Examining the effects of caffeine ingestion on human thermoregulation during exercise in the heat in habituated and non-habituated individuals

Purpose: In most thermoregulatory research, participants are asked to abstain from consuming caffeine prior to experimental trials due to an assumed confounding effect on core temperature, skin blood flow and/or sweating responses. Yet, evidence supporting this notion appears inconclusive. We aimed to rigorously assess the impact of acute caffeine ingestion on thermoregulatory responses to steady-state exercise in the moderate heat in both caffeine-habituated and non-habituated individuals. Methods: A total of 28 participants (Habituated (HAB), n=14, 4 women; Non-Habituated (NHAB), n=14, 6 women) cycled at a fixed metabolic heat production on two separate occasions: 60 minutes after ingesting i) 5 mg·kg-1 caffeine (CAF) or ii) 5 mg·kg-1 maltodextrin placebo (PLA), in a double-blinded randomised and counter-balanced order. Environmental conditions were 30.6±0.9°C, 31±1 % RH. Results: The rise in esophageal temperature from baseline (ΔTes) with CAF was greater in the HAB group (P0.05). WBSL was not between CAF and PLA (P=0.18). Conclusion: A greater ΔTes with CAF was observed in the HAB group but not the NHAB group, possibly due to an attenuated skin blood flow on the forearm and back with CAF compared to PLA in the HAB group only. These findings support the practice of requesting habituated caffeine-users to abstain from all sources of caffeine prior to participating in thermoregulatory research studies. However, the restriction of caffeine intake for non-habituated individuals (from incidental sources, e.g. chocolate, tea etc) seems unnecessary