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 (USG) and urine osmolality (UOSM) 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 USG and UOSM the main outcome variables. Meta-regression analyses evaluated the interrelationship among body mass (BM), fluid intake, and urine measures.
Results: Significant USG 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 UOSM 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 USG alterations (p = 0.044) during rehydration protocols.
Conclusions: There were likely small changes in both USG and UOSM readings across all experimental conditions, with moderate-to-large heterogeneity in all studies, except for USG readings during dehydration protocols. The meta-regression failed to provide conclusive evidence concerning the interrelationship among urine measures, BM fluctuations, and fluid intake.
Keywords: Dehydration, combat sports; Systematic review; Thermoregulation; Water.