Physiological Mechanisms of Heat Acclimation Training: A Longitudinal Study on Plasma Volume Expansion and Sweat Rate Enhancement

Based on the Journal of Applied Physiology, Medicine & Science in Sports & Exercise, and Sports Medicine among other international peer-reviewed journals, this article provides an in-depth analysis of the environmental physiological mechanisms of “heat acclimation training,” and combines Taiwan’s subtropical climate, mountainous terrain, and local race scenarios to offer evidence-based training and race preparation strategies.

In the field of environmental physiology, “heat acclimation training” is one of the key variables determining whether endurance athletes can perform to their full potential on race day. Many amateur and elite athletes focus their training on power output, pacing, and equipment, yet underestimate the enormous physiological impact of the environment—heat, cold, high altitude, humidity, and air quality. In fact, when two athletes have similar fitness and equipment, the one who better understands how to adjust strategies in response to the environment is often the one who can maintain pace in the latter stages of a race, avoiding collapse and mishaps. Taiwan’s geographical environment is particularly unique: hot and humid summers, cold and damp winters, extreme altitude differences from sea level to Wuling at 3,275 meters, plus urban air pollution and typhoon season—all of which make environmental physiology exceptionally valuable in the local sports community. This article will take you from cellular and systemic physiological mechanisms, through empirical research in top international journals, quantitative dose-response relationships, differences in responses across populations, to directly applicable training methods and Taiwan-specific scenarios, and finally debunk long-standing myths, so that your understanding of “heat acclimation training” is truly built on science rather than hearsay.

Review of Academic Research

Regarding the scientific exploration of “heat acclimation training,” the field of environmental physiology has accumulated rigorous and rich evidence. Below are several representative studies selected for their value in methodological design, subject populations, and strength of conclusions, which together form our current understanding:

1. Périard, Racinais & Sawka (2015). Scandinavian Journal of Medicine & Science in Sports systematically reviewed nearly a hundred heat acclimation studies, quantifying the time course of changes in plasma volume, sweat rate, heart rate, and core temperature over days.

2. Lorenzo et al. (2010). Journal of Applied Physiology had cyclists undergo 10 days of 90-minute daily heat training, resulting in a simultaneous improvement of ~5% to 8% in time-trial power output in both cool and hot environments.

3. Garrett et al. (2012). European Journal of Applied Physiology demonstrated that a 5-day short-term heat acclimation protocol can induce significant plasma volume expansion, suitable for amateur athletes with limited time.

4. Sawka et al. (2011). Experimental Physiology used a heat balance model to describe the interactive suppression of maximal oxygen uptake by skin temperature and dehydration.

Looking across these studies, it is clear that the scientific picture of “heat acclimation training” has deepened with advances in measurement technology. Early studies mostly manipulated temperature, humidity, or partial pressure of oxygen in environmental chambers, observing changes in maximal oxygen uptake, time to exhaustion, or time-trial performance; subsequent research introduced ingestible core temperature capsules, near-infrared spectroscopy, stable isotope tracers, muscle biopsies, and molecular markers, allowing us to move from “phenomenological observation” to “mechanistic explanation.” Notably, most high-quality studies employed randomized crossover designs, with each subject serving as both experimental and control conditions, greatly reducing noise from individual differences. However, extrapolation of research conclusions still requires caution: a single environmental variable controlled in the laboratory does not necessarily equate to the complex combination of heat, humidity, wind, radiation, and fatigue encountered on a real race course; nor do responses from well-trained subjects necessarily apply to the general amateur population. When interpreting “statistical significance” and “effect sizes,” we must also distinguish between average trends in the laboratory and practical significance for an individual on race day—a 3% performance difference may determine rankings in elite competition, while its meaning for a recreational rider is relatively limited. It is precisely this careful attention to levels of evidence and scope of applicability that forms the foundation of scientific training.

Core Mechanisms

The core of heat acclimation is the overall upregulation of the thermoregulatory system, which can be divided into two temporal phases. The first phase (days 1–5) is dominated by cardiovascular adaptations: repeated heat exposure stimulates aldosterone and antidiuretic hormone secretion, prompting the kidneys to retain sodium and water, and increasing plasma albumin, which together drive plasma volume expansion of approximately 10% to 12% within one week. The rise in plasma volume increases stroke volume and lowers exercising heart rate, significantly reducing cardiovascular strain at the same intensity. The second phase (days 6–14) is dominated by sweat gland adaptations: sweat gland sensitivity to acetylcholine increases, the sweating threshold shifts downward, maximum sweat rate can increase from 1.0 L/h to 1.5–2.0 L/h, and sweat sodium concentration decreases, indicating enhanced sodium reabsorption capacity of the sweat glands. At the cellular level, expression of heat shock protein HSP72 increases, protecting proteins from heat denaturation and raising the cellular heat tolerance threshold. Together, these adaptations lower the exercising core temperature plateau by 0.2–0.4°C, delaying central fatigue.

To truly understand how “heat acclimation training” affects athletic performance, one must return to the systemic integration of thermoregulation, cardiovascular, respiratory, metabolic, and central nervous systems. Environmental factors translate into measurable performance differences precisely because they act on one or more key links in this physiological chain—either limiting oxygen and fuel delivery, disrupting heat dissipation and fluid balance, or altering central perception of fatigue and motor drive. The table below summarizes the key effects of this topic at different physiological levels, helping you build a complete mechanistic picture:

| Physiological Level | Key Mechanisms | Significance for Athletic Performance |

|—|—|—|

| Cardiovascular System | Affects blood flow distribution, stroke volume, and circulatory volume | Determines oxygen delivery and stability during prolonged exercise |

| Thermoregulation/Fluid Balance | Regulates heat dissipation pathways, sweat rate, and electrolyte balance | Affects rate of core temperature rise and dehydration risk |

| Muscle and Metabolism | Alters substrate utilization, enzyme activity, and contractile function | Determines sustainable power output and timing of fatigue onset |

| Central Nervous System | Regulates perceived fatigue, motor drive, and cognitive decision-making | Influences “how hard it feels” and the ability to persist and make safe judgments |

Special emphasis should be placed on the two dimensions of “dose-response” and “temporal dynamics.” The same environmental exposure, at different intensities, durations, and acclimation states, can produce vastly different or even opposite effects—this is precisely why many popular recommendations are one-sided. The limiting factors of performance also shift dynamically with context: for short-duration high-intensity efforts, limitations mostly stem from anaerobic energy supply and local metabolism, while for multi-hour endurance events, the focus shifts to the combined effects of rising core temperature, fluid imbalance, glycogen depletion, and central fatigue. “Heat acclimation training” deserves in-depth exploration precisely because it can target certain of these limiting factors. Understanding the mechanisms allows us to judge “how to adjust for a given environment, by how much, and when”—rather than being led by the environment on race day. The more thoroughly you understand the mechanisms, the more flexibly you can respond across different situations such as heat, cold, high altitude, or pollution. This ability to adapt to context is precisely the dividing line between those who understand environmental physiology and those who train blindly.

Dose-Response Relationship

In environmental physiology, “dose determines effect” is a core principle. Environmental stimuli that are too low fail to reach the physiological threshold, producing no adaptation or impact; excessive exposure, on the other hand, may exceed the body’s compensatory capacity, triggering risks such as heat injury, hypothermia, and altitude sickness. The table below summarizes the dose-response correspondence for “heat acclimation training” and serves as the most important quantitative reference when designing training and race preparation plans:

| Dose / Condition | Physiological State | Effects and Key Points |

|—|—|—|

| 1–5 days, 60–90 min daily | Cardiovascular adaptation phase | Plasma volume +8–12%, lower exercise heart rate |

| 6–10 days | Sweat gland adaptation phase | Sweat rate +30–50%, core temperature plateau shifts downward |

| 11–14 days | Integration and consolidation phase | Sweat sodium −25%, benefits approach a plateau |

| From 1 week after cessation | De-acclimation | Plasma volume declines first; sweat gland adaptation is retained longer |

As the table shows, the effects of environmental exposure often follow a threshold-type or inverted U-shaped curve: before reaching the effective dose, adaptation or impact increases with dose; but beyond a certain critical point, there is not only no additional benefit—risk and cost rise sharply instead. This means that “finding your own optimal exposure” matters far more than “relentlessly pursuing extremes.” Whether it is the daily heat dose for heat acclimation, the altitude and duration of altitude training, or the exposure time in cold environments, there exists a sweet spot that balances benefit and safety. It is recommended to progressively test your responses under different environmental conditions during training (not on race day), recording core temperature or heart rate, perceived exertion, power data, and recovery status to build your own environmental response profile. Remember: the laboratory average is a starting point, not an endpoint; everyone’s body size, sweat rate, acclimation status, and genetic background cause the optimal dose to shift individually. Only by calibrating with your own data can you translate population science into a personal prescription.

Differences Across Populations

The impact of “heat acclimation training” is not equal for everyone. Age, sex, training status, body size, acclimation state, and genetic background all significantly modulate an individual’s magnitude of response to the environment. Applying a one-size-fits-all recommendation while ignoring these differences is one of the most common errors in applied environmental physiology.

| Population Dimension | Response Characteristics | Practical Recommendations |

|—|—|—|

| Beginners vs. Advanced athletes | Advanced athletes have more mature environmental adaptation and better tolerance, but less room for marginal adaptation | Beginners should progress conservatively, building baseline tolerance before increasing load |

| Male vs. Female | Differences in body surface area/body mass ratio, hormonal cycles, and sweat composition | Females should receive individualized assessment of heat dissipation and hydration, with attention to energy availability |

| Young vs. Older | Older individuals have reduced thermoregulation, sweat gland function, and compensatory capacity | Older individuals are more sensitive to extreme environments and require greater caution and longer adaptation |

| Body size differences | Body surface area, body fat, and muscle mass affect heat dissipation and heat production | Larger individuals dissipate heat less easily; smaller individuals cool down faster—each carries its own risks |

When interpreting individual differences, one must also be wary of a statistical trap: studies typically report “group mean responses,” but beneath the average often lies enormous individual variability. In the same environmental intervention, some may be strong responders, some barely respond, and some may even respond in the opposite direction. This is why even when a study shows “effective on average,” you still need to confirm through your own experimentation which category you fall into. It is recommended to conduct personalized controlled tests: across two training sessions under similar conditions, with and without a given environmental strategy (e.g., pre-cooling, heat acclimation), compare power, core temperature, heart rate, and subjective sensation, repeating several times before drawing conclusions.

Take Taiwan’s common amateur endurance population as an example: many are masters athletes and runners over 35 who train around work schedules. This group’s thermoregulation and recovery capacity are already somewhat inferior to young elites. When facing the hot-humid summer and high-altitude challenges, correct environmental strategies (heat acclimation, individualized hydration, gradual ascent) can yield relatively greater safety and performance gains. Female athletes, meanwhile, need to pay attention to the influence of the menstrual cycle on baseline core temperature and fluid regulation, as well as whether energy availability is sufficient. After understanding population differences, you will realize: truly professional environmental physiology advice is always an individualized prescription that is “different for each person,” never a one-size-fits-all slogan.

Practical Training Application

Theory must ultimately be translated into training plans and race courses. Below is a practical framework for converting “heat acclimation training” into concrete training and race-day operations:

• Pre-race environmental rehearsal: All environmental strategies must first be rehearsed in training—“never try anything new on race day” is an iron rule. Whether heat acclimation, pre-cooling, or hydration pacing, the body needs time to build tolerance and familiarity.

• Periodization mindset: Align environmental adaptation with the training cycle—build tolerance in the base phase, and target the specific temperature, humidity, or altitude of your goal event during the pre-race phase.

• Progressive exposure: Start with mild environmental stimuli and gradually increase the load based on bodily responses, building a personalized exposure dose and timing profile to avoid injury from a single excessive exposure.

• Objective data monitoring: Combine core temperature (or heart rate drift), power, rating of perceived exertion (RPE), and urine/body weight changes to objectively evaluate whether environmental strategies are truly effective.

• Holistic context: Environmental adaptation is one part of training, sleep, recovery, and nutrition; no single strategy can compensate for sleep deprivation, dehydration, or flawed training design.

Using a one-week training schedule as an example, you can rehearse different environmental scenarios in key midweek sessions and weekend long rides: high-intensity days focus on maintaining power quality in the target environment and testing heat-dissipation or insulation gear; long-distance days emphasize hydration and sodium pacing, thermoregulation, and long-duration gear tolerance. Through repeated rehearsal, your body can respond to the environment in a near-automatic manner on race day, freeing mental resources for pacing and tactical decisions. The most common mistake many people make is “only taking the environment seriously on race day while treating daily training casually”—this is precisely putting the cart before the horse. Daily training is the best laboratory for building heat tolerance, testing hydration doses, familiarizing yourself with gear, and cultivating an environmental coping rhythm.

It is recommended to integrate an environmental log with your training log, recording for each key session the day’s temperature, humidity, altitude, wind conditions, the environmental strategies used, bodily responses, and performance data. After weeks to months of accumulation, the value of this personalized database will far exceed any generic guide. Furthermore, do not overlook the often-underestimated aspect of “recovery in the environment”—heat or altitude slows recovery and accelerates fatigue accumulation. The quality of recovery between consecutive training days often determines whether you can steadily accumulate training volume without injury, and training volume is the most fundamental engine of long-term progress. Treat environmental management as a serious part of training rather than a last-minute accessory before race day, and both your progress and your safety will be markedly different.

Local Application in Taiwan

Taiwan’s unique climate, terrain, and race culture add a distinct local flavor to the application of “heat acclimation training.” In summer, high temperature and humidity mean the apparent temperature often exceeds 35°C, with relative humidity frequently above 80%; evaporative cooling is inefficient, and sweat and electrolyte losses far exceed the scenarios studied in temperate countries. This means recommendations from foreign literature often need to be adjusted upward. In winter, the northeast monsoon brings damp cold and strong winds, and mountainous areas carry risks of hypothermia and low temperatures. And the extreme elevation gain from sea level to Wuling at 3,275 meters puts high-altitude physiological challenges right at your doorstep.

Taking events such as the West-to-East Wuling climb, the Hualien–Taitung Cycling Tour, the Sun Moon Lake loop, the Taroko Marathon, triathlons across the island, and round-island challenges as examples, athletes should factor local and seasonal environmental conditions into their planning: summer events should start earlier to avoid high WBGT in the afternoon, and take advantage of the dense convenience-store network along routes to strengthen fluid and sodium replenishment; high-altitude sections should allow for gradual acclimatization, with attention to cold protection and signs of altitude sickness; winter wet-cold events require stronger windproofing, insulation, and hypothermia protection. By making good use of the Central Weather Administration’s forecasts for apparent temperature, humidity, wind speed, and air quality—converting them into daily environmental risk assessments and strategies—you can balance safety and peak performance in Taiwan’s changeable and demanding environment.

Common Myth-Busting

Myth: “Drinking plenty of water is enough to combat high temperatures; no special heat acclimation is needed.” In reality, hydration cannot replace physiological adaptation—even with adequate hydration, unacclimatized individuals still experience faster rises in core temperature and more pronounced performance declines; true plasma volume expansion and sweat gland remodeling require at least 5 to 7 days of continuous heat stimulus.

This type of myth spreads widely, often because it “sounds reasonable,” is easy to pass along by word of mouth, or is amplified by marketing and anecdotal experience. Yet the value of science lies precisely in testing intuition with rigorous evidence: many seemingly obvious environmental notions fail to hold up under controlled environmental chamber experiments and epidemiological investigations. The field of environmental physiology is especially rife with oversimplified claims that compress complex dosages, timing, individual differences, and risks into a single slogan. The next time you hear a categorical environmental recommendation, it’s worth asking: “What is the level of evidence behind this claim? Who is the target population? Are the dosage, timing, and safety margins clearly defined?” Cultivating this evidence-based critical thinking is more valuable than memorizing any single conclusion, and it is a key step for athletes toward a more scientific approach and avoiding environmental injuries.

Conclusion

“Heat acclimation training” is a topic in environmental physiology that combines both theoretical depth and practical value. As can be seen from the international journal evidence reviewed in this article, the environment’s impact on athletic performance is real and profound, but by no means unmanageable—the key lies in understanding the mechanisms, mastering the dosage, individualizing adjustments, and coordinating with overall training, recovery, and nutrition. For endurance sports enthusiasts in Taiwan, while grasping the scientific principles, it is equally important to integrate the local humid heat, extreme altitudes, and variable weather, transforming general rules into personalized prescriptions suited to oneself. May every cyclist and runner sweating on Wuling, in the rift valleys, and along the island-circumnavigation route protect themselves, push their limits, and enjoy the purest joy of sport through the wisdom of environmental physiology—whether in heat, cold, high altitude, or various other challenges. Before you next step onto the race course, don’t forget—your true opponent, besides the timer, is the entire environment beneath and around you.

Related Reading

• Physiological Responses to Cold-Weather Cycling: Research on Vasoconstriction, Muscle Metabolism, and the Respiratory Tract
• Designing Interval Training in Hot Environments: Research on Thermoregulatory Strategies During Recovery Intervals
• Mitochondrial Adaptations to Training in High Temperatures: Research on the Induction Mechanisms of Heat Shock Protein HSP
• The Impact of Climate Change on Cycling Events in Taiwan: Physiological Prediction Research on Rising Temperatures