Endurance running in hot, humid environments strains the body’s cooling and circulatory systems. Heat acclimation produces coordinated adaptations—cardiovascular, sudomotor, and thermal—that preserve performance when evaporative cooling is limited by humidity. Evidence-based reviews by Michael N. Sawka, United States Army Research Institute of Environmental Medicine, and guidance from Michael J. Casa, University of Connecticut, describe these physiological changes and their performance consequences.
Physiological mechanisms
Acclimation increases plasma volume, which helps maintain stroke volume and cardiac output at a given work rate, thereby reducing heart rate for the same pace. It also enhances sweat rate and shifts sweating to begin at a lower core temperature, improving heat dissipation when evaporation is possible. Skin blood flow regulation becomes more efficient, and resting and exercising core temperatures typically fall after acclimation. These combined adaptations lower cardiovascular strain and perceived exertion, enabling athletes to sustain higher intensities in the heat. In humid conditions the effectiveness of increased sweating is constrained because higher ambient moisture slows evaporation, but the cardiovascular benefits remain important.
Practical implications for humid marathons
For runners from temperate climates, structured acclimation — repeated daily exposure to heat for 7–14 days, often with moderate-intensity exercise — is recommended to trigger these adaptations. Michael J. Casa, University of Connecticut, highlights that combining heat exposure with progressive training sessions and attention to hydration optimizes benefits while minimizing risk. Acclimation reduces heat illness risk and often improves time-trial and endurance performance in hot environments by preserving pacing and delaying exhaustion.
Cultural and territorial realities shape implementation. Athletes traveling to tropical marathons should plan acclimation near the race venue when possible, accounting for local housing, work schedules, and access to cooling. Resource limitations and urban heat island effects can complicate acclimation strategies for recreational runners in some regions. Clothing choices, acclimation timing relative to competition, and in-race cooling and hydration tactics also vary by local climate and cultural practices.
Consequences of inadequate acclimation include earlier fatigue, higher core temperatures, impaired pacing, and increased incidence of heat-related illness. Conversely, well-executed acclimation supports safer and faster performance in humid marathons by stabilizing cardiovascular function and reducing thermal load, even when evaporative cooling is partially compromised.