Background To mitigate health risks associated with occupational heat stress, workers are advised to adhere to a work-rest regimen, and hydrate regularly. However, it remains unclear if beverage temperature influences whole-body heat exchange during work-rest cycles, and if responses differ in older workers who have a blunted heat loss capacity.

Methods Ten young (mean [SD]: 22 [3] years) and 10 older (60 [4] years) males performed four 15-min bouts of moderate-intensity cycling at a fixed rate of metabolic heat production (200 W·m−2), each interspersed by 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 bouts. Evaporative and dry heat exchange (direct calorimetry) and metabolic heat production (indirect calorimetry) were measured continuously to determine cumulative heat storage (summation of heat loss and heat gain) over the entire protocol. Rectal temperature was also measured continuously.

Results Relative to warm fluid, ice-slurry ingestion reduced cumulative heat storage in young (69 [181] vs. 216 [94] kJ) and older males (90 [104] vs. 254 [140] kJ, main effect: p < 0.01), but was unaffected by age (p = 0.49). However, rectal temperature was unaffected by beverage temperature in both groups (all p ≥ 0.15).

Conclusion We show that cold fluid ingestion is an appropriate administrative control for both young and older males as it can mitigate increases in body heat content during moderate-intensity work-rest cycles in dry heat.