Caffeine and diuresis during rest and exercise: A meta-analysis.
Level 1 - systematic review of randomized trials
Meta-analysis of controlled intervention studies
PubMed 25154702 · doi:10.1016/j.jsams.2014.07.017
What was done
Authors searched three electronic databases through November 1, 2013, to identify studies evaluating the effect of acute caffeine ingestion on urine volume in healthy adults compared to non-caffeine control conditions. Data from 16 studies yielding 28 effect sizes were pooled using a random-effects meta-analysis model. Subgroup analyses were conducted to evaluate the moderating effects of exercise status (active vs. resting) and participant sex.
What was found
Across all conditions, caffeine (median dose 300 mg) produced a minor increase in urine volume: overall effect size (ES) = 0.29 (95% CI 0.11 to 0.48, p = 0.001), representing an average increase of 109 ± 195 mL (16.0 ± 19.2%). Subgroup analysis showed this effect was present at rest (ES = 0.54, 95% CI 0.22 to 0.85, p = 0.001) but absent during exercise (active ES = 0.10, 95% CI -0.07 to 0.27, p = 0.248; difference Cochran's Q, p = 0.019). Females showed a significant diuretic response (ES = 0.75, 95% CI 0.38 to 1.13, p < 0.001), whereas males did not (ES = 0.13, 95% CI -0.05 to 0.31, p = 0.158; difference Cochran's Q, p = 0.003).
Why it matters
Concerns that moderate caffeine intake compromises hydration during physical activity appear unfounded, as exercise suppresses caffeine-induced diuresis. Athletes can use moderate caffeine doses for performance benefits without meaningful risk of exercise-induced fluid deficits.
Limits
The meta-analysis was limited to 16 studies with a relatively small total number of effect sizes. Total participant sample size, habitual caffeine use, ambient temperature, baseline hydration status, and fluid intake protocols were not reported in the abstract.