Fluid balance and hydration status in combat sport Olympic athletes: a systematic review with meta-analysis of controlled and uncontrolled studies

Abstract

Purpose: Athletes in Olympic combat sports experience body water fluctuations resulting from training and intentional dehydration when making weight. Despite the popularity of urine specific gravity (U SG ) and urine osmolality (U OSM ) measurement in characterizing fluid fluctuations, their utility remains questioned. This systematic review/meta-analysis examined the utility of urinary hydration indices in laboratory and field settings in Olympic combat sport athletes. Methods: 27 articles met the inclusion criteria for systematic review, 15 studies were included in the meta-analysis; with U SG and U OSM the main outcome variables. Meta-regression analyses evaluated the interrelationship among body mass (B M ), fluid intake, and urine measures. Results: Significant U SG alterations were observed following different sampling time frames: dehydration (ES 0.59; 95% CI 0.46–0.72; p = 0.001), follow-up period (ES 0.31; 95% CI 0.11–0.50; p = 0.002) and rehydration (ES − 0.34; 95% CI − 0.56 to − 0.12; p = 0.003). Direct comparison of laboratory (ES 0.20; 95% CI − 0.19 to 0.59; p = 0.324) and field (ES 0.35; 95% CI 0.14–0.56; p = 0.001) sampling showed marginally trivial and small effects. Small effects on U OSM were observed following dehydration (ES 0.31; 95% CI 0.12–0.74, p = 0.15), follow-up period (ES 0.39; 95% CI 0.08–0.70, p = 0.015) and rehydration (ES − 0.45; 95% CI − 0.60 to 0.30, p = 0.001). Meta-regression analysis suggests only fluid intake predicts U SG alterations (p = 0.044) during rehydration protocols. Conclusions: There were likely small changes in both U SG and U OSM readings across all experimental conditions, with moderate-to-large heterogeneity in all studies, except for U SG readings during dehydration protocols. The meta-regression failed to provide conclusive evidence concerning the interrelationship among urine measures, B M fluctuations, and fluid intake.