LAUSR

PURPOSE To assess the impact of acute caffeine ingestion on thermoregulatory responses during steady-state exercise under moderate heat stress conditions in caffeine-habituated and non-habituated individuals. METHODS 28 participants (14 habituated (HAB) (4 females); and 14 non-habituated (NHAB) (6 females)) cycled at a fixed metabolic heat production (7 W·kg-1) for 60 minutes on two separate occasions 1 hour after ingesting i) 5 mg·kg-1 caffeine (CAF) or ii) 5 mg·kg-1 placebo (PLA), in a double-blinded, randomized and counter-balanced order. Environmental conditions were 30.6±0.9°C, 31±1% RH. RESULTS The end-exercise rise in esophageal temperature (ΔTes) from baseline was greater with CAF in the HAB group (CAF=0.88±0.29°C, PLA=0.62±0.34°C, P<0.001), but not the NHAB group (CAF=1.00±0.42°C, PLA=1.00±0.39°C, P=0.94). For a given change in mean body temperature, rises in % of maximum skin blood flow were attenuated with CAF on the forearm (P=0.015) and back (P=0.021) in the HAB group, but not in the NHAB group (P≥0.65). Dry heat loss similar in the HAB (CAF=31±5 W·m-2, PLA=33±7 W·m-2) and NHAB groups (CAF=31±3 W·m-2, PLA 30±4 W·m-2) (P≥0.37). There were no differences in whole-body sweat losses in both groups (HAB: CAF=0.59±0.15 kg, PLA=0.56±0.17 kg, NHAB:CAF=0.53±0.19 kg, PLA 0.52±0.19 kg) (P≥0.32). CONCLUSION As the potential for both dry and evaporative heat loss was uninhibited by caffeine, we suggest the observed ΔTes differences with CAF in the HAB group were due to alterations in internal heat distribution. Our findings support the common practice of participants abstaining from caffeine prior to participation in thermoregulatory research studies in compensable conditions.