Proper Hydration at High Altitude

The standard dictum when advising persons who travel to high altitude, and thus expose themselves to a lower atmospheric oxygen concentration, is to stay "well hydrated," which translates into drinking sufficient liquid that they urinate frequently, with urine color being light (not concentrated). However, this recommendation has heretofore never been based on science, just on presumption and medical common sense. So, it is with great interest that I read an article in the current issue of Wilderness & Environmental Medicine, entitled "Hydration and the Physiological Responses to Acute Normobaric Hypoxia," authored by Alan Richardson, Peter Watt and Neil Maxwell (Wilderness & Environmental Medicine 20, 212-220 (2009).

The objective of the study was to identify how hydration status, above and below normal hydration levels, affects physiological responses and onset of acute mountain sickness (AMS) symptoms during acute normobaric (normal atmospheric pressure - equivalent to that at sea level) hypoxia (lowered concentration of oxygen in the air). In this study, eight males subjects completed intermittent walking tests in the condition noted after controlled normal hydration (euhydration), hyperhydration (too much water) and hypohydration (dehydration - too little water) protocols. During the measurement period of approximately 2 hours' exposure, heart rate, core body temperature, peripheral arterial blood oxygen saturation, urine osmolality (a measure of concentration and thus the state of hydration), and self-reported AMS scores were obtained.

The observations and analysis showed that the different states of hydration had a significant effect on all of these parameters, and that hydration state above (hyper-) and below (hypo-) normal hydration had detrimental consequences on physiological strain and onset of acute mountain sickness symptoms under the conditions studied.

This is very important work, and will undoubtedly spur further investigation. We are fairly familiar with the concept of hypohydration, which leads to dehydration and all of its deleterious effects upon performance and body functions. However, in the setting of high altitude, we are less familiar with hyperhydration (too much water), because we don't encounter it very often, unless it is induced by a doctor- or rescuer-led intervention. We suspect that fluid retention in general, when it occurs for whatever reason, may contribute to the accumulation of fluid in the brain (AMS) or perhaps even the lungs (high altitude pulmonary edema), but this has never been proven. The worsening of headache in this study (as a presumptive symptom of AMS and perhaps harbinger of fluid accumulation in the brain) in the hyperhydration group is a bold word of caution to us to attempt to achieve normal hydration, and nothing more, with our fluid replacement strategies. How best to do this? At the current time, the best we have in the field is maintaining urine color, specific gravity and/or osmolality (signs of urine concentration and thus state of hydration) at preferred values. However, with the advent of technologies such as that offered by Cantimer, we may soon have other methods by which to guide fluid administration, as thirst in and of itself is notoriously not sufficiently precise for this purpose.