Short-term heat acclimation prior to a multi-day desert ultra-marathon improves physiological and psychological responses without compromising immune status

Multi-stage, ultra-endurance events in hot, humid conditions necessitate a-priori thermal adaptation, often achieved through heat acclimation (HA), to improve performance by reducing thermoregulatory strain and perceptions of heat stress. This study investigated the physiological, perceptual and immunological responses to short-term HA (STHA) in athletes preparing for the Marathon des Sables. Methods Eight ultra-endurance athletes (age; 42±4 yrs, mass; 81.9±15.0 kg and body fat; 17.6±5.9%) completed 4 days of controlled hyperthermia STHA (60 min·day -1, 45°C and 30% relative humidity). Pre, during and post sessions, physiological and perceptual measures were recorded. Immunological measures were recorded pre-post session 1 and 4. Results STHA improved peak thermal comfort (-1,P=0.02), sensation (-1,P=0.03) and perceived exertion (-2,P=0.04). A dissociated relationship between perceptions of fatigue and Tre was evident after STHA, with reductions in perceived physical (-6,P=0.04) and general (-2,P=0.04) fatigue. Exercising Tre and HR did not change (P>0.05), however, sweat rate increased 14% (P=0.02). No changes were found in white blood cell counts or content (P>0.05). Conclusions Four days of STHA facilitates effective perceptual adaptations and lower feelings of fatigue, without compromising immune status prior to an ultra-endurance race in heat stress. A greater and prolonged physiological strain is required to confer optimal physiological adaptations.