European Journal of Nutrition

, Volume 58, Issue 2, pp 497–514 | Cite as

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

  • Damir ZubacEmail author
  • Armin Paravlic
  • Reid Reale
  • Igor Jelaska
  • Shawnda A. Morrison
  • Vladimir Ivancev



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.


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.


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.


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.


Dehydration, combat sports Thermoregulation Water Systematic review 



This research did not receive any grant from funding agencies in the public or commercial sectors.

Compliance with ethical standards

Conflict of interest

Reid Reale is an employee of the Gatorade Sports Science Institute, a division of PepsiCo, Inc.; which is a manufacturer of beverages, sports drinks and hydration products. All other authors certify that they have no affiliations with or involvement in any organization or entity with any financial or non-financial interest in the subject matter or materials discussed in this manuscript.

Disclosure statement

Damir Zubac, Igor Jelaska, and Vladimir Ivancev are employed by the University of Split, Faculty of Kinesiology, Split, Croatia, EU. Armin H. Paravlic and Damir Zubac are employed by the Science and Research Center Koper, Institute for Kinesiology Research, Koper, Slovenia, EU. Reid Reale is employed by the Gatorade Sports Science Institute, a division of PepsiCo, Inc. Shawnda Morrison is employed by the University of Primorska, Faculty of Health Sciences, Polje 42, 6310 Izola, Slovenia. The views expressed in this article are those of the authors and do not necessarily reflect the position or policy of PepsiCo, Inc.

Supplementary material

394_2019_1937_MOESM1_ESM.docx (24 kb)
Supplementary material 1 (DOCX 23 KB)


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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  1. 1.Faculty of KinesiologyUniversity of SplitSplitCroatia
  2. 2.Science and Research Center KoperInstitute for Kinesiology ResearchKoperSlovenia
  3. 3.Gatorade Sports Science InstituteBradentonUSA
  4. 4.Faculty of Health SciencesUniversity of PrimorskaIzolaSlovenia

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