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

PurposeAthletes 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.Methods27 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.ResultsSignificant U-SG alterations were observed following different sampling time frames: dehydration (ES0.59; 95% CI 0.46-0.72; p=0.001), follow-up period (ES0.31; 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 (ES0.20; 95% CI -0.19 to 0.59; p=0.324) and field (ES0.35; 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 (ES0.31; 95% CI 0.12-0.74, p=0.15), follow-up period (ES0.39; 95% CI 0.08-0.70, p=0.015) and rehydration (ES -0.45; 95% CI -0.60to0.30, p=0.001). Meta-regression analysis suggests only fluid intake predicts U-SG alterations (p=0.044) during rehydration protocols.ConclusionsThere 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.