Ice Slurry Ingestion and Physiological Strain During Exercise in Non-Compensable Heat Stress

INTRODUCTION: Precooling with ice slurry ingestion attenuates the increase in rectal temperature (T-re) during subsequent running and cycling. It remains unclear how this cooling method affects physiological strain during work while wearing protective garments. This study investigated the effect of ice slurry ingestion on physiological strain during work in hot conditions while wearing firefighter protective clothing. METHODS: In three counterbalanced trials, eight men (mean +/- SD; age = 21 +/- 2 yr, height = 179.5 +/- 3.5 cm, mass = 79.1 +/- 4.1 kg, body fat = 11.4 +/- 3.7%) wore firefighter protective clothing and walked (4 km.h(-1), 12% incline, similar to 7 METs) for 30 min in hot conditions (35 degrees C, 40% RH). Every 2.5 min, subjects ingested 1.25 g.kg(-1) (relative total: 15 g.kg(-1), absolute total: 1186.7 +/- 61.3 g) of a tepid (22.4 +/- 1.7 degrees C), cold (7.1 +/- 1.5 degrees C), or ice slurry (-1.3 +/- 0.2 degrees C) beverage. RESULTS: Heart rates (HR) were lower with ice slurry ingestion compared to both fluid trials starting 5 min into exercise (tepid = 158 +/- 14, cold = 157 +/- 11, ice slurry = 146 +/- 13 bpm) and persisting for the remainder of the bout (min 30: tepid = 196 +/- 10, cold = 192 +/- 10, ice slurry = 181 +/- 13 bpm). T-re was lower with ice slurry ingestion compared to cold and tepid trials (min 5: tepid = 37.17 +/- 0.38, cold = 37.17 +/- 0.39, ice slurry = 37.05 +/- 0.43 degrees C; min 30: tepid = 38.15 +/- 0.29, cold = 38.31 +/- 0.36, ice slurry = 37.95 +/- 0.32 degrees C). The physiological strain index (PSI) was lower with ice slurry ingestion compared to fluid trials starting at min 5 (tepid = 3.8 +/- 0.7, cold = 3.8 +/- 0.6, ice slurry = 3.0 +/- 0.5) and remained lower throughout exercise (min 30: tepid = 8.2 +/- 0.6, cold = 8.3 +/- 0.9, ice slurry = 6.9 +/- 1.2). DISCUSSION: A large quantity of ice slurry ingested under non-compensable heat stress conditions mitigated physiological strain during exercise by blunting the rise in heart rate and rectal temperature.