The Effect of Ice-Slurry Ingestion on Whole-Body Heat Exchange During Intermittent Exercise in Young and Older Adults

Abstract

Inadequate fluid replacement during work in the heat is associated with a blunting of thermoregulatory function and a reduction in physical performance. Regulatory agencies recommend the consumption of cold fluids to replenish fluid losses from sweating to maintain thermoregulatory function and productivity during prolonged work in the heat. However, the consumption of cold beverages has been associated with a reduction in sweat output which limits evaporative heat loss during prolonged continuous exercise in young men. Previous research has not analyzed how cold fluid ingestion influences heat loss responses during intermittent exercise or how age may modulate the effect of cold fluid ingestion on whole-body heat exchange. Thus, the purpose of this study was to assess the effect of ingesting a cold beverage (ice-slurry) on whole-body heat exchange in young (22 (3) years) and older (60 (4) years) adults during intermittent exercise in the heat and to ascertain if age modulated this response. Participants performed four 15-min bouts of moderate-intensity cycling at a fixed rate of heat production (200 W‧m⁻², representative of the typical work effort observed in many physically demanding occupations), each interspersed by a 15-min rest in dry heat (40°C, ~12% relative humidity). On separate days, participants consumed either ice-slurry (~0°C), standardized to provide a heat transfer capacity of 75 kJ‧m⁻², or an identical mass of warm fluid (37.5°C) before the first and third exercise bout. Evaporative and dry heat exchange (assessed via direct calorimetry), metabolic heat production (assessed via indirect calorimetry), body temperatures (rectal and mean skin temperature), and heart rate were measured continuously. Total heat loss (evaporative + dry heat exchange), rectal temperature, mean skin temperature, and heart rate reserve were assessed in the final 5-min of each exercise bout. In addition, cumulative heat storage (temporal summation of metabolic heat production, total heat loss and the internal transfer of heat to the cold fluid) over the entire intermittent exercise protocol was determined. All continuous measurements were analyzed in young and older males separately, whereas cumulative heat storage was compared between young and older adults. No effect of beverage temperature was observed on total heat loss, rectal temperature, mean skin temperature, or heart rate reserve during exercise or rest in young or older males (all P ≥ 0.15). While ice-slurry ingestion reduced cumulative heat storage relative to warm fluid ingestion in both young (216 (94) vs. 69 (181) kJ) and older males (259 (143) vs. 91 (104) kJ, both P ≤ 0.04). However, cumulative heat storage did not differ between groups (P = 0.49). We show that consuming cold beverages during intermittent work in the heat, reflective of commonly recommended work/rest cycles, is an appropriate administrative control. This is evidenced by a reduction in cumulative heat storage following ice-slurry relative to warm fluid ingestion, as well as no influence of beverage temperature on total heat loss in young or older males. As cold beverages are typically more palatable than warmer beverages, workers should be encouraged to consume cold fluid during work/rest cycles given that it may increase the likelihood of maintaining or restoring hydration status while performing work in the heat.