Early changes in body mass at high altitude and the development of acute mountain sickness.

Every year, millions of people travel to altitudes above 2,500 m for work or leisure, risking acute mountain sickness (AMS). The role of fluid retention in AMS development is controversial. This study examines the ability of simple hydration markers, collected within hours of arrival at high-altitude (HA), to predict AMS development. In 32 healthy, AMS susceptible individuals (female: 17; mean age: 38 ± 12 years), body mass (BM) and AMS severity were monitored over 21 h at HA (3,650 m). Fluid intake and toilet visits were recorded. BM changes within 3 and 9 h correlated with AMS severity the next morning (r²=0.168, p = 0.011 and r²=0.235, p = 0.003, respectively). Individuals with and without AMS showed different BM changes within 9 h (+ 1.18 ± 0.99 vs. +0.66 ± 0.65, p = 0.048, Cohens d = 0.62). Excessive early BM increase may indicate later onset of AMS, possibly related to fluid retention.