive capacity. For the heat-acclimatized, thermally tolerant competitor, aggressive pre-cooling may actually blunt some of the physiological priming — including elevated skin blood flow and earlier sweating onset — that their adaptations have conferred.
A more individualized approach, informed by an athlete's acclimatization history, genetic predispositions where testable, and empirical performance data across thermal conditions, is likely to produce better outcomes than universal application of cooling protocols derived from population averages.
The Practical Takeaway
Thermoregulation is not a fixed physiological ceiling — it is a dynamic, trainable, and individually variable capacity that deserves the same rigorous attention athletes devote to VO2 max development or strength periodization. For coaches and athletes willing to engage seriously with the science, structured heat acclimatization protocols offer a legitimate and evidence-supported pathway to performance gains that extend well beyond warm-weather competition. And for the subset of athletes who already run hot without consequence, understanding that thermal tolerance may represent a genuine competitive advantage — rather than a liability to be managed — is a meaningful shift in perspective that the research increasingly supports.
