Objective.—High altitude and exposure to cold are associated with significant levels of dehydration because of cold-altitude urine output, high energy expenditures, and poor access to water. The aims of the present study were to measure the fluid intake and urine output among military mountaineers during their stay at high altitude and to study the level of fluid intake and decrease in urine output in relation with acute mountain sickness (AMS).

Methods.—This study used an analytic prospective follow-up design of hydration-dehydration conditions of a group of mountaineers with similar characteristics (military group). Data collected each day included quantity and type of fluid intake, urine output in 24 hours, other fluid output (as diarrhea or vomiting), and symptoms or signs of AMS according to the Lake Louise consensus score. Values are given as mean ± SE. A 1-factor analysis of variance procedure and t test were used to compare variables.

Results.—The mountaineers consumed a variety of fluids, including water, tea, coffee, soup, Isostar, and milk. Daily fluid intake was 2800 ± 979 mL, with a maximum intake of 4700 mL. Daily urine output was 1557 ± 758 mL. When we stratify our sample at the median by fluid intake, a significant correlation is detected with mean balance and mean urine output. Mountaineers developing AMS demonstrated reduced urine output (mean 1336 mL) when compared with those without AMS (mean 1655 mL).

Conclusions.—We found that fluid intake was associated but insignificantly correlated with incidence and degree of AMS. Past research suggests that vigorous hydration decreases incidence and severity of AMS and other altitude illnesses. Our results also imply that aggressive fluid intake is protective, but our limited sample size yielded insufficient power to demonstrate a statistically significant difference.