During a marathon, a runner's core body temperature can reach 105°F—the same level as a severe fever—yet trained athletes safely sustain this heat through extraordinary physiological adaptation.

When a human runs a marathon, the body's metabolic rate can increase to 15 times its resting level. Research published in the Journal of Applied Physiology shows that skeletal muscles are relatively inefficient, converting only about 20–25% of energy into mechanical work while the remaining 75–80% is lost as heat. This thermal energy must be dissipated quickly to prevent cellular damage and organ failure.A landmark study by Dr. Alberto J. Salazar and colleagues during the 1980s monitored marathon runners and found that core temperatures frequently exceeded 102.2°F, with several elite athletes reaching 105°F. At these levels, the hypothalamus triggers vasodilation, which sends warm blood to the skin surface for cooling. This process depends heavily on the evaporation of sweat, the body's primary defense against hyperthermia during prolonged exercise.The key difference between a marathoner's elevated temperature and a clinical fever lies in the hypothalamus's "set point." During a fever, the body intentionally raises its target temperature to fight infection, whereas during exercise, the temperature rise is an unavoidable byproduct of physical work. If a runner cannot maintain balance between heat production and heat loss, they risk exertional heatstroke—a critical condition that becomes dangerous when core temperatures exceed 104°F for extended periods.Modern sports medicine now uses ingestible thermometer pills to track these temperature fluctuations in real-time. Data from the Boston Marathon and other major races demonstrate that environmental factors like humidity significantly impair a runner's ability to cool down. Despite these risks, the human body's capacity to function at 105°F underscores an extraordinary physiological adaptation to extreme physical stress.