A rise in body core temperature and loss of body water through sweating are natural consequences of prolonged exercise in the heat.

Despite the ability of humans to regulate body core temperature within a narrow range in a variety of scenarios, exercise under heat stress delivers a unique and integrative challenge to physiological function. The factors that include the thermal environment specify the compensability of a setting and if thermoregulatory responses could maintain heat balance. Under heat stress, persistent exercise capacity and performance are harmed as heat gain frequently exceeds heat loss. This impairment is mainly intervened by hyperthermia-induced adjustments in cardiovascular, central nervous system, and skeletal muscle function. Defeat or inability to restock excessive body water loss causes a reduction in blood (plasma) volume and a rise in osmolality that compromises thermoregulatory capacity and further worsens the rise in thermal strain, precipitating the development of fatigue. 

The judgment to discontinue exercise at a constant work rate or reduce self-paced exercise work rate in the heat, with or without hypohydration, occurs across various physiological and perceptual responses. These responses are specific to the exercise being undertaken, the intensity of effort, prevalent ambient conditions, contextual parameters, and individual expectations. 

Disagreement exists regarding the best way to hydrate during exercise, thus identical factors should be considered when deciding on which hydration approach to utilize. Factors like intensity and duration of the exercise task, ambient conditions, availability of fluids, and fluid needs must be considered.

Numerous active and/or passive techniques can be utilized to induce adaptations, with regimens of 10–14 days proposed to optimize responses. Pre- and per-cooling procedures may also improve persistence performance in the heat. These techniques utilize either internal (e.g., ice-slurry ingestion) or external (e.g., cold water immersion) cooling, with the preference of an appropriate technique specified by its efficacy, feasibility, and event-specific compatibility. 

Hyperhydration using water or co-ingested with sodium, glycerol, or creatine increases the body water content and has been suggested to maintain fluid homeostasis for longer during exercise in the heat. 

Although many of these approaches have been reported to provide benefits, further performance and mechanistic studies are warranted.

Source: Physiological Reviews. 2021;101(4): 1873-1979. https://doi.org/10.1152/physrev.00038.2020