Journal Title (Medline/Pubmed accepted abbreviation): J Appl Physiol
Page numbers: 1792-1800
Summary of Background and Research Design
Background: High altitude exposure greatly reduces aerobic ability, increases fluid loss, and can lead to acute mountain sickness (AMS). Increased fluid loss can result in hypohydration (greater than 4% body mass loss) and is a concern for athletes, military personnel, and even civilians participating in recreational activities at high altitudes.
Hypothesis/purpose of study: To determine whether hypohydration or altitude exposure would diminish aerobic exercise performance and whether the combination of these factors would exacerbate the results and affect symptoms of acute mountain sickness.
Subjects: 7 lowlander men (mean age, 25 ± 7 yr; body mass, 82 ± 11 kg; percent body fat, 18.7% ± 3.9%)
Experimental design:4 separate, counterbalanced trials conducted in a crossover manner
Treatments and protocol:Subjects were evaluated over several weeks to determine baseline measures of body mass, body composition, percent body fat, peak power output, and peak oxygen consumption at both sea level and simulated (via barometric chamber) 3,048 meter (m) altitude. Subjects performed 3 to 4 altitude exposure trials to additionally prepare for the experiments. Four experimental trials were conducted 3 to 6 days apart, and all subjects completed each of the trials: sea level-euhydrated; sea level-hypohydrated; 3,048 m-euhydrated; 3,048 m-hypohydrated. In the hypohydration conditions, subjects were dehydrated 4% of their body mass. Subjects maintained their respective hydration status overnight, and the following morning entered a hypobaric chamber at either sea-level or 3,048 m. While in the chamber the subjects performed 30 minutes of submaximal, steady-state exercise followed shortly afterwards by a 30-minute performance time trial. Following a 2.5-hour rest, AMS was measured with the Environmental Symptoms Questionnaire-Cerebral Score (AMS-C) and the Lake Louise Scoring System (LLS).
Summary of research findings:
- Hypohydration resulted in 6% to 7% reduction from baseline in total body water, 10% to 12% reduction in plasma volume, and an increase in plasma osmolarity.
- There were no differences between the hypohydration groups at sea-level or altitude.
- Total work in the time-trial performance was lower in the sea level-hypohydrated group compared with sea level-euhydrated subjects, but did not reach statistical significance.
- Hypohydration plus altitude was significantly lower than all 3 other trials.
- Percent change in performance was significantly lower in both the sea level- and 3,048 m-hypohydrated groups compared with the sea level-euhydrated group (P < .04 and P < .001, respectively).
- Results from the AMS-C symptom severity score tended to increase when subjects were at altitude vs at sea level, but the difference was not statistically significant (P = .07).
- Heart rate values were higher in the hypohydrated conditions for both the submaximal and the time trials, but did not significantly differ between the sea level and altitude conditions.
Interpretation of findings/Key practice applications:
Hypohydration at 3,048 m does appear to adversely affect aerobic performance in an additive manner with that induced by increased altitude. Together, these variables resulted in a 33% decrease in performance, which is equivalent to the sum of the 2 variables evaluated independently. Although hypohydration did increase the occurrence and severity of AMS symptoms, this increase was not statistically significant. However, there was wide variability in the AMS scores and the number of subjects was small. Additionally, the study could not distinguish between symptoms of hypohydration and AMS, which may influence AMS scores. Therefore, more research is needed to better elucidate the impact of hypohydration on AMS.