Author(s): BRAZAITIS, M., DAUKSAITE, G., SOLIANIK, R., EIMANTAS, N., Institution: LITHUANIAN SPORTS UNIVERSITY, Country: LITHUANIA, Abstract-ID: 516

INTRODUCTION:
Heat acclimation (HA) or acclimatization is a process where the body adapts to repeated exposure to heat, resulting in improvements to thermoregulatory mechanisms and heat tolerance (1,2,3). This adaptation leads to enhanced exercise performance in warm-to-hot conditions. The widely accepted consensus recommends HA sessions lasting at least 60 minutes per day, aiming to increase both body core and skin temperatures while stimulating sweating (1,2,3). However, here we propose that daily activation of nociceptor TRP channels through 5-minute immersion in 45°C water could be the primary mechanism driving HA induction.
METHODS:
Ten healthy young adult males and ten females participated in a study where they underwent daily 5-minute whole-body immersions in 45°C water for 14 days. A provoking test for heating tolerance was conducted two weeks before, one day after, and one month after the HA program. Changes in well-established HA parameters such as body temperatures, heart rate, sweating, subjectively perceived stress, and plasma prolactin concentrations in response to provoking heating were measured.
RESULTS:
The HA program significantly lowered both resting and post-heating body temperatures (p < 0.05), heart rate (p < 0.05), subjective warmth perception (p < 0.05), and prolactin activity response (p < 0.05) in both male and female subjects. These effects remained significant one month after completing the HA program. However, the HA program did not have a significant effect on sweating capacity in either group of subjects (p > 0.05).
CONCLUSION:
This study, for the first time, demonstrated that undergoing a repeated 14-session head-out hot-water immersion over 14 days induced HA in both sexes. Furthermore, it was observed that the HA effect persisted significantly one month after the HA program. Our study provides strong mechanistic evidence supporting the notion that short-term daily activation of nociceptor TRP channels is crucial for successful HA induction.

References
1. Racinais S et al., Consensus recommendations on training and competing in the heat. Sports Med. 2015.45:925-938.
2. Kelly MK et al., Heat adaptation for females: A systematic review and meta-analysis of physiological adaptations and exercise performance in the heat. Sports Med. 2023 53:1395-1421.
3. Sotiridis A et al., Are five 60-min sessions of isothermic heat acclimation sufficient to elicit beneficial physiological adaptations?