Changes in body water distribution and phase angle following rapid ascent to 3,680 m and their association with acute mountain sickness: a prospective cohort study

Background Acute mountain sickness (AMS) is a common pathological response following rapid ascent to high altitude, and its underlying mechanisms remain incompletely understood. This study focused on investigating the early changes in body composition following acute high-altitude exposure and their association with AMS. Methods A prospective cohort study was conducted on healthy individuals who flew from plain (500 m) to plateau (3,680 m). Multi-frequency bioelectrical impedance analysis was used to measure body composition indicators within 24 h before and 24 h after ascent. AMS was diagnosed and its severity assessed using the Lake Louise Score System (LLSS). Comparisons of body composition changes before and after high-altitude exposure were performed, and intergroup comparisons were conducted based on the occurrence of AMS. Spearman correlation analysis and multiple linear regression were used to explore the relationships between body composition changes and LLSS score. Results A total of 34 participants were included, and the AMS incidence was 61.76%. Compared with the plain baseline, after rapid ascent to high altitude, extracellular-to-intracellular water ratio (ECW/ICW) ( t = −2.41, p = 0.022) and phase angle (PhA) ( t = −4.78, p < 0.001) decreased significantly, and participants with AMS exhibited greater magnitude decrease in both ΔECW/ICW ( Z = −2.14, p = 0.032) and ΔPhA ( Z = −3.01, p = 0.003). Spearman correlation analysis showed that both ΔECW/ICW ( r = −0.447, p = 0.008) and ΔPhA ( r = −0.646, p < 0.001) were negatively correlated with the LLSS score. However, all results of ECW/ICW and ΔECW/ICW shown no statistically significant after Bonferroni correction. Multiple linear regression analysis indicated that only ΔPhA was an independent predictor of the LLSS score ( β = −0.507, p = 0.001). Conclusion After rapid exposure to high altitude, body water often shifted into cells and pronounced cellular-level dysfunction occurred. Susceptible individuals with AMS exhibited a more pronounced decrease in PhA. The change of PhA (ΔPhA), rather than the change of ECW/ICW (ΔECW/ICW), was an independent predictor of AMS severity. Monitoring ΔPhA may provide an effective, non-invasive method for early risk warning of AMS.