Author(s): SURAPONGCHAI, J., RAMLEE, K., AREERAK, K., Institution: MAHIDOL UNIVERSITY , Country: THAILAND, Abstract-ID: 2299

INTRODUCTION:
Sex differences in thermoregulatory responses during exercise in the heat remain incompletely understood, particularly under protocols that standardize relative exercise intensity. Ice ingestion is an established internal cooling strategy; however, limited evidence exists regarding frozen isotonic formulations and their potential sex-specific thermophysiological effects. This study compared physiological and perceptual responses between males and females during prolonged exercise in hot-humid conditions and examined the effect of isotonic ice bar ingestion.
METHODS:
Twenty healthy participants (10 males, 10 females) completed two randomized crossover trials: ambient water (AMB) and ice bar ingestion (ICE; −1°C) in an environmental chamber set at 32°C and 70% RH. Participants ingested 5 g·kg⁻¹ body mass during a 30-min pre-cooling phase and 2 g·kg⁻¹ during each subsequent rest interval. Exercise was performed at 55% VO₂peak to standardize relative intensity. Core temperature (Tc), skin temperature, heart rate (HR), whole-body sweat loss, perceptual responses (RPE, thermal sensation, thermal comfort), peak Tc, and time to exhaustion (TTE) were assessed.
RESULTS:
ICE reduced Tc during pre-cooling in both sexes (ΔTc: males −0.27±0.18°C vs −0.05±0.12°C; females −0.29±0.22°C vs +0.04±0.10°C; p<0.05), with no sex difference in cooling magnitude. Tc during prolonged exercise did not differ between ICE and AMB. Despite matched relative intensity, males exhibited greater metabolic heat production per BSA (240±13 vs 200±20 W·m⁻², p<0.001), higher peak Tc (38.9±0.3°C vs 38.4±0.2°C, p<0.05), and higher HR during mid-to-late exercise phases (p<0.05). Whole-body sweat loss was numerically greater in males (AMB: 1.7±0.4 L; ICE: 1.5±0.5 L) than in females (AMB: 1.1±0.3 L; ICE: 1.1±0.5 L). Females demonstrated longer TTE in both trials (p<0.05). ICE did not influence peak Tc, perceptual responses, or performance in either sex.
CONCLUSION:
When exercise intensity is standardized relative to aerobic capacity, males exhibit greater thermophysiological strain than females, reflected by higher metabolic heat production, peak core temperature, and cardiovascular strain. Although ice bar ingestion transiently lowers core temperature before exercise, it does not attenuate peak thermal strain or improve endurance performance. Because the protocol controlled relative, but not absolute, exercise intensity, greater absolute metabolic load in males likely underlies the observed sex differences during prolonged exercise in the heat. These findings indicate that matching relative aerobic intensity does not ensure equivalent thermal strain between sexes under heat stress, and the cooling effect of ice bar ingestion appears confined to the pre-exercise phase under sustained thermal load.