Background: Progressive exercise-induced dehydration may impair aerobic exercise performance (AEP). However, no systematic approach has yet been used to determine how pre-exercise hypohydration, which imposes physiological challenges differing from those of a well-hydrated pre-exercise state, affects AEP and related components such as peak oxygen consumption (VO2peak) and VO2 at lactate threshold (VO2LT) Objective: To determine, using a systematic approach with meta-analysis, the magnitude of the effect of pre-exercise hypohydration on AEP, VO2peak and VO2LT Design: This was a systematic review with meta-analysis of well-controlled studies. Data Sources: MEDLINE, SPORTDiscus and CINAHL databases and cross-referencing. Inclusion Criteria for Selecting Studies: (1) well-controlled human (= 18 years) studies; (2) pre-exercise hypohydration induced at least 1 h prior to exercise onset; (3) pre-exercise body mass loss in the hypohydrated, experimental condition was = 1% and = 0.5% than the well-hydrated, control condition; (4) following the dehydrating protocol body mass change in the control condition was within - 1% to + 0.5% of the well-hydrated body mass. Results: A total of 15 manuscripts were included, among which 14, 6 and 6 met the inclusion criteria for AEP, VO2peak and VO2LT, respectively, providing 21, 10 and 9 effect estimates, representing 186 subjects. Mean body mass decrease was 3.6 ± 1.0% (range 1.7-5.6%). Mean AEP test time among studies was 22.3 ± 13.5 min (range 4.5-54.4 min). Pre-exercise hypohydration impaired AEP by 2.4 ± 0.8% (95% CI 0.8-4.0%), relative to the control condition. Peak oxygen consumption and VO2LT, respectively, decreased by 2.4 ± 0.8% (95% CI 0.7-4.0%) and 4.4 ± 1.4% (95% CI 1.7-7.1%), relative to the control condition. Compared with starting an exercise hypohydrated, it is respectively likely, possible and likely that AEP, VO2peak and VO2LT benefit from a euhydrated state prior to exercise. Meta-regression analyses did not establish any significant relationship between differences in body mass loss and differences in the percent change in AEP or VO2LT. However, VO2peak was found to decrease by 2.6 ± 0.8 % (95% CI 0.7-4.5%) for each percent loss in body mass above a body mass loss threshold of 2.8%. Conclusion: Pre-exercise hypohydration likely impairs AEP and likely reduces VO2LT (i.e., the aerobic contribution to exercise was lower) during running and cycling exercises = 1 h across different environmental conditions (i.e., from 19 to 40 °C). Moreover, pre-exercise hypohydration possibly impedes VO2peak during such exercises.
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