Purpose: The present study determined the association between body fluid variation and the development of acute mountain sickness (AMS) in adults.
Methods: Forty-three healthy participants (26 males and 17 females, age: 26 ± 6 yr, height: 174 ± 9 cm, weight: 68 ± 12 kg) were passively exposed at a FiO2 of 12.6% (simulated altitude hypoxia of 4500 m, PiO2 = 83.9 mmHg) for 12-h. AMS severity was assessed using the Lake Louise Score (LLS). Food and drink intakes were consumed ad libitum and measured; all urine was collected. Before and after the 12-h exposure, body weight and plasma osmolality were measured and whole-body bioimpedance analysis was performed.
Results: The overall AMS incidence was 43% (38% males, 50% females). Participants who developed AMS showed lower fluid losses (3.0 ± 0.9 vs. 4.5 ± 2.0 ml/kg/h, p = 0.002), a higher fluid retention (1.9 ± 1.5 vs. 0.6 ± 0.8 ml/kg/h, p = 0.022), greater plasma osmolality decreases (-7 ± 7 vs. -2 ± 5 mOsm/kg, p = 0.028) and a larger plasma volume expansion (11 ± 10 vs. 1 ± 15%, p = 0.041) compared to participants not developing AMS. Net water balance (fluid intake--fluid loss) and the amount of fluid loss were strong predictors whether getting sick or not (Nagelkerkes r(2) = 0.532). The LLS score was related to net water balance (r = 0.358, p = 0.018), changes in plasma osmolality (r = -0.325, p = 0.033) and sodium concentration (r = -0.305, p = 0.047). Changes in the impedance vector length were related to weight changes (r = -0.550, p<0.001), fluid intake (r = -0.533, p<0.001) and net water balance (r = -0.590, p<0.001).
Conclusions: Participants developing AMS within 12 hours showed a positive net water balance due to low fluid loss. Thus measures to avoid excess fluid retention are likely to reduce AMS symptoms.