Dehydration (DEH) is believed to impair cognitive performance but which domains are affected and at what magnitude of body mass loss (BML) remains unclear.
Purpose: To conduct systematic literature review and meta-analysis to determine the effect size (ES) of DEH on cognitive performance and influence of experimental design factors (e.g., DEH > 2% BML).
Methods: Thirty-three studies were identified, providing 280 ES estimates from 413 subjects with DEH ranging from 1% to 6% BML. Outcome variables (accuracy, reaction time), cognitive domains, and methods to induce DEH varied. Effect sizes were calculated using standardized mean differences and multivariate meta-analysis.
Results: Impairment of cognitive performance (all domains/outcomes) with DEH was small but significant (ES = -0.21; 95% confidence interval [CI]: -0.31 to -0.11; P < 0.0001) with significant heterogeneity (Q(279) = 696.0, P < 0.0001; I = 37.6%). Tasks of executive function (ES = -0.24; 95% CI: -0.37 to -0.12), attention (ES = -0.52; 95% CI: -0.66 to -0.37), and motor coordination (ES = -0.40 to 95% CI: -0.63 to -0.17) were significantly impaired (P ≤ 0.01) after DEH, and attention/motor coordination was different (P < 0.001) from reaction time specific tasks (ES = -0.10; 95% CI: -0.23 to 0.02). Body mass loss was associated with the ES for cognitive impairment (P = 0.04); consequently, impairment was greater (P = 0.04) for studies reporting >2% BML (ES = -0.28; 95% CI: -0.41 to -0.16) compared with ≤2%; (ES = -0.14; 95% CI: -0.27 to 0.00).
Conclusions: Despite variability among studies, DEH impairs cognitive performance, particularly for tasks involving attention, executive function, and motor coordination when water deficits exceed 2% BML.