INTRODUCTION: Extreme environmental conditions are associated with adverse changes in health and performance. Changing environments during exercise can also impact nutritional needs. The IRONMAN World Championship (IMWC), held on the island of Hawai`i, sees athletes race 226.2 km through 4 climate zones across ~8 to 17 hours. The purpose of this investigation was to describe the environment that triathletes encountered during the 2019 IMWC and compare those conditions vs. the host-city temperature. METHODS: We recorded a 10-min mean, environmental measurement (wet bulb globe temperature (WBGT), dry bulb (DB)) experienced by the leader (LEAD) and athletes finishing in ~9 (FRONT) and 11.5 h (MID). Measurements were taken at transition 1 (T1), three bike locations (~56, 100, 145 km; B56, B100, B145, respectively), transition 2 (T2), two run locations (5, 27 km; R5, R27, respectively) and the finish. Online reported DB temperature was also recorded for Kailua-Kona. Solar irradiance (W/m2) was measured for LEAD. Measurements were taken at 12 time points for each group, from ~06:30 to 18:50. Duration in WBGT heat stress flag levels was calculated for bike and run segments. Box Cox transformation was applied prior to ANOVA, followed by Tukey`s HSD if significant (p<0.05). RESULTS: There were significant differences in WBGT (p=0.026) and DB (p<0.001) on course, but not between groups. WBGT (°C) ranges were 24-30 (T1), 27-31 (B56), 25-28 (B100), 29-32 (T2), 28-30 (R8), 25-30 (R27), 29-30 (FINISH). The online reported DB for Kailua-Kona was significantly lower than what athletes experienced on course (29.1±0.22 vs. 30.5±0.53 °C, respectively, P<0.001). DB (°C) for LEAD, FRONT, MID, respectively, was 26.0, 28.1, 28.6 (T1), 29.6, 31.3, 31.2 (B56), 26.6, 27.8, 27.9 (B100), 31.3, MD, MD (B145), 35.5, 33.3, 33.5 (T2), 29.1, 30.4, 29.9 (R5), 31.7, 31.3, 29.2 (R27), 32.6, 32.1, MD (FINISH). Online DB (°C) for LEAD, FRONT, MID, respectively, was 26.4, 27.2, 27.2 (T1), 27.8, 28.6, 29.2 (B56), 28.9, 29.4, 30.0 (B100), 29.4, 30.0, 30.0 (B145), 30.0, 30.0, 29.4 (T2), 30.0, 30.0, 29.4 (R5), 30.0, 29.4, 28.6 (R27), 29.7, 29.2, 27.8 (FINISH). Solar irradiance for LEAD ranged from 17-1005 W/m2. Duration (h) in high stress was 4.2 (LEAD) and extremely high 2.8 (LEAD), 7.8 (FRONT). CONCLUSION: While mean environmental conditions were similar between groups, athletes experienced changing conditions during the race. Importantly, leaders spent less time in high/extremely high heat stress conditions compared to slower racers, which may have implications for pacing strategy of the latter. Easing race intensity during the bike may, therefore, enable higher intensities during a run performed in less extreme environmental conditions. As environment impacts exercise performance and nutritional needs, when developing a racing and nutrition strategy for events in extreme environments, athletes should consider both the environment for the host city and how the environment may change based on the course and expected finishing times.
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