INTRODUCTION: The arduous nature of infantry training increases the risk of exertional heat illness (EHI), which in its severest form can be fatal. Widely considered EHI risk factors include low aerobic fitness, elevated wet bulb globe temperature (WBGT) and hypohydration (Roberts et al., 2023). Unfortunately, the evidence supporting hypohydration as an EHI risk factor lacks objective measures and comparator controls.
AIM: Prospectively examine the association between pre-exercise hydration status and EHI risk.
METHODS: Between June 2021 and July 2024, 805 males and 2 females enrolled on Royal Marine Recruit training provided written informed consent and completed data collection. Before a criterion loaded march, widely considered EHI risk factors were assessed e.g., low aerobic fitness, overweight status, previous EHI, heat acclimatisation status and physician diagnosed injury or illness within 28 days. The night before the loaded march (2100 h), participants swallowed a telemetry capsule for gastrointestinal temperature assessment. On the day of the loaded march, urine samples were provided upon awakening (~0530 h) and after breakfast prior to commencing exercise (~0700 h). The 6.4-mile loaded march took place under cool conditions (mean ± SD, WBGT 11.2 ± 3.6 °C) and was completed in 66 ± 2 minutes. Participants wore a long sleeve t-shirt, combat trousers and boots and carried a 14.5 kg load. Urine osmolality was measured using freezing-point depression and thresholds for hypohydration (>900 mOsm.kg-1; Maughan et al., 2007) and euhydration (<500 mOsm.kg-1; Dolci et al., 2022) adopted. EHI cases were classified as “mild” who reported exercise-induced headache, dizziness or nausea and “severe” who exhibited central nervous system (CNS) dysfunction (Roberts et al., 2023), peak gastrointestinal temperature typically ≥39.5 °C and, where available, evidence of end-organ damage. A total of 790 male participants were included in statistical analysis. N=2 females were removed due to insufficient representation in the sample population and N=15 males were removed who exhibited CNS dysfunction but for whom a peak gastrointestinal temperature was unavailable. Unadjusted and fully adjusted multiple logistic regression was used to examine the association between widely considered risk factors and EHI.
RESULTS: A total of N=118 participants were classified as mild EHI (15%) and N=40 as severe EHI (5%). Urine osmolality was similar upon awakening (710 ± 231 mOsm.kg-1) and pre-exercise (687 ± 244 mOsm.kg-1, t-test P>0.05) and the proportion of hypohydrated (23%) and euhydrated (22%) participants was similar at both time points (ꭓ2 P>0.05). Hypohydration was not significantly associated with EHI risk in regression models (awakening adjusted OR=0.78, 95% CI 0.44‒1.38, P=0.39). However, adjusted regressions showed that EHI risk was increased by other widely considered risk factors, including low aerobic fitness (OR=2.68, 95% CI 1.56‒4.61, P<0.01), previous EHI (OR=2.39, 95% CI 1.48‒3.86, P<0.01) and elevated WBGT (OR=1.84, 95% CI 1.18‒2.86, P<0.01).
CONCLUSION: Hypohydration was not significantly associated with exertional heat illness risk during arduous activity in cool conditions. Significant exertional heat illness risk factors did include low aerobic fitness, previous exertional heat illness and elevated WBGT.