Hydration and cardiorespiratory responses to a simulated 9.65 km foot march in healthy adults: Implications for research and training.

Foot marches are a common occupational task for service members and require walking long distances while carrying heavy occupational loads (e.g., 25 kg fighting load). The physical exertion associated with foot marches may induce sweat-related dehydration and altered thermoregulation, potentially leading to impaired performance and increased risk of exertional heat illness. Yet, many studies fail to assess hydration, even in controlled settings. The purpose of this study was to evaluate hydration and cardiorespiratory responses to a simulated, laboratory-based 9.65 km foot march and examine relationships between participant characteristics and outcomes. Sixteen (10 male, 6 female) adults with load carriage experience performed a 9.65 km foot march at 4.82 km/h on a treadmill in a temperature-controlled (22.2 °C) laboratory. Participants received no fluid during the march and 1 L of water during a 1h recovery. Hydration was assessed pre-, mid-, and post-march and post-recovery via urine specific gravity (USG), nude dry mass, and perceived thirst. Heart rate and breathing rate were averaged across each half of the march. Repeated measures analysis of variances (ANOVAs), paired t-tests, and Pearson correlations (α = 0.05) assessed changes and associations with characteristics. Participants became mildly dehydrated (USG: p < 0.001, ηp2 = 0.622; post-march 1.020 ± 0.005) with 1.5 ± 0.7% body mass loss. Heart rate (p < 0.001, ηp2 = 0.671) and breathing rate (p < 0.001, ηp2 = 0.788) were elevated while marching versus recovery but did not differ between march halves. Fat mass (R = 0.618, p = 0.011) and body fat percentage (R = 0.537, p = 0.032), but not aerobic fitness, were significantly correlated with USG changes. Even under controlled laboratory conditions, the fluid-restricted foot march induced mild dehydration, with greater adiposity associated with increased susceptibility. These findings underscore the need for hydration assessment, in both research and training settings, using measures such as USG, body mass change, and thirst to better manage dehydration risk and its implications for performance.