It has been speculated that such a relationship may be due to sub-clinical pulmonary edema.[35] Similarly, elevated heart rate has been associated with AMS by some[13] but not all[34] authors; the current data which is supportive of the relationship is consistent GSK-3 beta pathway with the hypothesis of altered autonomic cardiovascular control leading to AMS.[36] Alternatively, some other factor which elevates heart rate may cause AMS

symptoms, such as dehydration.[13] Although data on hydration state and AMS is contradictory,[10, 13, 14] the current data suggest that fluid intake reduced AMS symptoms during the expedition as a whole. However, fluid intake had little effect when investigating more specific and conservative small molecule library screening definitions of AMS, possibly because the majority of participants achieved an intake of at least 2 L per day, recently speculated as the minimum intake required to avoid AMS.[37] On the other hand, these findings may be due to fluid intake reducing dehydration-associated headache rather than altitude-associated headache per se, a finding consistent with recent experimental studies suggesting that dehydration induces headaches of similar severity to hypoxia.[38] Weaknesses of the study include lack of

clinician and microbiological ADAMTS5 diagnosis of illness. However, such methods to verify diagnosis of illness have recently been scrutinized and found lacking.[39] While self-assessment may lead to underreporting of illness due to social desirability bias, controlling for this weakness would have been unlikely to improve accuracy of the health logs.[40] Finally, this observational cohort study was non-interventional and did not

include a control group. The longitudinal analysis that allowed estimation of causality and the multiple time-point baseline period at lower altitude, which was longer than accepted incubation periods for general illnesses,[20] addressed this issue. Furthermore, the present study’s control period, completed under expedition conditions and where individuals acted as their own controls, may be a stronger design than using a control group residing at low altitude but under non-expedition conditions. In conclusion, upper respiratory symptoms and anxiety increasingly contributed to symptom burden as altitude was gained. Data were consistent with increased heart rate, decreased arterial oxygen saturation, reduced fluid intake, and upper respiratory symptoms being causally associated with AMS. These findings are of relevance to researchers investigating travel-associated illnesses common at altitude.